Thursday, January 26, 2012

Deformation and GPS

We get a LOT of questions about volcanoes, including how to Know if they'll Blow. There are a number of ways we can track magma movement at depth, including deformation and "LP's" - long-period seismic tremor that is indicative of fluid movement. At late stages of unrest, we will start seeing "VT's" - short-period volcanic tremor that is indicative of shallow rock-breaking - and increases in CO2 and H2S gases. There is at least the possibility that we can detect early movement of magma at 30 - 40 km depths using magnetotelluric systems, but so far there hasn't been funding to try this. As I write this, deformation reaches out the longest time ahead of all these detection systems to give us warning of an impending eruption.

The term "deformation" is used by specialists in ground movement in the geosciences; these guys themselves are called "geodesists". Geodesists measure movement as a component of strain along an active fault, to try to get a sense of the energy accumulating that could lead to an earthquake. Deformation is used in volcanology to look for - and then track - inflation in a volcanic edifice. Deformation is done in several ways:

  1. By surveying the ground with high precision. This has been done at Yellowstone since the mid-1920's, and those early data have helped us get a much better sense of how the huge caldera moves and breathes. 
  2. By deploying tiltmeters. Originally these were long tubes of water laid out over the ground. If the ground under the flank of a volcano started tilting, it would show up in amplified movement of water in vertical tubes at the end of the long tube. Modern tiltmeters are ultra-sensitive cylinders placed in a vertical hole in the volcanic rock, then packed in with sand. The signal from these devices and all the following systems is generally telemetered back to a recording and monitoring system. 
  3. By using radar satellites - this is called InSAR for Interferometric Synthetic Aperture Radar. If two images can be captured over the same volcano, they can be used to make interferograms. These are colored, Moire patterns - generated with enormous mathematical calculations to geometrically correct and ratio each pixel to another, called "rubber sheeting" - that will show inflation over the surface of a volcano and its environs. Each rainbow-colored ring-set represents one radar wavelength (typically 5 - 15 centimeters) of uplift. These often form a bulls-eye centered over an inflating volcano or deflating caldera, and I've seen several gorgeous examples at Ngiragongo volcano, in Central Africa; at Pavlof, Akutan, Okmok, Shishaldin, and many other volcanoes in the Aleutians, and at Mauna Loa and Kilauea volcanoes in Hawai'i. 
  4. Gravity level-lines. This is like survey leveling, but is done by making repeat measurements with a gravity meter over a line of stations every six months or so. All other things (including the water table) being equal, an inflating volcano will show up as a decrease in the gravity field - the gravimeter is being moved farther away from the Earth's center, and the pull of gravity falls off as (1/radius distance squared). I did this to monitor magma moving into the Harrat Rahat volcanic field east of Madinah al-Munawarrah ("Medina") in Saudi Arabia. Seismic telemetry also showed small earthquakes associated with this magma movement at the same time. The events died out by 1995, causing a lot of people to breathe a collective sigh of relief, but this kind of one-again-off-again restive behavior is not at all unusual for a volcano.
  5. Telemetered GPS. These use the same GPS satellites you and I utilize in our cars or when hiking, but the precision measurements made by geodesists (the formal name for the deformation guys) are made using different signals from the same satellites.
  6. We also instrument volcanoes with sensitive analog, and ultra-sensitive broadband seismic sensors. Some of these data are telemetered, some are recorded and just stored in the instrument box on a small hard-drive until retrieval the following summer. That is, unless bears decide to play ball with one. One over-winter seismic network campaign at Katmai in Alaska found 5 of 11 very expensive stations had been trashed by bears before they could get back and retrieve them. 
GPS is a fascinating field, and applies far beyond the earth sciences. A brief run-down might be useful here.

The Global Positioning System was first envisioned by DARPA - the Defense Advanced Research Projects Agency of the Department of Defense - during the 1980's. Navigation at that time was complex and difficult, and getting any sort of location precision over vast distances including oceans was very important to some people. Like, the people targeting ballistic missiles, for instance.

In the late 1980's I worked in the Venezuelan jungle, where our main form of navigation was using 1:250,000-scale airborne radar (SLAR) maps. These were assembled by flight strips - and it was not unusual to find splice errors as large as 3 kilometers. Basically that means I could be standing on a rock - and half of the rock was 2 miles along the strip edge from the other half of the rock. I have been on a helicopter traveling for an hour over trackless forest using a half-meter-sized roadmap of the country (except there are no roads in the jungle) and crudely-penciled lines with the azimuth and distance for the site we wanted to visit. If that helicopter's fuel line had a single bug in it, we would have dropped down into the trees. Even assuming we had survived such a crash (the incident statistics gave me a 50% chance of this), how would you call in a rescue helicopter? How in the world would you tell them where you were?!?

I first began using a GPS device in the early 1990's in Saudi Arabia. In the northern reaches of the country there is a vast plain that is dead flat for hundreds of kilometers in all directions. Some of our guys had accidentally strayed across the Iraqi border because there is no way to know where the line arbitrarily drawn by the British a century earlier actually was. The first GPS units were incredibly slow, the size of a Betty Crocker cookbook, and didn't always work - but the idea fascinated me. With a radio, I could then precisely tell people where I was. 

Since then, hand-held GPS devices have shriveled to matchbox sizes, strap to your wrist, and have maps built in. You can program them, collect precise tracks... the list of bells and whistles goes on and on.

But how do they work? What actually is out (or up) there?

The American GPS constellation has at any given time about 24 active satellites and a few loitering spares, and each one transmits a very faint signal on two freqs - digital signal for hand held use and another digital carrier that is used for precision location acquisition - I'm talking centimeter-size precision here. BOTH frequencies are encrypted... they belong to the military, and for a long time the signals were deliberately "fuzzed" - this was called Selective Availability, or SA for short. If you had the key and a certain type of book-sized device, you could get very precise locations - within 10's of meters. But DOD didn't want someone else using those signals to pop an artillery round on top of one of their military outposts. Even to this day, if you try to use a receiver and go faster than a commercial airliner (as in: a ballistic missile) it won't work. It has a built-in fail-safe.

In the meantime, the rest of the world has become incredibly dependent on the GPS constellation. I could never summarize adequately all the ways and places where it is used right now.

If you are surveying - or trying to see if two points on the opposite sides of a volcano are moving apart from other (uh-oh), then you need great precision. It can now be as good as a bit over a centimeter horizontally and 2-3 centimeters vertically. In part this difference in precision is because for horizontal solutions you can subtract the atmosphere effect from two different near-horizon satellites - and triangulate better. For vertical elevations, you have only satellites in one direction (not beneath your receiver).

GPS signals all use the same frequency, but the signals are encoded to separate the satellites. Both transmitted signals from each are encoded, so you can't use one for a ballistic missile guidance system unless you own the codes. As I said, above a certain aircraft speed, GPS won't work.

Well, the Russians certainly didn't want to be dependent on something that the Americans could fuzz - or even turn off. So despite their crushing economic difficulties, they turned the best Russian minds onto building their own constellation. This is called GLONASS, and the signal is not encoded, the energy transmitted is greater, so the signal-to-noise ratio is 5 times or 15 db better. Because of this, the signal penetrates tree canopy, so I could use it in the jungle! Woo-HOO! The GLONASS system also uses 3 different frequencies, so you can reduce ambiguities and calculate better differential atmospheric corrections.

These GNSS (Global Navigation Satellite Systems) are so precise that they routinely calculate and correct for relativistic effects! There are also huge atmosphere effects that must be compensated for - dense air masses here and and ionized layers there. GLONASS even works on new American and European hand-held devices when the GPS signals are poor due to a poor view of the constellation - if you've ever been in steep canyons in Utah or New York City, you will know what I mean here.

Not to be outdone, the European Union is now experimenting with their own GNSS (Global Navigation System) called GALILEO. This is a purely civilian system with three frequencies, and is scheduled to come online in 2015 - they are testing 2 satellites in orbit right now.

For the same reasons, the Chinese have started their own COMPASS satellite GNSS system, and it likewise is coming on line rapidly - there are 6 satellites in orbit already, and thee would have been more if a recent Russian rocket system hadn't crashed. Not to be left behind, the American version of GNSS - the only one that should technically be called "GPS", is being upgraded.

All four of these GNSS systems use L-band frequencies to resolve ambiguities and increase precision - and penetrate the ionosphere. What does L-band mean? Look at your personal GPS system and the smallest dimension on it will give you an idea of the wavelength for L-band.

The navigation problem is more than just triangulation - three satellites near the horizon would serve for this; two would give you two possible location solutions, three would mean only one possible solution. But there are four unknowns, since you are measuring how long a stretch of space and air that your signal must travel. The precision of your timing thus becomes utterly critical, the speed of light being so huge (300,000 km/second), and hand-held GNSS devices cannot carry $100,000 maser clocks. Thus, you must use a 4th satellite to help solve for the 4th unknown: 3 for position, 1 for a clock reference for your receiver

There are a few more complications. You really need to use a reference ground station to get really good differential distance calculations - to do good back-corrections for the changing satellite orbits, the complex and varying atmosphere, snow cover, etc... However, during the Tohoku earthquake in early 2011, all of Japan jerked eastward, so geodesists couldn't see the whole shift with really great precision because their reference station also moved.

So how does this help volcanologists? As I said earlier, if two telemetered GNSS receivers are moving away from each other, and there is a volcano in between them (this is happening right now with Mauna Loa, the largest volcano on Earth), then you are being given a warning that something is coming.

