Sinkholes and Plate Tectonics

Q:

Dear Geologist,

Our names are Liam and Allison and we are sixth grade students Preston Middle School in Fort Collins, Colorado. In Science class our current unit is Constructive and Destructive Forces. This has made us curious. We generated multiple questions that could only be answered by a professional geologist like you. For example, what forms sinkholes? Also, how were tectonic plates discovered? And finally, what do you do for your job? We ask this because we are interested in becoming geologists ourselves when we mature. Thank you for your time.

- Cordially, Liam and Allison



A:

1. Sinkholes usually occur because of dissolution of carbonate rocks. A variant on this is when the carbonate cement in a sandstone is dissolved away. As an experiment, drop a tiny bit of acid on a limestone (a carbonate rock) - or scratch the rock with a knife and pour Coke over it - and it will fizz. Florida and other states have a lot of limestone underlying their surface soils, and if there is even a slight acidity to the groundwater (for instance it filters through a swamp of rotting vegetation first), then it will slowly dissolve the limestone. As a practical matter, the sinkholes generally (not always) form when there is a dry spell. Then the water saturating the damaged rock under a house will drop lower, and without the water saturation, the roof over a solution cavern will more easily collapse.

2. The idea of Tectonic plates was first proposed by Alfred Wegener, a German geophysicist and meteorologist, in 1912, He noticed that the west coast of Africa would make a pretty good fit to the east coast of South America. In the 1960's, aeromagnetic data acquired by aircraft showed distinct, symmetric banding in the mid-Atlantic (paleomagnetism). Isaacs, Oliver, and Sykes in a paper published in 1969 showed that this could only be caused by the growth of the Atlantic floor as it spread apart. Iceland is just an above-water part of this mid-Atlantic ridge spreading center, which may extend over 25,000 kilometers around the Earth. In the 1990's people started using GPS to directly measure the actual motions of the tectonic plates. Where I am sitting right now (Vancouver, WA), the North American continental plate is riding up over the Juan de Fuca oceanic plate at about 2.5 centimeters a year. In Kamchatka, in the Russian Far East, the plate movement is greater than 8 cm/year. Because it's faster there, the Russians have many more large earthquakes and many more active volcanoes than we have here in the United States.

3. To answer what do I DO, you can check out the profile here: https://profile.usgs.gov/jwynn


I look forward to you joining the ranks of geoscientists - we need smart young people like you to move the field ahead. Who knows? Perhaps YOU will discover a way to predict earthquakes.

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Earthquakes only during the day?

According to my calculations, the 6th grade means students are around 11-12 years old. If so, then the Rising Generation is full of people a lot smarter than I was at that age. The example below is just one of many like it:

Q: Dear Geologist,

Our name is Arianah and Cray and we are sixth grade students at Preston Middle School in fort Collins, Colorado. We are currently learning about how the Earth’s surface changes over time. We are curious about earthquakes. We have a couple questions for you. Is there a common time when earthquakes happen during the day? Also, why did you become a geologist?

Yours sincerely, Arianah and Cray :D

A:

1. Earthquakes are essentially random. We understand why they happen, we understand where they happen, but we do NOT understand WHEN they will happen. There are always aftershocks following a main event, of course, but the main event cannot be predicted. Extensive research has shown that there is no correlation between earthquakes and certain times of the day or external* events - for instance there is no correlation with either the location of the Sun, or of the Moon, or with tides (alignments of celestial bodies, which cause neap tides or spring tides, is called syzygy). Some of the brightest minds on this planet have been searching for more than a half century for some evidence that main event earthquakes can be predicted, but without success. They can be forecast#, but not predicted.

2. I was a solid-state physicist and realized that if I didn’t do something drastic, I would be stuck inside a laboratory all my life with radioactive sources and high-pressure cells. This was brought very much to my attention one day when I had a high-pressure cell blow out and spew Cobalt-60 all over the inside of our lab, and had to call in a special Spill Team. Also, by this time physics as a profession was drifting into a dead end with string theory, and I saw relatively little value to humanity to spending billions of dollars to see if another exotic particle existed. I checked out breakoffs of physics, including astrophysics, hydro-geophysics, weather physics, and geophysics, and found the last one to be very exciting. It also got me out into exotic places, like the Venezuelan jungle, the southeastern Alaska panhandle, the Empty Quarter of Saudi Arabia, etc. Geoscience gives me amazing opportunities to visit these places and many more. But even more interesting to me is to be a detective – to be the first to discover something beneath the ground or the seafloor. I was the first to say where the groundwater was beneath the San Pedro Basin in Arizona and Sonora, Mexico, and the first to map where titanium sands lay beneath the seafloor off the coast of South Africa. That’s ever so cool. 

