Landscape Change – How Fast?

A major question from the beginning of geology as a science has been how fast does change take place? From the Literalist read of the Bible, it would seem 6,000 years is far too short a time to permit the development of tens of thousands of meters of sediment, with remains of primitive life-forms at the bottom and advanced life-forms preserved at the top. The first rough estimates of the rate of sedimentation were made in England, by thoughtful natural scientists measuring how fast mud accumulated in a pond. These early geologists had already mapped thick stacks – thousands of meters of distinctive layers - of sediment in cliffs, road-cuts, and quarries. They had seen the same sequences long distances away, implying the same sedimentary process was happening over a very wide area. Finally, they had realized that for mud and sand to accumulate to thousands of meters of thickness, would take at minimum many millions of years. This was really the first baby step of geoscience.

Q: Hello, my name is Jurgen and I am currently enrolled in an AP Environmental Science class and have a question about river formation. I hope you can answer my question.

How long does it take for a gully or rill to be formed into a river if there is a constant stream or supply of water running through the land?  Thank you.

--Jurgen P

A: Time for a gully to become a river can vary wildly from less than a hundred to many millions of years. Generally, most terrains are in some sort of equilibrium and don't change much over time – unless disturbed by something, like a tectonic event. This is sometimes called "punctuated equilibrium." The change of a feature from one form to another (like a gully to a river) implies a permanent shift in the rainfall regime - some form of climate change – or tectonic uplift.

Change from a gully to a river could also have a lot to do with human intervention. I've walked down 10-meter-deep, steep-walled gullies that were really mini-canyons (Arroyos) in SE Arizona. These apparently didn't begin to form until man introduced cattle in the late 19th Century. Early journals from some of the first visitors describe “grass that was belly-high to a horse.” These cattle quickly wiped out the native prairie grasses by over-grazing the landscape. When Arizona earned its statehood in 1912, it had a human population of about 12,000 people, but an estimated cattle population of perhaps 10,000,000. Soils started disappearing rapidly with no roots to hold them, and small rivulets began to rip through the landscape and form small canyons in less than a century. Events like this, and the 1930's Dust Bowl, lead to the formation of the US Bureau of Land Management and the US Soil Conservation Service during the 20th Century.

Tectonic uplift can also weigh in powerfully, but tectonic shifts are generally relatively slow - slow at least in typical human time-frames. The Grand Canyon only began to form (cut down through pre-existing Precambrian to Mesozoic rocks) about 70 million years ago. The actual timing of the initial incision and the final down-cutting is still being argued today by geologists as more evidence accumulates, but it appears to have been quite rapid at the beginning.

Unconformity? Disconformity?

Here's a purely geologic question by someone who has already taken at least one course in geology. The question opens up and highlignts the three-dimensional aspect of geology - and why mathematics (especially geometry) is such a fundamental prerequisite for studying geology. Some people persist in saying that a geologist is just someone who didn't do well in physics or math. The hard reality is that physics, math, chemistry, and English composition are the building blocks - the basic tools - of a modern geologist. Some of the most sophisticated geology being carried out these days is done with computers. Drill-hole information is fundamentally three-dimensional, and the ability to construct three dimensional landscapes from surface mapping and drill-hole intercepts is just so very cool. To rotate this 3D landscape on one or several computer screens, showing how individual components evolved in time in a single giant cubic space... is absolutely essential to numerically assessing any resources the land under the geologic map may host.

Q: I have a question regarding identifying unconformity on geological map. I have attached a map as an example. How do we identify unconformity on such 2D geological maps if each colour represents a different rock? Please advice.

Thank you and hope to hear from you soon. Regards

- Hazel A

A: I have not downloaded your map and looked at it in detail, but just looked at it via the attached thumbnail. We are discouraged pretty strongly from downloading and opening any files from unknown individuals that might potentially be vectors for malware. For the purposes of this Q/A, a map is not really necessary, however. 

I'd like, instead, to address your question on a somewhat broader level: The inherent problem with a geological map is that it represents the surface of the land. It's a view looking downwards from space, which is not always the same as looking downwards in time. Sometimes, with tectonic and erosional events, older in time doesn't necessarily mean deeper in the Earth.

An unconformity is a gap in sedimentary deposition for one of several fairly specific reasons: non-deposition, subsequent erosion, etc. It is not easily represented in a geologic map, which only shows just one sub-horizontal surface - the part exposed to the sky. An unconformity means that there has been a time break in the geologic record. This is quite different from a juxtaposition of different geologic units due to, say, a thrust fault (though they could both be involved at the same time). 

In practicality, this means that the geologist who produces the map must somehow indicate or convey any unconformity (or disconformity, or nonconformity, or paraconformity, etc.: see http://en.wikipedia.org/wiki/Unconformity ) in her/his *Correlation of Map Units* columns on the side of the geologic map.

For most people not intimately familiar with a particular local or regional geology, it would be very difficult if not impossible to determine if some break between units is an unconformity or a fault juxtaposition just from looking at a geologic map alone. A change in rock-type could mean any of several much more common things: a change in sedimentary regime (like an ocean transgression), an intrusive event (like a big granite body punching up from the Mantle), a volcanic eruption, any of several different kinds of fault, etc., exposed at the earth's surface. 

It comes down to the fundamental difference between a map view (looking down at the ground from space),  and a cross-section view (looking at the ground side-ways, as if a giant trench had been cut in the landscape). However, even in an exposed cross-section, considerable sleuthing is required to determine if a break is an unconformity or not.

Some Questions About Volcanoes

Some questions are just fun to get. Perhaps it’s the teacher in me that likes to see young eyes light up with intellectual excitement. I infer from the following that volcanoes first get talked about seriously in the 5th Grade. It's beyond the soda, vinegar, and food coloring lesson.

Q: Question for my 5th grade class!

My students have some questions,

-           Megan A

1. Why do volcanoes erupt?

A: Pressure builds up from rising low-density magma below the earth. The low density is caused by the heat from the Mantle and Core of the Earth convecting upwards, sorta like a pot of Cream of Wheat cooking, or a lava lamp. The path of least resistance is to break out through the Earth's Crust at its weakest point. Where are those weakest points? Well, where you now see volcanoes is a pretty good hint. Some geologists have speculated that when tectonic events leave faults, and two faults happen to cross, that may make the intersection a “target of opportunity” for rising magma. However, there are a number of other factors involved, including where is the magma rising, and from what source, is there some under-plating of the crust happening, are there some gross compositional differences in the crust, etc. 

2. What are volcanoes like?

Some volcanoes look like cones (Mount Hood in Oregon, Mount Fuji in Japan). Some look like giant bulges (Mauna Loa in Hawai'i). Some volcanoes don't look like much of anything. You just see black-gray lava that has broken out of fissures, then poured out and run across the land in all directions – but generally the "pouring" goes downhill. There are vast, nearly impassable volcanic fields in western Saudi Arabia. There are huge obsidian flows (volcanic glass, caused by lava emerging in water and cooling too rapidly to form mineral grains) at Medicine Lake volcano in California. These look like a giant painted the ground with swirling green-black glass.