In 1989 we didn't have such a warning before Redoubt volcano in Cook Inlet of Alaska erupted. A KLM Boeing 747 flew right into the ash cloud - and lost all four engines in rapid succession. I've got a recording of the captain's voice as she tries to guide her flight crew in Dutch and talk with flight control in Anchorage in English. Her voice rises steadily a full octave before she finally yelled "Anchorage we have lost all four engines, we are in a fall. We can use all the help you can offer." They managed to restart two of the engines, and made a rough landing at Anchorage International airport. No lives were lost - but the repairs to that Boeing 747 cost $80 million.

To put that in perspective, when I served as chief scientist for volcano hazards for the US Geological Survey, my entire science team budget was less than $20 million.

There was another interesting GNSS application that you will find fascinating - I sure did. When Mount St Helens erupted on 1 October, 2004, we had just a week of accelerating seismic racket on our network beforehand for a warning. The extrusion was first seen on October 12 - and by pure luck I got the first photo of the new "spine" from a helicopter orbiting the steaming and fractured Crater Glacier. The dacite extrusion - 700 degrees C at where it was coming up from the talus slope at its base - came out like a tube of squeezed gray toothpaste. It resembled the back of a whale, so that became its name: The Whale. It moved south through crumbling talus and ice until it hit the remaining south rim of the 1980 eruption. The geodesists wondered when it actually reached the wall - When Did The Whale Hit the Wall? A check of a GPS station on the other side, on the outside south slope of the volcano, answered the question. On November 17, 2004, that station suddenly started moving south. Was it an effect of snow on the antenna? No, because the only direction it moved was south - by about 10 cm. The entire crater wall was shoved southward by 4 inches.

I'll never forget the elation of scientists using GPS technology to answer a real question about an erupting volcano. But GNSS systems provide us more than just answers to our scientific curiosity.

In 2006 a sharp-eyed geodesist in Anchorage, Alaska, was routinely checking data from several GPS units installed on Augustine volcano in the middle of Cook Inlet, south of Anchorage. This had erupted in 1979 and nearly killed David Johnston, one of our brightest young geologists who was later killed during the 1980 lateral blast, the opening eruption salvo of Mount St Helens.

In August 2006 this geodesist noticed some differential movement apart - the first subtle inflation was starting - and notified the Scientist-in-Charge. A close checking and monitoring effort was triggered - and sure enough, the signal was real, showing above all the background noise - and it was continuing. Federal and State Emergency entities, along with the FAA, were put on notice. In Late December the first VT's started appearing on the seismometers. As they accelerated in frequency and amplitude, the USGS issued a warning: an eruption is imminent in hours or days. One day later, on January 16, 2007, Augustine erupted, and dusted Anchorage with ash. International flights were cancelled or re-routed for three days - but not a single aircraft was damaged, not a single life was lost.

Yeah! This stuff works!


Wednesday, January 25, 2012

Lava Tubes

If you've never walked through a lava-tube, you are in for a Bucket List experience. There are many in Hawai'i (of course), but there are others in the Pacific Cascades of America and even in Idaho. I have a permanent dent in my forehead, a trophy obtained while climbing through Upper Ape Cave, on the south side of Mount St Helens, in Washington, State.

WEAR HELMETS. Do as I say, not as I did.


I’m writing a sci‐fi novel and would like to know what kind of rock makes up a lava tube? As far as I can tell with my feeble mind, it’s basaltic rock, is this right? I’ve tried searching the internet and can’t find a definite answer. Can you help me?
Thanks ahead, J.R.M.

A: Yep, you're right. Higher-silica lava like rhyolite and dacite don't make tubes - but crusty domes instead. It helps if you understand how lava tubes are formed. I've walked through lava-tubes in Hawai'i and Mount St Helens, and you can see that everything - even the "bathtub rings", is basalt. At Pu'u O'o, part of the East Rift Zone of Kilauea, a friend used a police speed-gun to clock the yellow-glowing basalt magma at 40 kph (25 miles per hour) as it shot through the active tube past a skylight. As lava pours down a slope, it finds the natural drainages (or makes its own) and follows them. The lava on the edges of these flowing, yellow-red rivers starts to cool, and then starts to crust over. When the cover is complete, you have a lava-tube that is now insulated from the (relatively) cold air, and liquid lava can now maintain its heat and travel farther. After the the hydraulic pressure stops from above, the tube drains, empties out, and cools off. The inside isn't perfectly smooth, either; there are lots of irregularities, and these are fascinating. They give subtle insights on how magma flows - and "paints" and drips and leaves "bathtub rings" in the walls of the tube.

The roof can be up to 7 meters tall, and boulders and cold lava in the hot lava path get incorporated into the flow or partially dam it. There are parts of the roof that break off - exposing skylights - and these slabs travel down as solid chunks (at least for awhile) in the lava. I have a permanent dent in my right forehead from making a right turn into one of these lava "blades" hanging out of a roof in the Ape's Cave lava tube at Mount St Helens. In my office I have a USGS cap soaked in dried blood from this experience: sort of a trophy, and a reminder to be more careful.


Tuesday, January 24, 2012

CME Events - How They Affect Your Life

Let's switch briefly to the interface between Earth and Space. Specifically, the probably-novel-for-most-people idea of "space weather."

Two days ago, something not unknown, but not commonplace either, happened on the surface of the Sun. Something called a Coronal Mass Ejection (CME) event took place. These are complex phenomena, still rather poorly understood, but involve the Sun's powerful magnetic field and the huge energy being generated by hydrogen fusion. The result is a huge ball of ionized material shooting off into space. This particular one was aimed at Earth and Mars. It is the space equivalent of a Category 5 hurricane.

The Solar Dynamics Observatory satellites first picked up this solar flare erupting from the sun on January 22, 2012. Almost immediately there was a large burst of radiation, higher levels than have been measured since 1989. The highly energetic ionized particles erupted from the sun as part of this CME traveled well above the speed limit - at roughly 2,200 kilometers per second - and hit Earth on January 24 around 10am US Eastern Time.

Why would you possibly care?

Because there are some amazing side effects from one of these events that can directly affect you.

The huge ion bomb, when it strikes Earth's protective magnetic field, is partially deflected towards the poles. Auroras sometimes reach the mid-continental United States during these events. They set up huge telluric currents in the Earth's surface - these are short-circuited by the oceans, but the shallow continental crust is resistive to varying degrees. Basic physics says that if current is flowing, there must be a voltage difference causing it. Electrical power grids generally transmit electrical energy at very high voltages (in the 100,000 range and higher) in three "phases" - in other words, the power on each of the three lines held up by transmission towers is at 60 Hz (in North America; 50 Hz in Europe and the British Commonwealth), and each line's alternating signal is out of phase from the other two by 120 degrees. Think of a wheel turning through 360 degrees for each cycle, then each cycle will peak at 4 o'clock, 8 o'clock, and 12 o'clock in sequence.

But the power transmission system is never perfectly insulated - electrical charge inevitably "leaks", and for this reason each tower and power substation has a fourth electrical line called a "ground" to take care of that leakage. This is the fat round opening on the bottom of most electrical sockets. In our homes, we only "see" two phases and a ground after the voltage is stepped down by transformers to 220 volts and 110 volts. The assumption underlying all grounds is that the Earth is all at the same base voltage - the same reference point. However, if there is a large telluric current (this word means an electrical current flowing through the ground), then the voltage of each tower's "ground" and each substation's "ground" is going to be different. Inevitably something will become unstable - a ground-loop is set up - and an excessive voltage at some point is going to lead to an "arcing" or jump of current to someplace where it shouldn't be going. It's not unusual for transformers the size of a car or an SUV to explode violently when this happens. This is commonly seen in video of a city skyline as a tornado approaches: the transformers "pop" with a flash, one by one.

How could this affect you? The Canadian provinces of Quebec and Ontario experienced a huge and long-lasting blackout due to a CME event back in 1989. In mid-winter, if electricity is your source of heat, this could be a life-threatening event. If you survive, your water pipes will freeze and burst, and you will have heck to pay when it warms up again.

What else happens? The huge telluric currents overwhelm the carefully-designed electrical corrosion protection on oil and gas pipelines. Most readers are aware of the fiery inferno that happened in San Bruno, California two years ago when a corroding pipeline leaked natural gas that somehow ignited. Whole neighborhoods were consumed in a raging fire that took hours to subdue.

Death by fire, death by flood.

Do you use GPS in your car? You can expect that the GPS satellites, which are designed to withstand this sort of event up to a point, might malfunction. Some satellites can go off-line for awhile or even permanently if the damage is too severe. I hope you are aware that any commercial airliner you fly in depends on GPS. Same holds for weather and communication satellites. American Idol? You may miss an episode, but that may not necessarily be a bad thing...

Death by boredom. Are you starting to feel a pinch now?

Large and turbulent changes in the ionosphere during geomagnetic storms triggered by these CME events interfere with high-frequency radio communications... just what your pilot is using to communicate with the control tower. As of today (24 January) Delta Airlines has started re-routing pole-crossing international flights away from the poles, where the dipolar nature of the Earth's magnetic field allows these huge ion-storms to penetrate deeply into our Earth's protective atmosphere. This is why the auroras will be so bright and far-reaching tonight.

What about our astronauts in the International Space Station? They have a protective "safe room" to retreat to, but this is a partial mitigation at best. As I write this, all six astronauts in the ISS are being severely irradiated. The highly energetic particles during solar events like this cause temporary operational anomalies, damage critical electronics, degrade solar arrays, and blind optical systems such as imagers and star trackers. The latter are necessary to keep the solar arrays correctly oriented, and to keep one side of the ISS from broiling while the other side freezes.