* It has been shown that if you inject fluids into certain formations (e.g., deep sediments northeast of Denver, CO), you can trigger swarms of micro-earthquakes. Basically this is the ground shuddering to equilibrate and adjust itself to a slightly new stress regime. However these sorts of events are so small that they are almost never felt.They really are not earthquakes as the general public understands earthquakes.

# A forecast: in other words, there is an X% chance that there will be a magnitude Y event on the Z fault zone in northern California within the next 30 years. This is very, very different from saying that there will be a Magnitude Y event at Z location on X day - that would be a prediction. We can't do that.

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If there was ever a tornado going on when a volcano is erupting…

Then there are questions that could only come from children who are learning, but haven't learned much, and have no fear about asking questions about things that are scarey/fascinating to them. The following "improbable" is actually not that far from the probable.

Q: Hello my name is Thorin and i have a question for my geography class.

If there was ever a tornado going on when a volcano is erupting, can the tornado, so to speak, pick up the lava erupting from the volcano. I hope you guys respond to my question.  Thanks, - Thorin T

A: Hi Thorin,

Tornadoes don't usually occur where there are volcanoes. Tornadoes generally require pretty flat terrain or ocean (not common in volcano territory) and a complex mix of warm and moist air, with cold and dry air coming from another direction. Again, this is not common in volcano territory like the Pacific Cascades, Kamchatka, or Japan. That said, however, it has been reported that tornadoes can pick up cars and carry them hundreds of meters away - even kilometers - and then drop them. Three tornado-hunter scientists from Denver were killed that way this last May (2013).

So. IF there was an erupting volcano, AND a tornado passed through nearby, it's conceivably possible that a tornado could pick up a bit of lava. However, lava is pretty dense. By comparison a car is much less dense (it has a lot of air space in it). More likely, the tornado would pick up ash and tephra (tephra is cooled lava filled with gas bubbles, so it is not very dense) and throw these around. 

Something like this is not all that unreasonable, actually. There was a tropical cyclone (a Pacific hurricane) passing through the Philippines when Mount Pinatubo erupted in 1991. The effect of the hurricane-force wind, the HUGE amount of water that the cyclone brought with it, and the really huge volcanic eruption made quite a mess of the island of Luzon. It basically destroyed Clark airbase and badly damaged the nearby Subic Bay naval facility - and even though evacuations had been ordered, it still killed a lot of people. 

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Thorin later replied:

Thank you very much Jeff and it answers my question in great detail. Thank you for taking your time to answer my question and give me a very helpful answer. 

~~~~~

What states are safe from earthquakes?

Where are you safe? Really nowhere. If you live in the Pacific Northwest, tornadoes are extremely rare, and hurricanes non-existent - but you are at risk of a large subduction earthquake and floods. If you live in the Southeast, you generally don't have to worry about earthquakes, but you are certainly living in Hurricane Alley. When I first took up my 5-year assignment to serve as chief scientist for volcano hazards in Vancouver, WA, in 2002, I was frequently asked the question: will we have a big eruption soon? I told everyone that I had voted with my feet, made an informed decision, and bought a house in Clark County. Two years later, of course, Mount St Helens erupted. While it briefly threatened the Johnston Ridge Observatory north of it, it was never really a threat to people in Clark County... though an ash-plume erupted on March 5, 2005 did apparently intersect an aircraft flight-path near Roseburg, WA. At least three Boeing 747 aircraft have lost all or almost all of their engines when they flew through a volcanic ash cloud. The aircraft flying over Roseburg was diverted to a nearby airport, thoroughly checked out, and found to be OK however.

Q: I saw on your website that 39 states are endanger of earthquakes, what 11 are not and why?

 Kaitlyn S, 8th grade Endeavor Charter School student

A: Hi, Kaitlyn,

If you go to this web-page you will see a map of earthquakes in the United States:

http://earthquake.usgs.gov/earthquakes/states/seismicity/:

You can count 11 states that have no recent seismicity (no red circles) - but this doesn't necessarily mean they are earthquake-free. 

The longer answer to your question is that earthquakes happen where there is crustal movement: mountains rising (Hawaii and Utah, for instance), crustal slabs sliding past each other (California), or continents riding over oceanic plates (most of the West Coast). If you look at this map, you will see states that have no apparent earthquakes. This is because they are in the center of a very old, stable continental crust, and not on the tectonically active margins. However the map of RECENT seismicity could fool you into thinking that Oregon is relatively free of risk. In fact, the same subduction fault off the coast from northern California to British Columbia threatens the western sides of three American states and one Canadian province. This kind of fault may not rupture for centuries at a time. However, when it does break it will be a real attention-getter. 

One thing to keep in mind is that distance from a fault offers increasing protection: Western Washington State is at much greater risk than eastern Washington State, for instance. This is because the energy of the earthquake falls off with distance, just like sound does. Sit on the edge of your bed and have your brother kick the edge next to you. If he then goes around and kicks the other side, it will not feel nearly as uncomfortable to you. 

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Are tsunamis and volcanic eruptions a result of other catastrophic natural disasters?

Catastrophes are often related: a drought in southern California leads to wildfires, and within a year the denuded terrain is damaged further by floods and landslides because the vegetation that preserves and protects the ground surface is missing. A massive earthquake in Haiti leads to a devastating cholera epidemic, because people are displaced and water sources are compromised. The following query is from a thoughtful young man trying to understand some of these relationships.

Q: hello, my name is Brendan and i have a question related to geology in which i would like u to answer. so ya, here it is: Why do tsunamis and volcanic eruptions often act as a result of other catastrophic natural disasters? ya so please respond to this. Oh and btw I am a student at endeavor charter school, just to let you know. alright well thanks for your time and i hope to get a response. – Brendan J

A: Hi, Brendan - I can provide some brief answers.

Tsunamis are caused by SOME volcanic eruptions and by SOME earthquakes:

1. Thera volcano in the eastern Mediterranean erupted catastrophically around 1,500 BC. It triggered a tsunami that destroyed the Minoan civilization based on Crete. The 1883 eruption of Krakatau volcano in Indonesia sent a tsunami into the island of Java that scoured everything within 10 kilometers of the coastline and swept it all back to the sea. Contemporary descriptions said that you could walk across the Sunda Strait because of all the bodies (people, livestock) and logs floating there. MOST volcanic eruptions, however, do NOT cause tsunamis. For one thing, the volcano must be adjacent to an ocean for this to be possible. 

2. The tsunami that destroyed the Fukushima Di-Ichi nuclear powerplant, and destroyed much of the Sendai, Japan, coast, was caused by the Great Tohoku earthquake of 2011. The tsunami happened because a 200 km by 600 km slab of ocean floor was suddenly uplifted several meters by a sudden slip on a subduction fault offshore. The displaced seawater slopped onto the nearby coast, and a security camera showed a 15-meter (nearly 50 foot) wave crashing into the facility, wreaking nearly incomprehensible damage - that is still evolving as I write this. MOST earthquakes, however, do NOT cause tsunamis. There must be an uplift or down-drop of a large piece of ocean floor for this to happen.

Volcanoes are connected indirectly to subduction earthquakes - the Pacific Ring of Fire is an example of how they are related. A down-going section of oceanic crust, being over-ridden by a lighter-density continental crust (Sendai, Japan, and the coast of Washington State in the US, for instance), gives rise to volcanoes. The oceanic crust is heated up as it works its way deeper and deeper into the hot Mantle, and fluids in that down-going slab of oceanic crust contribute to partial melting. This is where lighter component elements of the oceanic floor float up until they burst through the overlying continental crust. Think of a lava lamp: it's the same general principle. Just offshore of North America, Kamchatka, Japan, the Philippines, and Indonesia (and many other places) there is a subduction fault where a continental section is riding over an oceanic crust. Just INLAND from these subduction faults you will find chains of volcanoes paralleling the same coastal margin - the Cascades Range extending from California to British Columbia is an example. One may not directly cause the other, but they are certainly related nevertheless. 

An earthquake was closely associated with to the 1980 eruption of Mount St Helens, but to this day geologists still argue about it. Did the earthquake trigger the eruption of the highly-unstable volcano? Or did the eruption cause the largest earthquake in recorded human history? 

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