3. What is lava like?

Lava is very hot initially when it first reaches the air - it glows yellow-red from incandescence in cracks and at the flow-fronts. You can walk on it, because it is denser than a human body, but it is pretty rough on your boots. It melts boot-soles while hot, and cuts them up when cold because lava (e.g., in Hawai'i) is really just black glass. As lava cools, it sounds like a bowl of Rice Crispies crackling. As the flow-front reaches trees and houses, it engulfs them and the very high heat sets them on fire. This often forms tree molds - molds of where trees once were before being engulfed by the lava, for instance in HAwai'i and at Newberry Volcano in central Oregon. On Mauna Loa, a fast moving flow-front in the 1950's burst out of a fissure high on the volcano's west flank. I talked with a man who watched the flow run down the volcano's flank and onto a forest. It clipped off the trees at the base, then stack them vertically like bunched toothpicks at the front of the flow as the whole thing raced downhill at 60 kilometers per hour into the Pacific Ocean. 

4. Have you seen a volcano erupt?

I was inside Mutnovskiy volcano in Kamchatka when it started venting. I watched Mount St Helens erupt several times in 2004-2005. I've walked over active (evolving, moving) lava fields from Kilauea volcano, tracking the growing flow-front using a GPS device.

5. Is your job dangerous?

Not any more dangerous than driving a car on a Friday night when there are drunks on the road. Most volcanologists know someone, a friend or a colleague, who has died while working on a volcano, so yes, volcanoes ARE dangerous, and must be treated with respect. Because volcanoes are so dangerous, we take extra precautions when working on one that is restive, and generally stay well away of they are erupting. It's sort of like wearing seatbelts when you drive in a car. If you don't you are being deliberately careless - and statistically you have a much higher chance of dying. 

NEGATIVE Earthquake Magnitudes?

Some questions require an explanation of a different kind of number that some students haven't yet seen before. These different ways of expressing numbers were developed to help explain very large things, very many things, very small things, or very complex things, among others.

Q: Hi, My name is Anthony. I was wondering how negative magnitudes can be recorded for earthquakes, and what is the smallest earthquake ever measured? Thanks
- Anthony N

A: Earthquake magnitudes are actually exponentials, so a negative exponential doesn't mean a "negative" value in the usual sense of the word. I'm hoping you've already had exponentials in school - or at least you can go ask a teacher what they are.

For instance,
10(exp)+2 = 10^+2 = 100.0
   The exponent here is +2 and it means one hundred. This is 10 to the second power.
10(exp)+1 = 10^+1 =   10.0
   The exponent here is +1 and it gives ten - ten to the first power.
10(exp) 0  =  10^0   =     1.0
   The exponent here is 0 and it means one - ten to the zeroth power.
10(exp)-1  =  10^-1  =     0.1
   The exponent here is a negative number, but it refers just to a SMALLER value than a non-negative exponent would. Here ten to the minus first power means one tenth.

The smallest earthquake ever recorded is a bit more difficult to answer. There are three parts to the answer:

1. It depends on the sensitivity of the instrument, and how close the hypocenter of the earthquake (the actual rupture point) is to the instrument. There are a lot of sensitive seismometers set up around the world as part of the global seismic network - they are designed to look for earthquakes in the magnitude 2 range or higher. There are also some really, really sensitive seismometers positioned on and around volcanoes. These are set up to look for earthquakes so tiny that earthquake people wouldn't really be interested in them - events so small that only one or two of the nearby instruments may even detect them, and no human would likely feel them.

2. I believe that the smallest recorded events are probably in the M= -2 range (negative two magnitude) for a very clean, noise-free station. That's also what two seismologists in my office tell me (independently!).

3. When you are looking at magnitudes this small, you are also dealing with a lot of noise: cars driving by on a nearby highway, people or animals walking nearby, wind vibrating trees and buildings, etc. In a sense, the smallest earthquake ever recorded is sort of meaningless, because it becomes harder and harder to even know if it's real - or just noise. Also, the smaller the seismic events, the more common they are. As an example, the US Geological Survey estimates that there were about 1,300,000 earthquakes worldwide in the 2.0 - 2.9 magnitude range. There are MANY more as you get to ever smaller magnitudes. See an earlier chapter on how many earthquakes are detected each year in each magnitude range (http://askageologist.blogspot.com/2012/11/earthquakes-how-often.html).

No one is really interested in most of the wiggles you see in these two examples:

http://www.avo.alaska.edu/webicorders/Veniaminof/
This is an instrument set up on Veniaminof volcano in the Aleutians. At 8:30am PDT on 22 October, I can see a few distant teleseismic events (distant earthquakes) and a lot of tiny events that may or may not be small volcanic earthquakes, or in some cases just small rock-falls from the crater walls. I can also see some large swings of the recorder that are instrument noise - probably electrical noise, either human-caused or natural, like distant lightning.

Whereas, if you look at Augustine volcano's webicorder for that same day, you see only a huge amount of wind noise:

http://www.avo.alaska.edu/webicorders/Augustine/
This is an instrument set up on Augustine volcano in Cook Inlet in Alaska. At 8:30am PDT on 22 October I could only see masses of blue "ink" on the plot that indicate a lot of wind noise on this station. There is so much noise on this seismometer record at this point in time that any "real" earthquake would be impossible to see.

~~~~~

How Many Volcanoes?

How many of X are there in the world? This is a common question that often provides a surprising answer. You might be surprised at how many drill rigs exist or once existed in the Gulf of Mexico, for instance (over 4,200!).

Q: how many volcanoes are there in the world?

- Josh S

A: The Global Volcanism Program of the Smithsonian Institution lists 1559 volcanoes with eruptions happening during the Holocene period (the last 10,000 years). This means they are listing active or potentially active volcanoes. There are MANY more volcanoes in the world than that, of course. Some of them are just older and long inactive, like parts of Craters of the Moon national monument in Idaho (however some features there are as recent as 2,000 years ago), or volcanoes that erupted in Venezuela over a billion years ago. Some volcanic features that are not on the Smithsonian volcano database are just too small to easily list. While serving as the chief scientist for volcano hazards in the US Geological Survey, I assigned one of my senior scientists to do a full assessment and summary of the Cascades volcanoes of Washington, Oregon, and California. I thought there might be 15 volcanoes there. He ended up with a list of over 3,500 - because he counted every focal point of volcanic activity including small scoria cones and maars. 

Is the Big One coming?

The following query arrived just a day before the M=6.0 Napa, California earthquake of 24 August 2014. Unfortunately, I could not respond to the obviously nervous individual until after that event took place. The bottom line is that non-human risks change little over time – they are just there, but they can be dealt with. However, people tend to obsess over what scientists call “high-impact-low-probability” events – like a shark attack or an earthquake along the so-called Pacific Ring of Fire.

Q: Hello I live in California and currently I'm getting very scared with all the current activity around the ring of fire. My question is whether this is normal activity or warning signs for bigger earthquakes to come, or even the "big one"?

-          Dolores L

A: The activity you are noticing is normal - it has been going on for millions of years. Sometimes it seems more exciting in some locations than normal, but this is still normal.

When you write of a "big one", this is also normal: we are expecting a subduction mega-quake in the Pacific Northwest anytime in the next one day to 300 years. Some older schools are being earthquake-retrofitted in Oregon as I write this. The Earth's crust is an active place, and the truth of the matter is that no matter where you live there is some degree of local (potential) hazard everywhere, whether hurricanes, volcanoes, domestic violence, earthquakes, drought, floods, car-accidents, or tornadoes.

The rational way to deal with these is to evaluate each hazard carefully – and there are public agencies like the USGS that do this all the time, very conscientiously. Once you know what the risks are – and most of the "big ones" are high-impact-low-probability – then you can plan accordingly. I bought my home on a slope for the view, but I checked the foundations carefully before I paid for it. I also paid an extra 15% earthquake premium on my homeowners insurance.

To put things in perspective: people react strongly to learning that a swimmer was killed by a shark, but 10's of thousands of sharks die brutally at the hands of humans each year rather than the other way around. Compare the 1 - 3 human shark-bite deaths to over 35,000 highway deaths in the United States last year. While seat belts, speed limits, and no-texting are theoretically enforced, no one seems to get particularly excited about this huge killer.

Another perspective: My father, before he died of lung-cancer, lived in a high-rise apartment in San Francisco less than 10 miles from the San Andreas Fault. I once asked him if he worried about it much? His response opened my eyes. “Listen,” he said, “I could enjoy the view of the Bay from here, or I could hunker down in a basement somewhere and worry constantly. Long ago I chose the former.”

Bottom line: the world is NOT about to end. Study your own personal risks, and then take rational precautions to mitigate them as much as possible without going overboard. Just taking any steps will lessen your worries, because you will be actively doing something about them. This could include putting up a supply of drinking water and food to last you during a local or regional disaster. Or better yet, a years supply so you can also help your neighbors. 

RUBIES!

The US Geological Survey does not employ gemologists – while there have been several within our ranks historically, they have been amateur gemologists who have pursued their interest on their own time. Nevertheless, gems DO come from the ground, and could reasonably be construed to be an ultimate product of geology. The following question is typical of the kind we receive about gems.

Q:           Is there somewhere in California near Modesto that I can have a rock collection looked at? We are almost positive that we may have found some raw rubies! They have passed the scratch test and are very heavy and hexagonal shaped. 209-xxx-xxxx

- James

A:            We can't do gemology for you - the US Geological Survey is tightly constrained to work on only particular national objectives that Congress sets, including mineral resource assessments, volcano hazards, etc. 

My recommendation to you is that you contact a local gemological society and ask for guidance. I would NOT recommend going to any jewelry store, as they only focus and specialize on the end products. 

You might try:

http://www.americangemsociety.org/ ... but keep in mind that this is a trade association of retail jewelers, independent appraisers, suppliers, and selective industry members, and only incidentally will they have any component that might be of help to you. 

           You would probably do better with:

http://www.gemsociety.org/ 

...or even better with a local society of educated amateurs, like the San Diego Mineral and Gem Society:

http://www.gemsociety.org/

Finally, please keep in mind that there are beryls and other igneous minerals like garnet and eudialyte that can easily be mistaken for rubies by inexperienced people. A true ruby is a pink to blood-red (so-called “pigeon blood”) colored gemstone, a form of the mineral corundum (aluminum oxide). Ruby has a hardness of 9 on the Mohs scale, and is considered one of the four precious stones, along with sapphire, emerald, and diamonds. The red color in a ruby is caused mainly by the presence of the element chromium in the crystal lattice.

~~~~~

The Dust Bowl, King Solomon, and Country-Western Music

Sometimes – even after all these years – I am amazed to learn of yet another series of cultural connections all tied together by geology. The following Q&A is just one of many examples of this. Another example is how modern Venezuelan politics and its history are underlain both literally and figuratively by its ancient Archaean geology. This Venezuelan example would take too long to share here, but it can be found in a book my wife and I have written titled “2 Worlds, The Real Venezuela: Living on the Edge of the Jungle and the Rise of Hugo Chavez” (http://www.barnesandnoble.com/w/2-worlds-the-real-venezuela-jeff-wynn/1112567534?ean=9780615428444). By the way, the girl holding the monkey in the cover photo is our youngest daughter.

Q: I just watched a show on the history channel. Part of the show covered the "Dirty 30's" and how the "Dust Bowl" helped shape human kind today. My question is, of the possible millions of tons of top soil blown away. Where did it go? Did the eastern sea board states get a foot taller during those years or what? Thank you for taking the time to answer my question.  Sincerely,
- Bart A

A: The Dust Bowl was caused by improper farming practices that destroyed the native prairie vegetation and their root systems - and didn't replace them. The US Soil Conservation Service (Now the Natural Resources Conservation Service, or NRCS) was created to first understand, then mitigate the consequences of plowing fields in prairies. In 1933, its original incarnation the Soil Erosion Service was created within the Department of the Interior, with Hugh Bennett as chief. Bennett was a visionary soil expert who had been publishing scientific papers on the subject since the beginning of the 20th Century. He practically invented soil science.

       To answer your specific question, yes: the soil goes elsewhere, and other places grow “taller”, though you might not be able to easily recognize this. Winds tend to scour certain localized areas, and then distribute the soil and dust outside these areas - but much more widely. Thus it may not seem like parts of the rest of the country developed deeper soils, but they did - though very marginally. The airborne distribution process drops out the heavier particles at shorter distances, while the finest dust can theoretically blow around the world. You might speculate where some of the dust that always seems to find its way into your house originally came from.

Two anecdotes may help you understand this better.
1. Satellites can see dust from the western Sahara blow thousands of kilometers out into the central Atlantic Ocean. Ultimately it collects in the abyssal ocean depths, where coring has actually measured it and its growth rate. It may even have a part in the Atlantic Hurricane development process.

2. While I was working in central Saudi Arabia two decades ago, I was asked to provide geophysical assistance to a geologist trying to evaluate a small ancient mine (SAM) named an-Najadi. This mine was one of at least 852 small artisanal gold mines found in Saudi Arabia today, and apparently formed the major source of gold reported in King Solomon's treasury.

    While visiting the site I saw that the geologist had used a backhoe to dig trenches. His objective was to get down through the soil to the bedrock, to figure out what the bedrock structure was - so he would know which direction to point or orient his evaluation drilling program. At the bottom of one trench I saw two round stones - they turned out to be grindstones used by the ancient miners to crush the quartz grains holding the tiny fragments of gold that the miners were after. I am looking at those two stones in my office as I write this.

    On the sides of the trench I saw three white lines, and asked the geologist about them. They turned out to be slaked-lime floors of ancient dwellings. I could see the oldest one, then one a meter higher stratigraphically above it - that therefore had to be at least a thousand years younger - and then one above that. The trench was 14 feet (4+ meters) deep. The lowest level was occupied by miners in Solomon's time - 3,500 years ago. The 14 feet of soil above the lowest dwelling level arrived since that time and is called "loess", a German word for blown-in soil. Most of this soil had been blown in from the west. It had come from the eastern Sahara, and crossed the Red Sea to get there. I have personally experienced sand storms in that area that kept aircraft from landing - because pilots could not see the ground for up to three days at a time. Operation Desert Storm in 1990-91 was rushed forward in time to beat these seasonal wind storms called the Shamal. These storms happen during the period of monsoon storms that lash the Arabian Sea for several months in the Spring every year.