For the vast majority of us, there will likely be no manifestation of anything unusual. But then, when Hurricanes Andrew (1992), Isabel (2003), Katrina (2005) and Felix (2007) struck the eastern United States, most of the rest of us felt nothing... until the forced immigrants began to arrive.


Monday, January 23, 2012

How do You Know if a Volcano is Going to Erupt?

How do you know if a volcano is going to erupt or not? Are volcanoes predictable?  Predictability is an important thing for humankind. If you are being shot at - by an  errant asteroid (like Tunguska, 1908), a hurricane, a tornado, an earthquake, a tsunami, or a volcano - there's some consolation if you can at least predict the event. A warning siren would be nice. This may give you enough time to collect the kids, the dog, the family photo albums, and Aunt Dottie's genealogy list - and beat it out of Dodge.

For the record, we can predict this much:

  • Asteroid impact: Something that would destroy a continent, up to 30 years' warning.
  • Asteroid impact: a "city buster" 30 to 50 meters in diameter, which could obliterate Washington, DC in seconds: hours of warning - if at all. These are very hard to detect because they are so small. They are so destructive because of the 25,000 km/hour speeds and phenomenal kinetic energy this translates into.
  • Hurricane: a week's warning that it is forming, moving towards you... and a day or two warning that you are about to get badly hammered.
  • Tornado: hours max, and perhaps as little as 5 minutes'  warning in the Midwest of the United States. And this is with the most advanced Doppler Radar network on the planet.
  • Earthquake: no warning - they are still unpredictable. You can at least know if you are in an earthquake hazard zone, and in some parts of California, you can get an actual percentage likelihood that you will get hammered in the next 30 years.
  • Tsunami: if you are in Hawai'i and the tsunami is triggered in Chile, then up to 9 hours warning. If you are in Indonesia and the tsunami is triggered by your personal subduction fault, then you get less than 20 minutes warning. Then it becomes: how fast can you run, and how far is it to the nearest high-point? The mayor of Minamisanriku, Japan, had less time than this to get to the communications tower on top of the town hall after the great Tohoku earthquake hit. Wave after wave swept over and gutted the multi-story, steel-framed building, killing all still inside - but he survived with scars on his hands from hanging onto the steel tower. Most people growing up around the Pacific Rim or Indonesian Archipelago are taught this warning from earliest childhood: if the ground shakes, run to high ground as fast as you can. 
  • Volcanoes: As much as 6 - 10 months' warning before an eruption - but many of the restive events that trigger warnings end up with a "fizzle" - it goes quiet again. For this reason, the warnings are graded, advanced in stages: Yellow, Orange, Red. If there is going to be a violent eruption, then deformation, gas, and seismic monitoring networks (if installed beforehand) can warn you with "days to weeks", and then as the signals ramp up, with "hours to days" timing, but the magnitude of the eruption is still very difficult to estimate - and with that, its consequences. The destruction of Armero, Colombia, happened about 45 minutes after the first phone-call from up the canyon towards Nevado del Ruiz volcano saying that something "sounding like 10 diesel locomotives" was passing and moving in the direction of Armero. The mayor told people not to worry. The Catholic Bishop told people to go to the cathedral for protection. But NO one can outrun a Lahar. Only the foundations of the cathedral survive. There is an elaborate acoustic flow monitor system down-stream and west of Mount Rainier in Washington State. School children routinely have evacuation drills - they must run a mile to a bridge over a busy highway to get to higher ground. They have just 45 minutes from hearing the first automated siren.


How do you know if a volcano is extinct or going to erupt again?
Rylee from Mrs. King’s class.

The 169 volcanoes in the United States and its territories are classified by USGS volcanologists as Very High Threat, High Threat, Moderate Threat, Low Threat, and Very Low Threat. There are 18 volcanoes classified as Very High Threat.

These categories were developed after many years of careful mapping, dating, and analysis. They are based on a number of criteria, including the history of the volcano - such as how recently did it erupt? How many times has it erupted in the past 10,000 years? How far out do old eruptive products reach? How many human beings are now exposed to danger in these areas if there is another eruption? Each volcano is sort of like people or bears: they each have their own unique "personalities". Some, like Kilauea volcano in Hawai'i are mostly effusive: they tend to flow lava with little explosive behavior. Others have a long and violent eruptive history, like Yellowstone. 640,000 years ago Yellowstone erupted and laid out a blanket of ash - that ash is over 20 meters (66 feet) thick hear Colorado Springs, CO - over 1,300 kilometers (800 miles) away from the volcano! I have personally pulled a camel's tooth out of the base of that off-white-colored deposit (called the Pearlette Ash Formation) where it had smothered all living things under it.

NO one can outrun a 20-meter-thick blanket of ash that reaches out and covers a continent.

A key point here is that the volcanoes and their eruption products must be age-dated. They must also be carefully mapped to see how far the eruptions reached in the past. It's pretty safe to say that if a volcano hasn't erupted in 10,000 years it's PROBABLY dormant. However, Mount St Helens last erupted in 2004-2006, and before that in 1980-1986. Kilauea volcano in Hawaii has been erupting continuously since 1982, so it's pretty safe to say that these two are DEFINITELY going to erupt again. Those are the two extremes, but a volcano called Four Peaks in Alaska erupted in 2006 after being dormant for many thousands of years... so even apparently dormant volcanoes can surprise us with little or warning - and the warning comes only if they are instrumented. Would YOU spent ~$100,000 to instrument a volcano that last erupted perhaps 10,000 years ago? Something in between Mount St Helens and Four Peaks would be Mt. Edgecumbe near Sitka, Alaska. It hasn't erupted in at least 5,000 years, so it's hard to say if it's extinct or not.

What have we done to protect the American people - and to prevent a volcanic eruption from becoming a volcanic crisis? We have put seismometers and telemetered GPS instruments on almost all of the most dangerous volcanoes. Cleveland volcano in the remote Aleutian Chain (which erupts frequently) is an exception. It has not been instrumented yet because we don't have enough funding to do so - but we watch it daily from satellites. Also, there are no towns nearby, so it was given a relatively low priority. Dangerous volcanoes close to human population centers are all instrumented in some way or another as of this year (2011). This way we will ALMOST always be able to provide some warning, even if only a few days.

When Mount St Helens erupted on October 1, 2004, we had about a week's warning from suddenly increasing micro-earthquake activity. As far as our records show, it was dead silent the day before the first volcano-tectonic earthquakes started (I was standing on the 1980-86 Dome just two months earlier). As a result, the Johnston Ridge Observatory five miles away was evacuated in time and no one was hurt. JRO was named for David Johnston, one of our PhD volcanologists who was killed by the 1980 eruption - when it erupted catastrophically while he was monitoring it. That won't happen again as long as we can keep doing our job protecting the American people.

We are, after all, the United States Geological Survey. Without funding, however, even the most dedicated scientists on the planet are helpless.


Saturday, January 21, 2012

Volcanoes - Where ARE they?

Volcanoes fascinate everyone - even volcanologists who spend their lives mapping and monitoring them are just itching to get back into one. At the Cascades Volcano Observatory, interns working for next to nothing are knocking at our electronic doors all the time. People who see restive volcanoes all the time, however, are definitely more circumspect. It's interesting to me that the Volcano Disaster Assistance Program (VDAP) international emergency response guys go out of their way to maintain a healthy distance. In conversations, when you delve a bit into it, something stands out. Virtually every volcanologist at CVO knows someone, once a friend, who is now dead because of a volcano.

Volcanoes kill. They have killed far more people than any earthquake. I'm not talking only about something fiery and explosive here. In 1984, the city or Armero, Colombia, was destroyed in just a few minutes by mud. A Lahar killed about 23,000 people just that quickly. A Lahar is a river of volcanic mud set loose by glaciers, water, and volcanic debris. Think of a 5-meter-high wall of wet concrete coming at you at 50 kilometers per hour. Another killer is a pyroclastic flow: a red hot cloud of gas and debris is a relatively common occurrence on the flanks of stratocone volcanoes, and nothing can stand in their way. In 1902, Mount Pelee on the Island of Martinique sent a fiery cloud - in French this was called a nuee ardent - that killed over 30,000 people almost instantly. These have great reach. In May, 1980, Mount St Helens destroyed about 140 square kilometers of forest in southwestern Washington State. The initial blast following the triggering earthquake may have actually reached supersonic speeds. Beyond the zone of meter-thick trees flattened, scattered, and shredded like match-sticks, there was a zone of trees, kilometers wide, that turned red-brown and died - just from the scorching heat.

So where are volcanoes located? In surprisingly predictable places. Stratocone volcanoes - the generally dangerous, explosive ones - are found all around the Rim of Fire. Wherever there is a continent being rammed up over an oceanic plate by tectonics - you will find volcanoes just inside the continent margins. The down-going slab of oceanic plate undergoes something called partial melting. This is material mixed with volatiles like water and CO2 from the former seafloor, that melts and floats up through the Earths crust just inside the continental margin. The Cascades volcanoes in the Pacific Northwest are classic examples. Virtually ALL of Japan is another aggregate-volcano example.

Where continents are splitting apart is another place where you will find active volcanoes - east Africa and the middle of the Red Sea are loaded with them, for instance. Iceland is merely an above-water part of a 25,000 kilometer-long oceanic spreading center that tracks the length of the Atlantic Ocean and winds all the way around the world. That's a 25,000-kilometer-long volcano.

