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.

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