    This all tells us that over-grazing in the proto-Sahara, starting thousands of years ago, had already stripped most of North Africa of its protective vegetation, leading to the ever-expanding "desertification" process we see that continues today... and which Oklahoma experienced for a short period in the 1930's.

    Soil conservation is an important lesson we learned from the Dust Bowl of the 1930's. Some of the Oklahoma economic refugees migrated to California, and a country western band leader brought his musical tradition from there - and lived across the street from me when I was a child in Bakersfield, California. I still remember Bakersfield being one of two major Country Western music centers in the country after Memphis... and I can still remember the disparaging name of "Okies" being used for these poor migrants decades after they arrived.

    I hope this answers your question, and perhaps puts it in a wider context.

Hot Magnetic Oxygen Water World

There is another version of the Anthropic Principle, one that applies only to the planet Earth. We may be more alone, or unique, in this universe than the Drake Equation - the calculation of the possibility of other life out there in the universe - may have led us to believe.

The deepest hole ever drilled into the Earth's crust reached down to about 7.6 miles (12 km) below the Kola peninsula of northern Russia. The technology available to humankind cannot get below that depth (and that depth took 24 years of drilling and billions of Rubles to achieve). The rocks are so hot and plastic with overlying rock pressure at those depths that the hole closes in on the drill bit - and partially fills the shaft back in from the sides as the bit is drawn back to the surface to be replaced. So... the maximum depth achieved by humanity's best effort is less than 1/10,000 the Earth's diameter, or the distance of a short commute on a Monday morning. We actually know more about galaxies, comets, and the moons of Jupiter and Saturn than we do about what lies below our feet on our own planet. No matter how you look at it, we cannot really touch virtually all of the world beneath our feet.

In other words, everything we think we know about the interior of the Earth is obtained by very indirect means, and a lot of this is from mathematical modeling.

To see below the depth of the Kola well, we must rely in electrical geophysical methods like magnetotellurics (which is one of the things that I "do" as a geophysicist; it can detect resistivity layering down to perhaps 50 km or so), and on earthquakeseismology. For nearly a century seismologists have traced the powerful vibration signals from very large earthquakes as these signals propagate and refract through the Earth. By comparing the time of arrivals elsewhere around the planet - and whether just P-waves, or P-waves and S-waves together make it - they can discern contrasts in density and other physical parameters as these change with depth. P-waves (or primary waves) are pulses of energy, momentarily compressing the material they pass through. It's the blast wave from an explosion expanding outward. S-waves (or secondary waves) are shear waves, oscillating material back and forth, sideways, as they pass through the material. Think of how you would move your hands forward and backward to tear a piece of paper. A key feature of S-waves is that they cannot propagate through a liquid. Think of trying to use your hands to tear water. By the 1920's seismologists had used the initial earthquake seismic information and some density calculations to conclude that there is a solid iron core to the Earth, surrounded by an outer liquid iron part of the core. The outer liquid core is overlain by a hot and plastic Mantle, and finally by a relatively thin crust serving as a very thin solid shell above them both. All living things live on or just beneath the top of that crust.

The methodical genius who first figured all this out was a quiet Danish lady named Inge Lehmann, who died in 1993 at 104 years of age.

Seismology and magnetotellurics show us the layering in the Earth with depth. Indirectly we also know that the center of the earth is very hot. After all, there are volcanoes and fumaroles, and the deeper you mine in places like South Africa the hotter it gets. Nearly everywhere scientists have measured temperature in wells, a thermal gradient exists: deeper means hotter. But we also know there is a lot of heat below us for several other reasons, including plate tectonics. SOMETHING has to be powering whole continents to be able to wander around. And then there's the magnetic field of the earth.

What distinguishes Earth from Mars and the Moon? A magnetic field, an atmosphere, liquid water - and life. The last requires the first three in our limited observations so far. Without a magnetic field to deflect it, Solar radiation would  sterilize the Earth and disrupt any attempt for life to gain a foothold. Solar radiation would also strip away any atmosphere, which is apparently why Mars doesn't have much atmosphere left to speak of. Mar's atmosphere is only a few percent of the density of our own atmosphere - though there is evidence of much more at one time in the distant past.

What distinguishes Venus from the Earth? Venus has an atmosphere, but it has fallen under a runaway Greenhouse Effect - too hot for water and in fact so hot that raw sulfur is a liquid on its surface. The Earth lies in what is sometimes called the "Goldilocks Zone" where it's not too hot and not too cold, between roasting Venus and frigid Mars. Water on Earth not only exists, but can exist in all three states (solid, liquid, and gaseous). This is not so for Mars or Venus, neither of which has a magnetic field, nor plate tectonics, nor significant water.

It has been apparent for quite awhile that the Earth's magnetic field is the reason why life exists on our planet. A magnetic field, however, requires some sort of dynamo to create and sustain it. How to power this? Well, if there are enough radioactive elements - or sufficient heat from the collapse of the proto-planetary disk to form our planet - well then maybe there is enough energy to drive a dynamo. However, this requires a lot of assumptions that scientists cannot test - they can't drill deep enough.

There is another problem: hot things tend to cool when surrounded by colder things... like interplanetary space. A magnetic field driven by an internal dynamo cannot last forever.

Hot things cool in two ways: by conduction and/or by convection. Conduction is like the metal pot you cook your cream of wheat in. Heat transfers from a hot source beneath to a cooler part above without any motion of particles involved. With convection, however - the bubbling cream of wheat - the heat is transferred by particles moving in three-dimensional loops called hydrothermal cells. You see them as bubbles driven by steam in the sauce pan. A hotter particle of the wheat from the bottom, in contact with the metal pan, rises because it is hotter (and thus less dense) than the particles above it, thus transferring heat from the bottom to the top of the cream of wheat. If the stuff cannot convect - if it's not liquid enough - then it will get hotter and hotter until it burns. It not only tastes terrible, but the sauce pan is a bear to clean up afterwards. In the same way, the solid iron core can only conduct heat out; like the metal sauce pan it cannot convect heat. However, the liquid iron outer core and the hot and plastic mantle above it can convect heat - and these convection cells of highly conductive material must be the source of the magnetic dynamo. The convection cells in the mantle are also what's driving whole continents around across the face of the Earth.

Remnant magnetization in rocks 3.5 billion years old, however, proves that the Earth's magnetic dynamo has existed for at least that long. The oldest known life is found in stromatolites - clumps of cyanobacteria - just about that old. This is not a coincidence. If there was no protective magnetic field, the stromatolites and then algae (and Earth's atmosphere) would not have survived Solar winds and radiation. But 3.5 billion years is a long time for something to stay hot enough to drive a magnetic-field-producing dynamo. Older computer models based on relatively low thermal conduction assumptions for iron seemed to suggest that it would take awhile for the solid iron core to give up its heat. This could conceivably sustain a dynamo lasting that long.  According to these older models, the heat from the core would take billions of years to conduct out to the outer liquid core and Mantle where a different form of heat transfer - the much faster convection - takes place.

