Friday, December 27, 2013

Sinkholes and Plate Tectonics


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


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:

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.

Sunday, December 22, 2013

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

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.