Another kind of volcano is the "MORB" kind - Mid-Ocean-Ridge Basalt. This you find creating islands on top of oceanic plate that is drifting over a mantle hotspot, like Hawai'i in the middle of the Pacific ocean, or Reunion Island off the coast of Madagascar.  In fact, the largest mountain on Earth is not Mt Everest - but Mauna Loa on the Big Island of Hawai'i. It is 13,800 feet tall, standing in 25,000 feet of water, and its immense weight has depressed the ocean floor by yet another 7,000 feet - this adds up to 14,000 meters in all. Mauna Loa and Mauna Kea don't look at all like a stratocone volcano - instead, they look like the laid-down shield of a Greek or Roman soldier: a long, broad hill that just happens to be bigger than any ordinary hill you will ever see in your life.

Volcanoes are grand, mesmerizing monsters - but at least their eruptions are moderately predictable (more on that later).


I was wondering what type of lava/pyroclastic events are common of composite/strato-volcanoes? I thought it was Aa lava because of its high viscosity. Would it be more common for bombs or cinders to erupt from the volcano? I am not sure about this and was hoping that I could get some help.
Andrew B.

The short answer is all of that and a lot more. Most volcanoes erupt a wide range of compositions over their life-spans. The compositions range from very dark-looking basalt to very light-colored rhyolite, and that range of compositions is partly defined by the silica content (and this silica content is what primarily controls the types of feature you see), and partly by the other minerals in the extrusion. Water, in the form of rain or mountain-peak glaciers, and dissolved gases in the magma itself also figure prominently in that form that you will eventually see. A good place to roam around and research this sort of stuff is


Friday, January 20, 2012

Glacial Lake Missoula

We sometimes receive locality-specific questions, and often they are for localities far away from where the A-A-G responder lives.  This is an example of one that is close to where I live - and which I have personally climbed many times. By happenstance this question also opens the door to a phenomenal event: perhaps the largest flood in the history of the Earth.

I live in Portland, and there is an odd mountain east of here on the Columbia River called Beacon Rock. Someone told me that this was a volcano.
(no name, just an email address)

By a happy coincidence, I personally know what you are talking about - because I have climbed it many times. Beacon Rock is the core of a volcano that erupted about 55,000 years ago. You notice it doesn't look like a volcano? We will talk about volcanoes that don't look like volcanoes in a later chapter, but this one once did "look" like volcano.

A classical stratocone volcano like Mount Hood, Mount Fuji, etc., is formed not all at once, but over time. Lava works its way up to the surface and if it gets there quickly enough and with sufficient violence it will fragment into ash and tephra. Volcanic ash is self-explanatory, but tephra is basically lava foam: as the lava rises to the surface the pressure on it drops, gas dissolved in it comes out like a shaken bottle of Coke, and the result is small fragments from pea-sized to car-sized that are full of air-filled vesicles. These tend to drape down the sides and build up like cone around the core, which may just pour lava down its flanks in between tephra falls. This all means that the core of these volcanoes is generally solidified, hard lava, while the sides are looser stuff.

About 12,000 years ago a huge ice dam built up near what is now Missoula, Montana. It backed up a huge lake, a lake that grew until the ice dam couldn't hold it any further. One thing about an ice dam: when it fails, it fails very, very quickly. A lake variously estimated to be the size of one of our modern Great Lakes unloaded ALL of its water in a very short time. And it was a lot of water.

The wave that roared out into what is now eastern Washington State completely re-shaped the land. It left a region called the "Scablands" - strange hills that don't look right to anyone who doesn't live there. They don't have a normal dendritic (like veins on a maple leaf) drainage pattern, and this terrane confused geologists for a long time. Then someone saw them from an airplane and recognized Ripple Marks - the ripples you see on a stream-bed. Only these ripples were gigantic, thousands of times bigger.

People noticed things like Beacon Rock, which stands up like a thumb from the north edge of the Columbia River, before you get to Bonneville Dam. It has nearly vertical sides, and a visionary man named Henry Biddle bought it for $1 in 1915 before it could be dynamited for road gravel. With friends, he built a trail - complete with hand-rails, to the top. It stands almost 300 meters (850 feet) high, and affords a truly sublime view of the Columbia River Gorge and Bonneville Dam (you can't quite see the top of Mount Hood to the south because of the high scarp of the Columbia River basalts on that side). Beacon Rock stands up like it does because all the tephra was stripped from it by the first of the (now believed to be up to 72 separate) Missoula Floods.

Geologist mapping central Washington State found some phenomenal scours in the Columbia River basalt field there. Geologists mapping the Columbia River Gorge found even more startling things: boulders the size of a Volkswagon bus that had been lifted up over a 400 meter-high ridge on the south side of the river... and dropped on the other side. As they published their maps and talked among one another, they all started thinking: what kind of water force could do that? And amazement grew.

The early pioneers entered Clark County in southwestern Washington, and the Willamette Valley in northern Oregon and found wonderful flat valleys unlike the canyons they found farther north or south. They had no idea during their lifetimes that they were planting crops high above where there had been steep canyons just 12,000 years earlier.

This was all something too big to put numbers on... until side-scan sonar was used to map the seafloor off the coast of Oregon in the latter half of the 20th Century. Oceanographers found a debris fan stretching more than a hundred kilometers south and west of the port of Astoria, the gateway to the Columbia River. This debris fan has a volume of over 5,000 cubic kilometers. To put this into human perspective, this is a cube of debris 17 kilometers on an edge; imagine a block of rock, 100 square miles in area, standing up 50,000 feet into the Stratosphere. Specialists believe that almost all of this came from just the first and largest flood. By now, mapping had allowed geologists to find the source, and to thus put a name on what is believed to be the largest flood that ever happened in the Western Hemisphere.

The Missoula Floods.

Beacon Rock is just a tiny but very distinctive piece of evidence in a puzzle extending many hundreds of kilometers, from the Pacific Ocean to the Rocky Mountains, for one of the greatest landscape-changing events of prehistory. My house on Prune Hill, in Clark County, sits on something called the Troutdale Formation: I can walk outside and put my finger on fragments from one of the greatest catastrophic events of all time.


Thursday, January 19, 2012

Why is the Earth 70% Covered With Water?

This is a good time to get into Tectonics - the movements of the Earth's crust. There are some questions that are basic to both kids and adults, like why is the sky blue? And why is the Earth 70% covered with water?


It has been over thirty years since I took a class in historical geology, but I have always been fascinated with the concept of continental drift. My question is: Given the evidence for continental drift in the earth's early history, is it likely that continental drift is typical of the geological history of all planets? In the search for extrasolar, water-bearing planets, is it likely that some of them-- if they do indeed have oceans--might exhibit water/land mass configurations similar to Pangaea with one huge continent on one side of the planet and a huge ocean on the other? I have also thought it strange that the surface of the earth is roughly 70% water and 30% land, whereas the rest of the terrestrial planets are--what--90%+ land? Why is the surface of the only significant water-bearing planet predominately water whereas the other planets don't show even trace elements of liquid water? 
Thank you for considering my inquiry.
--Amado N.


Continental drift apparently "turned on" sometime in the early history of the Earth. It took awhile for the accretion process to segregate itself into a silica-rich crust, an iron-and-olivine-rich mantle, and an iron core. It took even more time for the proto-continents to form - to "float" from isostacy above the denser oceanic crust. It then took awhile for heavy radionuclides accumulating in the deep Earth to create enough heat, and to begin the convection process that now drives the crustal segments around in what we call continental drift.

I have just a modewrate understanding of the other "solid" planets in the solar system, mainly from tracking the scientific literature in magazines such as Nature and Discovery. However, as I recall Mars and the Moon are thought to not have continental drift - this implies either less radionuclides remaining in their interiors for whatever reason, or it implies that the cores are cooled and solid. I think this holds for Mercury and Titan also, but too little is known of each to be sure of this. Titan does have an atmosphere, even an ocean and landmasses, but the chemistry is radically different - it's primarily a weather analog, as the main liquid (gas) is methane. Io is a tiny moon that lies deep within Jupiter's gravity well, and the huge gravity gradient there drives monster tidal forces that will make this planet a font of volcanism forever - but that's a different process. It's like squeezing PlayDough forever will keep it warm and pliant.

I'm unaware of water - for certain - on any other solar system planet - the solid planets are 100% "land" as far as I have ever heard, though there is a deep suspicion from visual evidence that Europa and perhaps Enceladus have watery oceans beneath their strange crusts. In the case of Venus, the temperature (~900C) is too high for liquid water, and in the case of Mars the gravity is so weak that most volatiles like water have long since wafted off into space.

The reason we have 70% surface water coverage on Earth stems from several unique circumstances that we find with our planet. One is the fact that the Earth lies in a relatively narrow "Life Zone" of moderate temperature where water can have a triple-point: be gaseous, liquid and solid. Mars is so cold that if water were somehow still present in any significant amounts on the surface, it would be solid* due to the very cold temperatures, though there is growing evidence of water not only in ancient times, but perhaps still present beneath the crust today.

The other reason is that the Earth is so large that gravity and weathering together enforce a pretty flat surface. The tallest mountain on Earth is Mauna Loa (not Everest), and it is only 14 kilometers "tall" from where it sits on the Pacific Ocean floor. The radius of the Earth is about 6,370 kilometers - so our tallest mountain is only slightly more than 0.002 of this: two tenths of a percent. Our planet is a very smooth blue marble! If there were 100-kilometer-deep trenches on Earth instead of what we now have (the deepest trench, in the Marianas, is just 12 km deep), then I suspect that the relatively small percentage of water on this planet would all pool in a relatively few of those sorts of structural lows. In other words, we would have far less surface covered by water if the Earth's surface was proportionally as rugged as Mars' surface is. To help you better understand the powerful effect of the greater gravity and weathering on Earth, Olympus Mons, the best-known volcano on Mars, stands 25 kilometers above the surrounding plains!