In the last several years, however, scientists have been forced to re-evaluate what they think they know about the center of the earth. Several years ago, another piece of information became available from some Japanese extreme-high-pressure experiments. Iron at pressures and temperatures we calculate must exist in the center of the Earth has a far higher thermal conductivity than anyone had thought could be possible. According to milecular orbital theory, if you smash material together hard enough, it frees up electrons and changes its conductivity. This means that the Earth's heat-driven dynamo should have burned out billions of years ago. In other words, the Earth's magnetic field would have then died, and the atmosphere and any nascent life would have all disappeared before most of the geologic record could even take place. Think of dead Mars.

Speaking of geology, fluid and gas inclusions in ancient rocks tell us that around 2.5 billion years ago the Earth's primordial atmosphere of CO2 and nitrogen transitioned to an oxygen-nitrogen atmosphere. The world as we presently understand it began then. In part we can blame this on the stromatolites and photosynthesizing plant life that was expanding at that time.

In the 1970's a few scientists offered what seemed like a ridiculous idea: the Moon formed well after the Earth formed. It formed in its current size and shape when a large Mars-sized planetoid crashed into the proto-Earth and splattered material into space around the Earth. That material blasted into multiple orbits then coalesced to form the Moon, leaving a very different - and very hot - planet Earth behind. Computer models show that this is easily feasible. If so, then the Earth would have glowed like a small star from the massive infusion of heat from all the kinetic and potential energy of the collision. This idea is now taken seriously for several reasons, but mainly because the rocks on the Moon are sooooo much like the rocks on the Earth, and sooooo different from rocks on Vesta, Ceres, Mars, and Venus. We can discern these by optical spectroscopy, coupled with sampling meteors that the spectroscopy says must come from those places.

Could that ancient impact hold the answer for why we have such a long-lasting magnetic field around our planet? That seems to be the best explanation at this time. If so, then life exists on this planet because of some pretty amazing circumstances: 

• it exists in a narrow Goldilocks Zone, 
• it was given a huge heat boost by a collision from a large planetoid, and 
• its crust was given a lot of water from impacting comets that allowed it to be less solid, more flexible, and have an ocean of liquid water. 
• Photosynthesis then started early and gave this planet an oxygen-nitrogen atmosphere, and finally
• The Earth's magnetic field lasted a very, very long time.

Those are a lot of things that had to come together at just the right time for life to form and evolve here.

There are so many coincidences - like the Anthropic Principle that allows molecules - and thus life - to exist. It seems remarkably like our Earth has its own local version of the Anthropic Principle: just the right features and additions at just the right times to allow life to form and evolve over an extended period of time.

Bruce Buffett, a geophysicist at Berkeley puts it this way: "The more you look at this and think about it, the more you think it can't be a coincidence. The thought that these things might all be connected is kind of wondrous." (Discover, July/August 2014, p. 41)

With all the exoplanets being found in solar systems nearby in the Milky Way Galaxy, what is the likelihood that one of them could have all these coincidences? Since Galileo, humanity has been humbled to know that it isn't the center of the universe. 

However, it appears that we certainly are unique.

~~~~~

Windshield Time

Q: Thank you very much. Your answer was more than adequate. Not only did you answer my primary question but also preemptively answered some follow-up questions I may have come up with.

My only remaining question is how the extremely deep oil reservoirs they are finding were formed. I've read some of the oil is at depths that would seem to pre-date the Carboniferous age.

I'm a plumber but I love pondering things such as this during my frequent "windshield time ". I appreciate you taking the time to explain this to me and to do so in a way I can understand.

Thank you very much,

- Patrick D

A: You can call yourself a "Plumber" if you want, but you are clearly and instinctively a natural scientist. That's the only definition that would apply to someone who ponders the world around them to such a deep extent during "windshield time" as you call it. I was involved on an expedition that crossed the Empty Quarter desert in Saudi Arabia and had two formally-designated scientists (we had PhD's). However, most of the other 15 expedition members got deeply into what we were trying to map at the Wabar asteroid impact site (Gene Shoemaker and I published this in an article in the November 1998 Scientific American). Our expedition companions first started asking questions, then offering ideas - and as a scientific TEAM we did the partial crater excavations and the surface mapping of the site. There were 17 people on that science team.

To answer your other question, there was carbon on this planet from its original formation. Some is magmatic in origin - things like carbonate volcanoes, or crystallized carbonate magma more commonly called "carbonatites".  This is primordial carbon that is thought to come from the mid-to-upper mantle. There is a carbonatite in southwestern Afghanistan that stands out from the surrounding rocks both chemically and structurally like a big red flag. There is another, a real monster, in southern Venezuela (Cerro Impacto is ~10 km across, but is NOT an impact feature). These things often have unusual levels of Thorium and Uranium in them, often in concentric zones. There are also Kimberlite Pipes - these are generally but not always tubes that carry diamonds up to the surface from the upper Mantle.

However, most oil & gas deposits come from sedimentary deposition of swamps and their occupants during ages that reach back as far as life existed. The carbon in the vegetation and animal life was buried to increasingly greater depths by later sediments, sequestering it and getting it out of the atmosphere. This usually happened in large basins, and the accumulating weight of these sediments often caused the basin to bow and get deeper in the middle. As an example of how fast this accumulation can happen, I was visiting an ancient mine site in the western Arabian peninsula. This was one of ~862 small ancient mines that provided King Solomon with his gold about 3,500 years ago. In that 3,500 years, dust and sand blowing across the Red Sea from the Sahara have buried the original mine site in nearly 4.5 meters (14 feet) of loess, silt and dust that we now have to dig down through to access the original shaft. And this accumulation was on flat ground! When surrounded by eroding mountains, a basin’s sedimentation can build up much faster than this.

With increasing weight overlying organic-containing sediments, both pressure and heat rise. Natural temperatures at the bottom of a 12,000-ft diamond mine in South Africa are about 60 C (140 degrees F). Eventually you get enough heat and pressure to "cook" the organic sediments - oil geologists call this process "maturation" among other things. When converted to a liquid these relatively less dense, carbon-rich fluids (oil and gas and water) tend to migrate upward, following weak zones in the sediments overlying them. They will do this until they either escape (the Gulf of Mexico is full of natural "seeps") or they get to a blockage that traps them: for instance some sedimentary salt from a dried-up ancient sea. THIS kind of natural trap is what the oil companies are looking for using sophisticated seismic prospecting and imaging systems.  

~~~~~

Seafloor Ooze, Subduction, and Oil

When I was a young man, I thought that having my PhD meant that I was now a scientist, that the advanced academic degree was somehow the dividing line between scientist and not-scientist. If I had been a little better at history, I would have realized that some of the greatest minds in science – people like Michael Faraday and James Clerk Maxwell – did not have PhDs. What they DID have was a tendency to think about things. The following two queries came from someone I call Patrick the Plumber Scientist.

Q: I've read the seafloor  "ooze" contains a fair amount of carbon based material. When this ooze is carried along with the seafloor downward in subduction zones wouldn't the combination of heat and pressure along with the presence of water form hydrocarbons aka oil?

- Patrick D

A: You are an unusually thoughtful person to arrive at that conclusion. Not all the ooze, as you call it, actually goes down with the subducting oceanic crustal slab – some of it gets scraped off and in some cases rafted onto a continental margin. You can find some of these strange remnants on the northern California and southwestern Oregon coastal area, among many other places in the rest of the world.