There is still a relatively incomplete understanding of why we have rocky planets vs. gas giant planets in our Solar System - or for that matter, what the cores of those gas giants may consist of. Jupiter has a powerful magnetic field, suggesting a conductive core - iron? Mercury probably almost certainly has no water or volatiles because of the intense heat and ferocious solar wind that it is subject to.

* For decades now NASA and astrogeology academics have speculated that there is water beneath the
surface of Mars. The mantra for a long time has been "Follow the Water." I was once a co-principal investigator for a NASA-USGS proposal to prove this. We proposed a balloon-carried electromagnetic "snake" designed to traverse the planet under its day-night thermal transition and thin-air storm regime, all the while sounding the surface for conductive material. In the case of a conductor on Mars, the likely cause would be water or sulfides - if widespread enough, a conductive anomaly would have to be water. Water will be absolutely crucial to ever establishing a human presence on that planet... assuming that the vast fuel, and cosmic and solar radiation problems can ever be dealt with.


Wednesday, January 18, 2012

Glaciers Flow Uphill

A rather large amount of the terrain we lay our eyes on in the mid-to-high latitudes have been shaped by glaciers. A major clue is to this action is when we see "U-shaped" valleys, or scoured "circs" - like someone took a giant spoon to the side of a mountain made of Extreme Moosetracks ice cream. These tell a practiced eye that something big and remorseless has ground everything smooth. Most glaciers are not small things, either. I have been in a fjord in Norway that has scoured edges 3,000 meters up from the current water-line! For American readers that represents a moving mass of ice about two miles deep. The huge weight of these glaciers at their height 10,000 - 16,000 years ago, but now removed, has caused parts of Scandinavia to uplift - float up - by as much as a meter in a century.


Hi, I live <in> Minnesota which experienced substantial effects of glacial movements during the ice ages. My question is: How were the gravitational forces sufficient to move the glaciers south into the upper Midwest given the current elevations above sea level of Hudson Bay (400 ft), Duluth (1430 ft), and Minneapolis (840 ft). Thanks Jim T.

You can't think of glaciers as slow-moving water - they are plastic, with high viscosity. Precipitation moisture falls and "pancakes" instead - compresses, forming glaciers, with the center of the precipitation building up steadily, and the edges moving outwards. If you pressed down on the center of a still-fluid core of a pancake (before it was cooked through), it would extend or "moosh out" sideways - this is a displace-and-inflate behavior we also see in lava in Hawai'i. If the pancake filled the entire griddle or pan, and there was confinement at the edge or lip of the griddle (the analog of the broad increased elevation at Duluth), then the plastic material would rise above and flow over this barrier. If the elevation-increase is an isolated mountain, the glacier will instead slowly move around it on both sides - but still lap up locally at its edges.

Keep in mind that behind the continent-scale glaciers was just more glacier all the way back to central Russia - so the ice front could only move southward in North America. And believe me, nothing epitomizes the word "inexorable" quite like a glacier on the move.

Hmmm. It's early in the morning, and I must be still thinking of breakfast to come up with a pancake metaphor.


Tuesday, January 17, 2012

Earthquake Precursors?

This section is a sort of asterisk * to a previous section about earthquake prediction. Before I proceed, I'll emphasize again that earthquake prediction is still not technically possible, despite legions of self-anointed amateur "expert" opinion to the contrary.

Most earthquake prediction research focuses on precursors: something, some clue, that might provide a warning. There are abundant stories out there of strange animal behavior, strange electrical phenomena, strange groundwater phenomena before an earthquake. In almost all cases this information has been gathered after an event, in an effort to understand what happened. In the vast majority of cases, there is no independent instrumental record to back up the precursor "discovery" - it's just the say-so of one or more individuals, after the fact.

An additional, more serious problem is is that these phenomena apply to one earthquake but not all earthquakes. Unfortunately each earthquake, like bears, humans, and gold deposits, is different - each has its own "personality," so to speak. Each has its unique rock, fault, and tectonic environment.

One physical concept that won't go away: tracking strain buildup. This concept is like crack cocaine for an earthquake research scientist. The idea is this: if you can see a change - especially an acceleration - in strain buildup, this might mean an impending earthquake is looming. Unless you can see an asymptotic (accelerating) pattern, however, you won't have much chance of saying when the fault will break. Measuring strain is technically difficult, and this makes the problem even less tractable. To do it correctly, you would need to measure strain as a function of time over a large horizontal region - and in a perfectly-funded world, also at depth. While there are proxies for measuring strain, they are different for each area, and hard to calibrate. Normally, professional scientists use - you guessed it - a strainmeter.

This latter is one of several goals of SAFOD: the San Andreas Fault Observatory at Depth. This is a hugely expensive endeavor, requiring much more energy by the (IMHO brilliant) principal investigators to just gather funding - than to even carry out the technical effort itself. A drillhole has already been punched down parallel to the San Andreas Fault, then at depth it was "whipped" (bent and re-directed) to pass through the fault (really a fault zone, a region of sheared-up rock several hundred meters thick) at depth. Besides sampling the rock, pulverized or otherwise at the fault zone itself, the plan was to install strainmeters in the fault zone.

Now the interesting news, published only recently (late 2011) in the Journal of Geophysical Research (Solid Earth). A group of scientists (Katsumata, principal investigator) investigated the earthquake catalog for events preceding the Magnitude 8 Tokachi-Oki earthquake of 2003 just off the Pacific coast of the Japanese island of Hokkaido. This is a nontrivial exercise, because there are hundreds of seismometers in Japan alone, plus they used everything in the general region - a lot of data to manage, much less wade through in a systematic fashion. The authors' statistical analysis shows a significant decrease in seismic activity in the region during the four years leading up to the earthquake. This can be construed as evidence of strain not being released by small events, but instead the strain was accumulating and perhaps even accelerating.

Of course, this is for just one fault, and statistics are famously arguable. This is also not the first effort to look for a quiescence signature. Therefor, you can expect much more additional work to be done on the earthquake catalog for, say, the Great Tohoku Earthquake of 2011.


Monday, January 16, 2012

Earthquakes and Tectonics

The subject of earthquakes leads naturally to tectonics - large-scale movements of the Earth's crust. It's hard to imagine rocks in the Alps with marine fossils in them getting to a kilometer or two of elevation above sea level without some violence associated with the process. Nevertheless, there are tectonic events that are not felt but can still be detected using sensitive equipment - the so-called "slow earthquakes."

Surprisingly often, people will respond with a thanks - and perhaps even another follow-on question.


Hi, I am a 5th grade student in Atlanta, Georgia and I was exploring your website when a question came up. I saw the email address, wrote it down, and told myself, go for it, so I did. My Question: What is the name of the mountain range in Utah that was created by earthquakes? Please respond.
-Natalie H.


All the mountain ranges in Utah likely had earthquakes associated with their formation. Relatively few tectonic uplift events are aseismic. However, "slow creep events" apparently happen a lot more frequently than scientists had ever anticipated, and this probably applies to most every mountain ranges worldwide. Though not felt, these are still technically earthquakes, and can be detected with telemetered GPS arrays.

I suspect, however, that you are thinking of the most obvious fault and mountain range, both named Wasatch, rising east of Salt Lake City. It's hard not to look at the Wasatch Front from I-15 and not think "That looks like it was sliced with a knife!"

Thank you that really helped!! :)


Sunday, January 15, 2012

Rewriting History

Of course, anything that involves money, including damages, or possible fame will attract the cunning and unscrupulous. We see these on the fringes of most science: the self-anointed "expert" who has a "secret" means for doing something - in this case, for predicting earthquakes.

The following is part of an actual example sent to Ask-a-Geologist. The writer attempts to rewrite history, by claiming after the fact that he "predicted" a series of events. I am only showing a part of the "question", since the entire thing would take several pages of false claims. A singular characteristic of these miraculous "claims" is that they are missing any substantive information on HOW they made their "prediction." They want you to pay first, of course.


Hey Rog, I am actually at a
real computer for a change!
I am getting more and more
serious about this 29th and
30th quake although,of
course,I am not sure it will
hit Christchurch but that is
where the Godly intuition
points as of now. It might hit
Christchurch the 22nd of
February if you read the
below information supplied a
journalist in 2009.

(My reply to the AAG Coordinator, Rex Sanders):

Rex, I don't think this one warrants a response. In fact, ANY response from the USGS would only serve this individual's efforts to recreate history. Do you see a lot of these? It's clear that the individual wants to claim association with the USGS to strengthen his "credentials" and any reply will only serve this dishonest purpose. ==Jeff


Saturday, January 14, 2012

Weather And Earthquakes - Can They Be Predicted?

This following question opens up the whole prediction/triggering question about earthquakes. Since the questioner threw in weather as a possible factor, I'll address that too. It's all part of the Earth we live on, all part of geoscience.


Hello..I was just wondering first if the increased number of earthquakes is a sign of something bigger to come and since the earthquake in Japan knocked our earth off of its axis a few feet is that the reason for the severe weather we have had lately like all the floods in the south and tornadoes and severe storms any info would be great…thanks so much.
Wendy M.


Available information (from the Global Positioning System) indicates that the Earth's axis was tilted about 10 cm by the Great Tohoku earthquake - about 4 inches. It requires some very sophisticated equipment and a lot of measurement time to arrive at that tiny amount of offset. To put things in perspective, if you stepped 10 cm north, I'm sure you wouldn't notice a change in the weather.

While there is evidence that continents were at hugely different latitudes in ages past (freshwater swamp dinosaur skeletons have been recovered from Antarctica), a 10-cm tilt change will not cause any measurable effect on weather. A long and slow tilt in the Earth's axis has been documented over time, but the operative word here is "slow" - we're talking in the millions of years slow. There is also the complication that the continental plates have been moving around at the same time. Since we didn't have observers using sextants to track where Polaris is/was 50 million years ago, these are understandably hard to sort out. Making matters even more complicated, the Earth's axis wobbles - the pole star was not Polaris, but Vega, 12,000 years ago. This effect is called nutation.

One scientist I know made an estimate that at least $15,000,000,000 has been spent over the past half century, in science agencies all over the planet, to be able to predict earthquakes. So far, at least - and I'm acquainted with some of the truly brilliant people who have worked on this problem - there has been no payout. Scientists can forecast, but not predict, earthquakes. By forecast, I mean to give a 30% statistical probability that the Hayward Fault in the San Francisco Bay Area will rupture in the next 30 years. They can calculate $165,000,000,000 in damage when it does. To predict means that I could tell you when to sell your house in Oakland - and that can't be done. You can very roughly predict a volcanic eruption, and your timing precision will improve steadily from "weeks to months" to "hours to days" as the event approaches, and as the physical evidence of inflation and ruptured rocks from seismometers and GPS telemetry accumulate.

You can predict a hurricane up to a week ahead of time, and a tornado up to hours ahead of time. But you can't predict earthquakes.

There is an on-going discussion about earthquakes triggering other earthquakes. Large earthquakes have been shown to "light up" restive volcanic areas like Yellowstone and Long Valley with clusters of increased micro-earthquakes. However, the current scientific consensus is that distant earthquakes do not have any effect on faults not part of that earthquake's own fault system. In other words, the earthquake in Chile in the Spring of 2010 did not trigger the earthquake in New Zealand in the Fall of 2010, and that one didn't trigger the Great Tohoku earthquake in March 2011 near Sendai, Japan. Among other things, there were months separating each one. Researchers have also studied syzygy - the effects of Sun and Moon tides - on earthquakes and have found no statistical correlation.

All THAT said, there HAS been a measurable, undeniable (and steadily ramping-up) increase in the carbon dioxide content of the Earth's atmosphere in the last several centuries - since you asked about weather. CO2 has a measurable greenhouse effect on atmospheric temperatures. Methane - there are far more cows on the planet than there were a century ago - has 37 times more of a greenhouse effect than CO2 for the number of molecules released. Virtually all scientists not paid to say otherwise readily acknowledge that there is a large anthropogenic component to this greenhouse gas increase - i.e., humans burning hydrocarbons, destroying forests, raising cattle, etc. are mostly responsible for these increased gases in our atmosphere.

It's still being argued - mainly through ever-increasingly complicated mathematical models - how much this has actually changed our weather. There are a huge number of variables involved, so one model may disagree with another in detail - but not in gross conclusions. There are certain undeniable influences on weather (the Solar flux and the great ocean currents, for instance). However, you and I may not remember huge hurricanes and tornado clusters from our childhood, but that may just be our imperfect memory. The apparent increase in wild weather events may also be an artifact of how records have become increasingly more detailed and complete over time. Keep in mind that earthquakes and probably to some extent anomalous weather events are essentially (or at least largely) random. They don't come like a ticking clock, but often in clusters, and our very human minds remember the most recent cluster best.

Using a statistically more reliable approach - comparing Atlantic hurricanes and their strengths for say, the 19th Century against the 20th Century - we are also hamstrung in that there were far fewer people to record (or even see) hurricanes 150 years ago, and thus correspondingly fewer and sparser records kept then. Scientists would say that there is a bias in the data - a sampling bias. It's sort of like trying to predict the snowfall from stories you heard your grandfather tell you when you were a kid. You know: he walked to school in three feet of snow, and both ways were uphill.


Friday, January 13, 2012

Really, Really Big Earthquakes

Perhaps we can get into some of the details of earthquakes by considering the largest one (so far) of this Century:

I have a question. Is it true that the Japan earthquake from this week moved the earth's axis 10 cm, moved Japan 8 feet and sped up the rotation of the earth? 
Laura P.


You are both informed and astute. Northern Japan DID shift roughly westward nearly 3 meters, and the Earth's axis DID shift 10 centimeters. To be fair, the Earth's axis is slowly shifting all the time, but not by this much, nor this fast.

To add another element to your perspective, a magnitude 9 earthquake event means that the crust east of Sendai Japan ripped at least 300 kilometers laterally, and probably at least another 200+ kilometers down-dip --> in the direction of the thrust fault.  In the direction of the northeastern Japanese coastline. While geologists have found bisected mountain ranges crossing the San Andreas Fault that have been shifted ~120 kilometers between what once were contiguous points, that sort of movement doesn't happen overnight - or the Coast WOULD be Toast, to quote a deservedly-maligned and utterly forgettable movie. The San Francisco earthquake of 1906 had a right-lateral tear nearly 6 meters in one place south of the city - but not over such a large distance.

Perhaps more astounding is the fact that by May 2011 there had been over 150 aftershocks (and a few close-in foreshocks) with magnitudes from 5 to 8 associated with the Great Tohoku Earthquake. Keep in mind that a magnitude 7 earthquake releases about 10 times more energy than a magnitude 6 event - the magnitude scale is logarithmic. Nevertheless, there was still a HUGE amount of energy released AFTER this event, now called the greatest crisis in Japan's history since WWII. If all those aftershocks had dumped their energy at the same time, I hesitate to think of how much worse it could have been.  This event was also pretty shallow - aftershocks continued (looking at an earthquake app on my Droid keyed to the USGS database) in a depth-range from 5 km to 60 km deep. The shallower the event, the less crustal dampening occurs. That means more of the "bang" you will actually feel.

The only event I can think of that is comparable in magnitude is the 1960 Chile earthquake... which caused truly cataclysmic damage at the time. It also destroyed downtown Hilo, Hawaii. I've seen Tsunami run-up markers 15 meter above sea-level in what is now a city park; a geologist friend calls it "unplanned urban renewal". That destruction was caused by the tsunami that followed the subduction event far away in Chile. Crustal dampening minimizes the shaking/rolling damage from the earthquake itself, but the tsunami effects will reach hundreds of times farther.

Some people have been questioning why the Fukushima nuclear reactors failed. Any engineer tries to design their creation to survive a certain level of event, but you could never plan for everything. What architect would have realistically anticipated someone flying 767 aircraft into their high-rise buildings? Fundamentally, it comes down to this: you could never afford to build ANYTHING if you planned for EVERYTHING. The death toll in Japan had reached beyond 18,000 people the last time I checked, and whole regions have been evacuated due to the radiation exposure risks.

Most worrisome of all, however, is the fact that the Fukushima Daiichi reactors No. 2, No. 4, and perhaps at least one other may have been so fatally damaged that, even with all the fail-safes built into these things, their cores have gone supercritical: overheated, and melted down through their containment structures.

Remember Chernobyl in 1986?  The heat and pressure of this fatally-compromised old-style reactor blew out upwards - and dusted most of Europe with radioactive iodine and strontium. Epidemiologists estimate that this contributed perhaps ~7,000 additional thyroid cancer deaths in Europe since then. There is a (currently failing) protective concrete structure above (and at enormous risk and expense also below) the Chernobyl reactor. As far as the Russians have told us, the radionuclides never got into the groundwater. A core meltdown at Fukushima, however, is something else: it could melt its way down to the water table and gain access to the sea. Huge steam explosion would be one of the minor consequences.

In volcanology, we call water-touching-magma events Phreatic Explosions - they blast huge tonnages of rock and ash to great altitudes and (under prevailing winds) lateral distances. Of all the dangerous radionuclides in a reactor core, however, plutonium is the most toxic - microgram for microgram, it is more toxic than botulin.  Imagine large quantities of plutonium getting into the coastal waters off NE Japan... a seafood-consuming nation. Think of the food-chain accumulation process. Now think of the downrange consequences of plutonium getting into the Humboldt current and crossing the north Pacific.

It's too late to ask the engineers and planners "what could you possibly have been thinking?" to build a nuclear reactor so close to a major subduction fault. However, Japan didn't have all that much land to build one on in the first place - nor had the Japanese ever experienced an earthquake of this magnitude in all their recorded history. The general rule of thumb for geologists and volcanologists is that history is the key to the future: map the deposits and calculate the explosive force of previous eruptions, and you'll have a good idea of what to plan for in the future.

That doesn't work always, of course. It sure didn't work for Japan.

Nor do I have much patience with the emotional, knee-jerk screamers out there. There are no other energy solutions out there free of "sin" - reasons driving antagonists for coal, tidal energy, solar energy, and especially windmills are legion. I once saw a bumper sticker in Tucson, around the time that a mining company wanted to develop a property, and a lot of environmentalists (for some, that's an alternative religion) fiercely objected.  The bumper sticker said in large block letters "BAN MINING", and in smaller letters below that "Let the Bastards Freeze in the Dark." There are people who object to clear-cutting old-growth forest. I suspect that they ALL live in wooden houses, and I would be willing to bet that they don't bike everywhere they have to go. Bikes are made using mining, metals, and chemical energy, anyway.

I suppose I'm saying that we have to balance all things, and a measured dialog between interested parties will always work out some sort of optimal solution. Note that I didn't say perfect solution.