At some point the carbon from the seafloor muck that DOES go down with the oceanic crust probably passes through an oil/hydrocarbon maturation phase, but at depths and circumstances where it could not be economically extracted (even if it could be located). The muck continues down even deeper with the oceanic crustal slab to depths where even greater heat and pressure subsequently break it down to even more primitive constituents. With the water and sulfur also found in these seafloor sediments, this leads to partial melting – the lighter constituents rise through the crust (like a lava-lamp), somewhere in-board of the subduction zone to form volcanic chains like the Cascades, the Kamchatka Peninsula, the Andes, the Indonesian Archipelago, etc.

The magma that actually rises is driven at least partly by CO₂ and H₂S gases that derive from that original seafloor muck and seawater. These constituents, along with the iron, manganese, and silica of the Mantle, comprise the rising magma.  As it comes closer to the surface of the Earth, the pressure decreases and the gases come out of solution (like uncapping a bottle of soda) in that rising magma to form bubbles. This has been studied in one of our laboratories in a hot-high-pressure cell. The increasing nucleation of bubbles expands the magma volume and this causes the whole mix to accelerate upward faster and faster toward the surface. There it can often reach a runaway explosion that we call a Plinian eruption (named after Pliny the Elder, who died at Herculaneum trying to rescue friends during the eruption of Mt Vesuvius). This bubble-filled magma becomes a froth exploding violently upward into the atmosphere; it cools in the air to form the ash and tephra that (along with effusive lava) form the slopes of stratocone volcanoes like Mt Fuji, Mt Hood, and Mount St Helens.

Volcanologists work hard to measure and track volcanogenic H₂S (the burnt-match smell) and CO₂ gases to get a sense of where a restive volcano is in its possibly-pending, probably-not eruption. When Mount St Helens erupted in 2004-2006, it was relatively non-violent (though you would have died if you had been inside the crater at the time). An earlier almost-eruption in 1998 never quite reached the surface. Seismologists could see the volcanic conduit below MSH "light up" with the rock-breaking activity of a magma approaching the surface, but it never broke through. In the intervening 6 years, apparently these gases largely escaped, reducing the explosive danger from the volcano when it finally did erupt on October 1, 2004. One way to know if the CO₂ is volcanogenic, or from the modern atmosphere, is to measure its isotopic makeup. Atmospheric CO₂ has ¹⁴C ("Carbon-14"), ¹³C, and ¹²C isotopes. Volcanogenic CO₂ has only Carbon-12 (¹²C), the stable isotope in it. The other two radio-isotopes have long since decayed during the millions of years passed while the carbon was deep inside the Earth. 

~~~~~

When Will the World End?

I have received episodic queries asking if the world is about to end? Sometimes these correlate with apocalyptic movies being released. Sometimes they are triggered by an uneducated conspiracy theorist (an oxymoron) somewhere with nothing better to do than to look at seismic data freely available on the web. For instance, does the latest seismic activity in Yellowstone portend the end of the world? That one turned out to be an instrumentation issue not understood by the conspiracy theorist. Do the huge earthquakes off the coast of Chile and Japan mean that the End Times are approaching? We’ve all seen trailers for movies like “Volcano” (“The Coast is Toast”), and “2012”, and I have little patience with these attempts to make money.

But when will the world really end?  Or at least become unrecognizable to us, or even uninhabitable?

Current understanding of the evolution of the Sun suggests that it is about 5 billion years old and will likely continue burning for another 5 billion years. It may start fusing helium to carbon and turn blood red before then, but the time is so distant as to be irrelevant to us.

What about things heading south on somewhat shorter time scales? An article by Wolf and Toon (http://onlinelibrary.wiley.com/doi/10.1002/2013GL058376/abstract) suggests that there will first be a “moist greenhouse runaway” event, followed by the loss of all water from the surface of the Earth, followed by a runaway thermal greenhouse situation – like Venus is currently experiencing. The Sun increases its energy output by roughly 1% every 100-110 million years. In other words, it will continue growing slowly hotter on the planet Earth (see an earlier chapter on the Faint Young Sun Paradox here: http://askageologist.blogspot.com/2012/06/snowball-earth-faint-young-sun-paradox.html).

As solar output grows, the Earth’s surface temperature should steadily rise. When it does, water vapor concentrations in the lower atmosphere will increase, and this will lead to an increase in water vapor in the Stratosphere. Solar radiation there will break down water molecules, and the Solar Wind will then blow them away into space, leading eventually to a waterless surface.  This may be what happened to Mars billions of years ago, made to happen faster and earlier due to its weaker gravity. 

Some earlier research had suggested, based on computer simulations, that a “moist greenhouse runaway” process would start about 170 million years from now, and that a full thermal runaway (the “Venus Effect”) would start around 650 million years from now. However, Wolf and Toon factor in ocean-atmosphere moderating effects from those same surface waters, and calculate something more like 1.5 billion years before the onset of the “moist greenhouse runaway” event. 

Somehow I find this difficult to worry about.

What about bad things happening on shorter time scales? For instance, what is climate change really leading to? There is no shortage of either Climate Doomsday or Climate Rubbish prophets. A recent article in EOS (Transactions, American Geophysical Union, Vol. 95, No. 18, 6 May 2014, Wuebbles et al, link here: http://onlinelibrary.wiley.com/doi/10.1002/2014EO180001/abstract) provides several illuminating graphs included here for interested readers. Figure 1 shows the severity of weather in the United States on a decade-by-decade basis starting in the 1950’s. It’s hard to argue with a graph like this: climate change is clearly well underway (see the earlier chapter on Climate Change – is it real? Here: http://askageologist.blogspot.com/2013/07/climate-change-is-it-real.html).

Figure 1. Extreme weather events in the United States by decade since the 1950's (Wuebbels, et al., 2014).

Figure 2 actually lays out the consequences for climate change: what things will look like for different parts of the country for the 2070-2099 timeframe. A short summary: it all gets hotter (no surprise), and the precipitation generally increases (surprise), except for the southwest, where precipitation will decrease (no surprise). More and greater hurricanes are projected (no surprise), but the numbers of severe tornadoes and severe East Coast winter storms have not increased in six decades and may not with the increasing CO2 and methane in our atmosphere (surprise). The minimum temperature in Alaska will be between 12 and 15 degrees (Celsius) warmer – not bad for people like me who don’t like white stuff on the ground. Perhaps more surprisingly, the northern tier of the Continental United States will get warmest – by about 6-11 degrees Celsius by the end of the century. Mean precipitation will stay pretty much the same in the Southwest – but it will be 6-8 degrees Celsius hotter, leading to drier conditions even with that precipitation. This will make those Phoenix afternoons somewhat less survivable as the century develops. 

Figure 2. What we can expect, region by region, from climate change if CO2 and methane continue to be produced by fossil fuel consumption at current rates (Wuebbels et al., 2014). 

What about economic impacts? The American Breadbasket of the central and northern plains will be seriously threatened by increasing drought conditions. Perhaps we should stop wasting 10% of our corn crop for ethanol. 

What can anyone do on their own? You should consider investing in land in the Canadian Prairie Provinces – but NOT anywhere near a modern coastline. Estimates of seawater rise vary – but they are all on the positive side, and low-lying areas like the Jersey coast, Florida, and New Orleans will be the Big Losers. An attempt to rationalize flood insurance following Hurricanes Katrina and Sandy lasted just two years – then appeals to congresspersons for relief from dramatically increased flood insurance rates “won” again. The end result is that people are rebuilding low-lying areas, and the American taxpayer will be expected to bail them out at enormous expense yet again.  Hurricane by hurricane. Science deniers apparently don’t believe in gravity, either.

Ultimately, if the world was going to end in 1,000 years, how would that be different from 1,000,000 years or 1,000,000,000 years? How would you change your life?

If you’re rational, you would not worry about the End of the World too much - unless you live on the Jersey Shore, or Florida, or New Orleans. If you are both rational and responsible, you would consider replacing your gas-guzzling SUV for something that gets better mileage. If you are still bothered, go help at a Sharing House for people who cannot get enough to eat, and you’ll feel quite a bit better afterwards.  

You will have increasing opportunities for this with time.

Aquamarine

Some people may be sitting on a gold mine – literally. I’m acquainted with some once-hard-scrabble ranchers in Arizona whose lands sat atop what would eventually become a gold or copper mine. They live in large houses and drive late-model pickups now. Other people may have stumbled on a rare fossil (a woman in Montana accidentally stumbled onto what turned out to be the most complete T Rex fossil ever found), or a rock that turns out to be a gem in more ways than one. 

Q: I have an aqua marine stone, approx 15 pounds . I would like to know it's value.
- Terry M

A: If you mean "aquamarine", then there are several possibilities:
a. a pale blue or greenish gem variety of beryl,
b. an aquamarine sapphire,
c. an aquamarine topaz, or
d. an aquamarine tourmaline.

15 pounds of any of these would be worth quite a bit, depending on the grade and quality. However, in the US Geological Survey we do highly applied research in geology and geophysics (some field offices work on ecosystems and biology). We have very specific line-item assignments in this agency, assignments set by Congress, and they do not include dealing with gem stones. As a result, we have never hired a gemologist per se as far as I know. 

I wish I could provide more help, because this is fascinating to me. In Bangkok, Thailand, there is a Wat (temple) that houses something called the "Emerald Buddha" that is apparently a carved statue of rough-grade emerald. In several senses of the word, this is a priceless artifact. Your stone would not be on par with this (it's not carved or sculpted I assume), but it is still worth something - if only as a source of material that gemologists can cut/extract high-quality raw gems from. 

13 Questions

I suppose we’ve all heard the expression of saving the best for the last. Here is another example: a series of remarkably eclectic questions from a group of 12-yr-olds in Indiana. These kids are ‘way out ahead of where I was at that age.

Q: My 7th grade science students had several questions they would like answered. They have been working hard studying the forces of the earth and have generated these questions collectively.

- Brianne G (7th grade science, Otter Creek Middle School)

A: I'll try to respond to each question below them.

Q1. How do tectonic plates move if they are right next to each other? (Madison)

A: The plates move with difficulty, as you might expect. The forces driving them, however, are immensely powerful – the strongest forces in this part of the solar system. Take an Oreo cookie and try to slide the two dark sides apart - it's not easy. Once it "goes", it goes with a sharp jerk... just like the San Andreas fault. Now take one of the freed-up dark sides and break it in half. Try to force one half back against the other, perhaps with a little skewed sliding action, on a flat table. There is a lot of breakage at and near the contact between the two halves... just like the Himalayas or the Andes range in South America.

Yes, the plates are right next to each other, but they must move in response to the ginormous forces driving them. The result is faulting (and huge earthquakes) and mountain uplift. It's a rough life if you are the lead edge of a tectonic plate.

Q2. How do fossils stay around after all this time? (Marisa)

A: Fossils "stay around" only if they are silicified of form a mold with another material around them. That is, the original bone or scale or feather material is (a) imprinted on a long-lasting mold in the resistant material around it, or (b) slowly converted after death to a rock made up of mostly silica - the same stuff in your car window. This happens because fluids from rain move through the rocks all the time, and they carry small amounts of dissolved silica from the overlying rocks that these fluids percolate through. The original bone material is more easily dissolved, and thus slowly "replaced" by the silica. This is especially the case if the water is slightly acidic – for instance if it came from a swamp above the buried bones. The bony/silica fossil record "turns on" at the beginning of the Cambrian period, about 542 million years ago. Before that, all life forms were apparently soft tissue, but even some of *those* were imprinted onto silica-rich (or at least weathering-resistant) material that formed molds, for instance a carbonate mud that then solidified. These sort of imprints are why we now know that some dinosaurs had feathers. 

Q3. Is there any evidence that all the continents will form back together and make a super continent? (Kobe)