The coast from Vancouver Island to California has its own mirror image of the Tohoku Subduction Fault.  Cores of Bouma sequences (laminated mud) off the Oregon and Washington coastline tell us that huge M ~ 8 subduction events (that is, earthquakes caused by continental crust riding up and over a down-going slab of denser oceanic crust) have occurred at least 7 times in the past 3,500 years. The last one happened in January of 1700 AD, and sank whole forests near Seattle below sea level in Puget Sound. That event caused something called the "Orphan Tsunami" in Japan - a wave that came out of a clear blue sky without the earthquake warning, ergo, the name. The Japanese already knew to run to high ground if the earth shook. The resulting tsunami obliterated a number of villages on this same northeastern Japanese coastline.

Scientist know that earthquakes on major faults do NOT follow a clock-like regularity, so we could say that we are "due" for a big event in the Pacific Northwest sometime. However, that means it could happen tomorrow or in 2300 AD. We still bought our house on a hill (for the view). We could spend our lives living in fear and trying to find some safe place to build our homes in this country... but there just ain't a place safe from all possible natural disasters. There are other ways to prepare, however, and these we do.

My Dad lived for many years near the top of a 25-story apartment building in San Francisco. As a young geoscientist, I asked him how he could live only 10 kilometers from the San Andreas Fault in such a building? His answer taught me yet another lesson: Jeff, I could worry about that and fan my ulcer problem. Or I could take what precautions are reasonable, and just enjoy this amazing view of the San Francisco Bay every day. He survived the Loma Prieta earthquake of 1989, and admitted to some sleepless nights for months afterwards.

However, my Dad died of lung cancer. A biopsy found asbestos in his lungs, which he traced to the insulation on the pipes in the basement of his building where he stored his bike. When 80 years old he would bike 80 kilometers in a day over the hills of San Francisco, cross the Golden Gate Bridge, and take a ferry back from Sausalito. He was in better shape than I was, 33 years his junior.

So I will continue to savor my view each evening before I go to bed. I will also be contributing money to help my brothers and sisters in Japan, something that I expect they may very well do for me or my children in the future.

We all live on the same planet. We're all part of the same family.


Thursday, January 12, 2012

The Earliest Measured Earthquake

One question about earthquakes gives me an excuse to provide a bit of fascinating history:

Hello...Can you please tell me if the first ever seismatic recording was on Earth or the Moon?
Thank you!  :)

The earliest measured earthquake occurred in China, and was detected using an elegant seismoscope invented by the Chinese philosopher/scientist Chang Heng around 132 A.D. I've seen a replica of this device, a bronze urn with eight downward-facing bronze dragons, each attached on the outside in the 8 cardinal directions. Each dragon holds a ball loosely in its mouth, and below each dragon's mouth, arrayed around the urn, there is a small ceramic cup. The idea is that the ball will pop out and fall out into the cup in the direction of the traveling earthquake (Raleigh) wave that rolls through the site hosting the seismoscope. If you travel in the opposite direction, you should eventually arrive at the epicenter. This simple device is immensely cool, and implies a marvelous observation-based understanding. It draws visiting school children to its glass display, in the US Geological Survey National Center lobby in Reston, VA, in droves. 

Prior to that instrument, earlier reported events were noted in Chinese literature -- one such event dates back to the Xia dynasty in 1831 B.C.

The first Moonquakes were detected by solar-panel-powered seismometers placed on the lunar surface by Apollo astronauts during their explorations on the Moon's surface (1969 to 1972).


Wednesday, January 11, 2012

Was That an Earthquake?

A lot of people have questions about earthquakes:
Did I feel one?
WILL I feel one?
Can you PREDICT one?
Should I move away from here?
These are fairly typical questions that we receive all the time. For the next few chapters we'll look at what we know and what we don't know about earthquakes.

I live in New Mexico. At Manzano State Park on March 23, 2010 at or around 3:35 was there an earthquake?? 
    We felt one.
    Marty N.

If you go to the following website, you can check on any recent earthquake in the last seven days in the US above a magnitude of 1.0 (M = 1 is generally so small it's not even felt by humans):

You can zoom in on the map that pops up to get more detail. The nearest events to you in the past week were in Oklahoma and Utah, and they are so small that there is zero chance that you may have felt them.

There are other things that can cause "quake-like" vibrations, including a heavy truck or a train passing nearby, or blasting in nearby quarries or during road-building. Once I was reading the Sunday paper and felt my house start shaking. I knew there was a fault nearby, so I called my team's seismologist, who checked (and educated me about this and other seismic websites). He found nothing. I went back to reading and a half hour later I felt the shaking again. THIS time I listened with more than my backside on the couch and heard the clothes washer upstairs reaching the maximum of it's spin cycle. A bit of further checking showed me that about half-way into the fastest spin-rate, the unbalanced washer (it had my heavy Jujitsu Gi in it) was reaching the natural oscillation frequency of my wood-frame house.

That was an Ah-HAH! moment for me. Even scientists are always learning... that's probably the coolest thing about being on this planet.


Tuesday, January 10, 2012

The Earth's Magnetic Field - and Why it is Important

We have had a number of questions about the Earth's atmosphere and also about its magnetic field. The following is a quick summary explaining how these may in fact be related - and why Mars lost its atmosphere:

Last year NASA approved a new orbiting satellite mission to Mars, called Mars Atmosphere and Volatile Evolution, or MAVEN for short. There is growing evidence that Mars once had water on its surface - and water cannot exist if there is no atmosphere, because it would evaporate quickly. There are unmistakable rivulet marks on rocky Martian slopes, evidence of sedimentary layering in giant Martian basins, minerals detected by various means that can only form in the presence of water - all discovered by orbiters and semi-automated rovers like Sojourner, Spirit, and Opportunity.

If Mars once had an atmosphere, then where did it go? More important: WHY did it go? Planetary scientists speculate that Mars lost its atmosphere when it lost its magnetic field.


Most people have no idea how important our Earth's magnetic field is. If you've seen videos of the Northern Lights, you have seen our planet's magnetic field at work: it traps the huge CME (Coronal Mass Ejection) blasts that come out of the Sun during the more active times of the Solar Sunspot cycle. The magnetic field bends these highly energetic charged solar particles until they come into the atmosphere near the magnetic poles.Those beautiful shimmering curtains of light you see are highly energetic particles literally tearing up the upper atmosphere.

Dr. Robert Brown was my adviser at Berkeley when I was a young student there. He was a great guy, and never hesitated to stop whatever he was doing to answer the physics questions I always had. His own experiments as a physicist concerned sending balloons into the troposphere to collect some of these energetic solar particles. They were so energetic, he told me, that he had actually captured iron nuclei with so much energy that all the electrons had been stripped off! That sort of stuff does terrible things to soft tissue - it's a burn that goes far beyond a simple sunburn. It is hard ionic radiation, and it kills.

Without our planet's magnetic field, the latitudes humans live at would be constantly blasted by CME's. Without the atmosphere, those deadly high-energy particles would blast right down to the ground, killing all life, sterilizing the planet's surface like the Moon.

So how are the magnetic field and the atmosphere related? If the magnetic field is turned off, the CME's would not be deflected, but would scour our atmosphere unimpeded and blast it off the face of the planet into interstellar space, instead of just nibbling at the atmosphere around the poles. Point a blow-torch at something and what happens? You've got the picture. That's what scientists believe happened to Mars sometimes in its ancient past: as its smaller core cooled, its magnetic field died, and the solar wind and CME's stripped the atmosphere away.

Let's go a little deeper - literally. If the interior of the planet cooled, convective movement of the hot liquid material inside would slow and stop - and with it, the magnetic field created by that conductive fluid dynamo. Interestingly, our Earth's magnetic field might or might not have something to do with radiation. Heavy radioactive minerals, that sank to the mantle and core of the Earth during planetary formation, will still decay. The heat has to go somewhere, and geomagnetic specialists believe this sets up convection like what you see in your kitchen: heat rises because hotter materials expand and have lower density.

Much if not most of the original heat in the inner Earth probably comes from gravitational collapse during the formation of the primordial Earth, however. There is also compositional convection thought to be going on in the core itself as it cools: metallic iron plates out onto the solid mostly-iron inner core, leaving lighter materials to float up towards the mantle. The Earth's magnetic field is a complex thing made up of a dipolar field and a secular field: a big, steady north-south magnetic dipole, and something added on top of this that "drifts" the north magnetic pole westward about 0.2 degrees each year.

An interesting aside from my days as a high-pressure solid-state physicist: rocks that are resistive at room temperature generally become highly conductive and plastic when heated and pressurized (even diamond), and the moving conductor that results is what generates the Earth's magnetic dynamo.

Perhaps we can also look farther out. Gene Shoemaker, my great friend and the brilliant father of Astrogeology, felt very strongly that life exists on Earth also in large part because of Jupiter. Yes, Jupiter. As the largest planet by far, it has a huge gravity field - as proven by the impact of Comet Shoemaker-Levy 9 in 1994. The dark blots that SL9 caused in Jupiter's banded atmosphere were as large as the planet Earth! Jupiter and the Sun drag comets coming in from the Oort Cloud to themselves, and away from the inner worlds - the Sun by itself would just draw more bombardment to the inner solar system if it was by itself. Gene actually thought that the Drake Equation - which is an estimate of the probability of life outside of the Solar System - should be modified to include a factor he called Fj - the Jupiter factor. Like a big brother on the playground, Jupiter protects the Earth from the Oort bullies.

So we are alive because of heat, perhaps some of it due to radioactive decay - and a Big Brother planet orbiting outside of the inner solar system.


Monday, January 9, 2012

Asteroid Impact Physics

Here's a bit more fascinating grain to the asteroid impact issue; this question was asked of me by a fellow scientist in the US Geological Survey after HE tried to answer an asteroid impact question as another volunteer on Ask-a-Geologist. For context, he asked me what was the typical velocity these incoming asteroids are bringing with them. Keep in mind that kinetic energy goes as mass times velocity squared, so double the speed and you quadruple the energy delivered. The velocities we are talking about are orders of magnitude faster than a rifle bullet. They are so fast that the atmospheric air stacks up in front of them until all but the nickel-iron meteors are blown apart from the accumulating atmospheric stress - this typically happens at about 8 km to 12 km altitudes.

This is what happened at Tunguska in 1908: the (probably stony asteroid) object broke apart from the growing stress on it, abruptly increasing the atmospheric ablation (burning) hundreds of times as more surface area was suddenly exposed to the air. This sudden flash was so powerful that it set the forest beneath it on fire. Seconds later the blast-wave blew out the fires, and stripped the branches off the trees in the center of the blast zone - it is littered with standing, burned poles with roots to this day. All the trees outside of this core zone beneath the blast were flattened in a radial pattern. A man in a trading post at Yeravan, 40 kilometers to the south-southeast, had the back of his wool shirt set on fire - then he was bowled over and knocked across the clearing of his trading post.


This stuff is all mind-blowing to me.
I don’t suppose anyone has done any wind-tunnel tests of FeNi objects traveling at 25 km/s, which can be used to calibrate the model. 
Larry M.


Hard to imagine a wind tunnel that wouldn't detonate at a tiny fraction of those velocities.

In the 1970's and 1980's, Dave Roddy (USGS-Flagstaff) participated in some NASA-funded experiments using bullets against metal plates to better understand the impact physics.

A couple of things they figured out included the fact that a crater is roughly 20 times the diameter of the incoming object... it's unclear to me how much the velocity controls the crater-diameter/bolide-diameter ratio, however.

Another take-away: the impact craters are always circular until the incoming angle drops to 15 - 22 degrees from the horizontal surface plane. The reason for this is that the object is going so fast that it first buries itself. Then the huge residual kinetic energy from the very high velocities has to go somewhere, so it blows up for lack of a more descriptive process term. There is a rare elongated "skip" crater on Mars that is now believed to have brought the Nakhlite meteorites to Earth. Nakhlites are firmly believed to be of Martian origin by meteorite specialists (from their isotopic content); I bought a small piece of one for my wife as a birthday gift; she worked for the Mars Society and participated in several habitat simulation efforts, including one on Devon Island next to the huge Haughton asteroid impact crater in the high Canadian arctic.

Incidentally, the burial process is an amazing thing in itself. My all-time prized rock sample lies in a shelf in my office for all to see. It consists of loose sand converted instantly into laminated sandstone from the impacting shockwave of the Wabar asteroid. It was then blown up into the "jetting" curtain - the ever-widening cone of high-temperature mixed material spurting out from around the impact point. This jet material Gene Shoemaker and I called "Glass" - from chemical analysis, it's very uniformly 90% local sand and 10% FeNi asteroid, and probably represents temperatures greater than 10,000 degrees Centigrade. My "Instarock" sample was then "painted" by the jet-cone Glass, and for all the world it looks like a piece of white sandstone with black lumpy paint spattered over a large part of it. Other samples I have include Instarock that was completely covered with the Glass - the sandstone quickly turned to molten glass - but the heat was so high that it then bubbled and turned to a vitreous white glass froth inside the paint "shell". These samples will actually float in water!

The largest of the Wabar craters in the Empty Quarter of Saudi Arabia is 116 meters in diameter. In most ways it is indistinguishable from the Sedan atomic crater blasted out of the desert at the Nevada Test Site in 1962 - save one. It has a circular crater shape, overturned surface materials, shocked quartz minerals including coesite and stishovite - but no radioactivity.

Craters typically have little or no contents of the original bolide in them - just fall-back material from an eruption cloud that can reach typically the stratosphere. Only about 15 of the known 182 proven impact craters on Earth have any remnants of the original bolide associated with them. That's because the huge kinetic energy associated with the impact blows everything out of the crater - and a long distance away. I have found molten glass beads that rained down as far away as 850 meters from the Wabar craters. If you had been present during the impact and survived the blast, you would have still died in the rain of molten glass that reaches out at least 9 football fields away.

Daniel Berenger was a wealthy 19th Century engineer who hoped to mine the iron asteroid at the bottom of Meteor Crater in Arizona - early explorers had found bits and pieces of iron long distances away all around the crater. After drilling and sinking a shaft, he came up with nothing - and threw away his family's fortune after refusing to give up.

At Wabar the craters are circular, but there is an asymmetric ejecta field, with Instarock preferentially distributed on the southeast side of the craters. This was a key clue to determine the incoming direction. I was later able to verify this from historical records. I have Henry St. John ("Abdullah") Philby's book on my book shelf; he was the first European to visit the Wabar site in 1932. Only after spending several days at the site did he finally figure out that Wabar wasn't volcanic in origin, but instead asteroidal in origin. The object was actually seen as a fireball in the sky passing south of Riyadh, heading into the deep desert in a south-southeastern direction. Pieces fell out of the flight path about 25 kilometers northwest of the final impact zone, and they are chemically identical to rare metal fragments found near Wabar. Bedouin apparently visited the site shortly afterwards; they probably could see the stratospheric-height, atom-bomb-like mushroom cloud from 100's of kilometers away.


Sunday, January 8, 2012

I Think I Found a Meteor Crater

This is a good place for an initial examination of something that may figure in the future of the human race: asteroid impacts. This is also one of the most common queries that we receive over time.


I think I found a meteor crater? Is there a national listing?  It is below the ice age cap so erosion would be minimum, and below tree growth so not visible by plane. 
I could mail you a graph. It looks like moon landscape.

Here are a few links that may be useful:

The first link is probably the most complete database on meteor craters on Earth currently being maintained; it started as an effort by the Geological Survey of Canada, but was picked up by the University of New Brunswick when the GSC program was de-funded a number of years ago. The database currently has 182 verified impact structures in it - and this covers only the ~30% of the Earth not covered by water.

The second link is an article I published in Scientific American more than a decade ago that provides some of the identifying characteristics of a meteor crater.  These features separate impact craters from the vastly larger numbers of circular structures on the Earth's surface that have nothing to do with a meteor impact.

Ask-a-Geologist receives several inquiries a week from people who have looked at Google Earth or other satellite imagery and feel they have found a previously-undiscovered impact crater. While the Moon is covered with impact structures, the Earth is not. The reasons for this include plate tectonics, volcanism, and sedimentation (features being buried), and weathering (features being erased over time). The Earth has a very active crust, and as a result only very young or very large impact features are represented.

During his lifetime, my SciAm article co-author Gene Shoemaker spent every Austral winter he could with his wife and sweetheart Carolyn working and mapping in Australia. He did this for a very cold-blooded reason (no pun intended): Australia is a large continent, and of all the planet Earth the continent least covered by forest, vegetation, or ice. Gene figured that if he could complete a full and informed assessment of the impact structures of Australia, he could get a statistical handle on the true number of asteroid and comet impacts that our planet has suffered.

From that, one can calculate how OFTEN we are being bombed from space, and how BIG the bombs are. That's one way to predict our future. If someone is pointing a gun at you, there is some consolation in knowing how bad a shot the gunman might be.

The Earth's atmosphere is also a very powerful protective blanket. Only about 5% of the meteorites found on the Earth's surface are the Iron-Nickel bodies that penetrate the entire atmosphere with sufficient velocity to create a crater on impact. Nickel-iron meteorites represent just 2% of the stuff floating between Mars and Jupiter, but because they are more distinctive are more easily recognized when they land on the Earth than the far more common stony and chondrite meteorites.

The rest (stony meteorites, chondrites, comets, etc.) are generally broken up in the middle to upper atmosphere, such as the Tunguska object that blew up over Siberia in June of 1908. This explosion burned, then flattened a forest the size of Rhode Island - nearly 2,200 square kilometers - but left no crater. As a general rule, the cratering record on the Earth only "turns on" at features above about 5 kilometers in diameter - when the object coming in is big enough to penetrate the Earth's atmosphere. As another general rule, these craters are roughly 20 times the size of the object that caused them - more on this in a later chapter.

The 99.99% of other circular features on the earth are caused by volcanoes, sedimentary processes like sinkholes, and intrusive magmatic bodies. The distinctive characteristics of a true impact structure include shatter cones, shocked-quartz minerals like coesite and stishovite, upturned and overturned rims, and if large enough, a central rebound peak. The largest known such features are ~300 km in diameter, and in most cases are severely weathered, folded, or otherwise obscured. Only about 15 of the 182 known impact structures have any remnants of the original bolide present, which makes them "meteorite impact structures" as opposed to just "impact structures" - again, this is a distinctive characteristic of our planet, which has a potent protective atmospheric blanket, active plate tectonics and weathering going on all the time.

In my studies related to the Wabar event (1863, in the Empty Quarter of Saudi Arabia), I have discovered records that indicate there have been at least five asteroid impacts on the Earth's surface between 1863 and 1947. At least two of these have been nickel-iron objects, and all five would be classified as "city busters" by virtue of the kinetic energy they delivered to the Earth's surface. By an amazing coincidence, all five happened in extremely remote, unpopulated areas of the planet.

I'm reminded of the poem by T.S. Elliot, called "The Hollow Men" that ends
"This is the way the world ends: This is the way the world ends: This is the way the world ends: Not with a bang but a whimper." 

But maybe it WILL be with a bang.