A: If you were able to stay around long enough, you would see the Pacific Ocean slowly squeezed out of existence by the continents over-riding it from nearly all sides. Tectonic plates do not always go in the same direction all the time (note the Hawaiian Island chain and the Emperor seamounts before them - they form volcanic chains that meet northwest of Oahu but are are different angles). If we assume that the plume swelling up beneath Kilauea, Mauna Loa, and Loihi volcanoes right now is "fixed" in position with respect to the Mantle, then it's clear that the Pacific plate must have changed movement directions sometime before Oahu Island appeared.

This is a long answer that I can put in shorter form: no geologist can say for sure. The surface of the earth is incredibly complex, and it has been evolving (changing) with time. Even if you made the assumption that all the plates will keep moving in their current directions (a bad assumption: it's clear that no one can say that if you just look at previous geologic history), it would still be messy to try to figure out where things will finally end up.

But it does look like a conglomeration of continental crustal plates is one future possibility... if you don't worry too much about the details of what a "super-continent" or its complicated margins actually are, and have 500 million years to wait around and see...

Q4. How far can a sinkhole go down? (Wyatt B.)

A: The sinkhole can go down as far as the bottom of the soluble carbonate rock that it forms in. That is to say, if the rock below your house is largely limestone, and the water table drops due to a drought and water is able to move more easily through it in a lateral direction and expose more voids, then theoretically all that limestone can be dissolved away. In fact all the limestone will not be dissolved, because not all the limestone will be exposed to water moving through it - that's why you have sinkholes and not sink-counties. There are clearly geometric considerations, in other words, plus the water dissolving the limestone has to be at least slightly acidic and stay acidic even as it mixes with the carbonates. However, there is always a bottom edge of any limestone or carbonate geologic unit. That bottom is as far as your house can possibly go down, at least vertically.

Note that I have not addressed here the issue of sinkholes caused by human mining of salt domes.

Q5. How do you define a resting volcano? (Evan N.)

A: Generally if a volcano has not erupted in 10,000 years (that is, within the Holocene period) then it is considered dormant. This isn't a perfect rule, as "Fourpeaked Volcano" in Alaska erupted in 2006, and apparently had been dormant for more than 10,000 years beforehand. To know if a volcano is dormant or restive, you must do careful geologic mapping to identify all previous flows – including buried flows – and age date them. The US Geological Survey has gotten very good at doing this, but its funding has been steadily declining, making it harder and harder to map and instrument all the potentially dangerous volcanoes in the United States and its possessions.

Q6. How do volcanoes create islands? (Chris M.)

A: Volcanic islands (and Guyots) are formed by magma welling up beneath the seafloor and breaking through. There are several reasons why this might happen, including seafloor spreading, like what we see in the middle of the (expanding) Atlantic Ocean (Iceland is an example), and mantle plumes, like we believe are building the Hawaiian Islands. Over time the lava and other volcanic materials will build up until it all breaks the sea surface, creating an island. There are many examples of this through geologic history, as well as modern, currently evolving features like the Hawaiian Islands. Another proof that this process is active are the huge pumice "rafts" seen floating in the Pacific Ocean by mariners periodically since the 17th century. Southeast Alaska, in fact, appears to be a series of volcanic island arcs that have been "accreted" (slammed together) since the Ordovician period as the North American continent encroached upon ocean seafloor where they originally formed. These island arcs were essentially "rafted" onto the approaching continental margin.

Q7. How hot are volcanoes on average? (Peyton L.)

A: It depends on what the material is and how close it is to the source. Rhyolite (silica-rich lava) can be solid at the same temperature where basalt (silica-poor lava) is flowing as a liquid. Some estimates of the bottom of the crust put temperatures at around 900 Celsius, but the bottom of the mantle is estimated to be at least 4,000 Celsius. Lava at Kilauea volcano in Hawaii is mantle-derived, and can be at least 1050 degrees Celsius – it glows bright yellow to your eyes even in broad daylight. With time at the surface, it cools to a dull red and then to black... but it is still hot enough to destroy boots for weeks afterwards. I lost a pair of boots walking out a lava flow lobe. An interesting anecdote: the cooling gray-black lava sounds like Rice Crispies after you pour milk into a bowl of it. This comes from thin flakes of volcanic glass popping loose as the cooling flow contracts.

Q8. How do we know when there's about to be an earthquake? (Devin A.)

A: We do not know. Moreover, some of the best minds on the planet working on this earthquake prediction problem say that we may never know. After over 150 years of intensive scientific study, no one has ever been able to figure out how to predict an earthquake. We can forecast an earthquake – that is, we can estimate a 63% probability that there will be a magnitude 6.7 earthquake in the San Francisco Bay Area within the next 30 years. However we cannot predict when it will happen, nor exactly where, nor can we say how big it will really be.

Q9. Where can we find faults, even when they aren't on a boundary of tectonic plates? (Nic)

A: There are several ways to find faults – and they are everywhere, so it should usually be easy. Geologists can map faults in the field (broken rock, or mismatched units adjacent to one another are evidence), and geophysicists can "see" them via small earthquakes if the faults are active (for instance, most faults in southern California are presumed to be active). If the faults are buried under dirt, swamp, water, or forest, it becomes very difficult to map them, and sometimes geologists must rely in digging deep trenches across ground segments where they think a fault may lie. This is expensive to do and dangerous to then crawl into and map, and thus is not commonly done unless fault timing is really critical (for instance, in southern California). The 1994 Northridge earthquake in Los Angeles was a big surprise to everyone. The seismic data indicates that it was caused by a break on a flat-lying fault lying many kilometers below the ground surface. This fault was not exposed anywhere, and was thus labeled a "blind" fault.

Q10. What do you have to do to become a geologist? (KC)

A: There are people who are just interested in rocks and fossils and land-forms all their lives, and they might rightly consider themselves to be geologists if they are studying these things as amateurs. However, to be a *professional* geologist – someone who can be hired by a company and paid to do geology work – you would need a college degree in geoscience or geologic engineering. A bachelor's degree might not get you more than an entry-level, low-paying job, so a Masters degree or PhD might make more sense. In either case, you cannot be a "real" geologist until you have studied physics and chemistry, learned math to at least the calculus level (you will need a *lot* of trigonometry for structural and economic geology), and you will need to get good grades in English. English!?! Yes – if you cannot write a clear and coherent report it makes no difference how much you have studied, because no one will know what you know, or what you have discovered. I personally know a brilliant PhD geophysicist who could not write his way out of a paper sack. He never got promoted, because he had to depend on others to write his reports and scientific research papers for him.

Q11. Why is your job as a geologist so important? (Hunter)

A: Are you warm and comfortable right now? Do you drive a car? Did you eat breakfast? Are you reading this with a hand-held device, or a computer, or with electric lights? Then thank a geologist who helped find the petroleum to power your car, and coal to power generators, and the copper and other critical elements for the wires and the computers and the harvesters. USGS geologists saved at least 800 lives in 1980 when they told Washington State that Mount St Helens was about to erupt catastrophically - and governor Dixie Lee Ray ordered that a "red zone" be set up around the volcano. USGS geophysicists saved the lives of hundreds of thousands of people in the Philippines in 1991 when they monitored and then recommended a massive evacuation around Mount Pinatubo. They also saved billions of dollars of aircraft that would have been destroyed by the ash and ejecta from the volcano at Clark Air Force base. We have a tsunami watch system all around the Pacific Rim now, saving untold lives. In 2004 we did not have a tsunami warning system in the Indian Ocean, and 250,000 people lost their lives after the Aceh, Indonesia, mega-quake.

Q12. How and why is the water in the ocean salty? (Andrew)

A: Try pouring a pitcher of water through fresh crushed rock. Then pour it through again and again. The mineral content in the water will continue to rise until pretty quickly you can taste it.

Another way of looking at this: put a tiny amount of salt in a pot of water and you may not even be able to taste it. Now boil the water down to just a few milliliters of water remaining. That remaining water will be very salty to your taste.

Over billions of years rain has poured down on the continents and leached out all the easily dissolved minerals, which then passed down through rivers to the sea. Salt is easily soluble, and even if found in just tiny amounts on the continents, it will just keep accumulating and accumulating in the seas over time. It’s more complicated than this, or course, because there are chemical interactions and changes involved, but you get the idea.

Q13. How does the crust of the Earth divide into plates? (Megan)

A: There are really two questions here, a how and a why. One way how you can divide up the crust into plates – to figure out where the plate boundaries are – is to use continuous GPS units to track which direction the ground beneath one station is moving with respect to the ground beneath another station. If the distance between two stations doesn't change, then to first order they are on the same plate. If they do change, you either have different tectonic plates (a boundary between the stations), or a volcano between the stations is about to erupt. In the case of two relatively close GPS stations on a volcano, if they are moving away from each other, then the volcano is inflating. How fast do plates move with respect to each other? The Kamchatka Peninsula of east Russia is moving about 8 cm/year eastward. The North American continent is moving roughly westward at about 2.5 cm/year. The Caribbean is moving about 2 cm/year with respect to South America. These are movements that are easily detectable with modern GPS technology.

As to why the crust is divided up, try closely watching cream of wheat cooking. Watch the surface of the goop in the sauce-pan... and notice that when there is heat applied below it, the surface moves in complex ways. The surface is responding to convection (one form of heat transfer), which moves heat (and with it material) from the bottom of the pan to the top surface, displacing material already at the surface in complex ways. This is continental drift writ small.

Q: We appreciate your time answering these questions, and we are looking forward to any replies!

A: It's my pleasure. Helping young minds grow may be the most important thing that you and I can accomplish in any given day. However, you get to do it all day, 5 days a week.

Reply (next day): Thank you! My students will be thrilled to hear these answers! :)

- Brianne

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