Tuesday, February 5, 2013

Detective work

Geologists are like detectives. We are trying to piece together the story of how and when the rocks formed and how and when they have moved. Every rock has a story to tell about the environment when it formed. The rocks in these mountains also tell the history of motion on faults underground.

In answer to Dayleni Grullon’s question, one of the rocks we study in this part of India is limestone. You may remember that I passed a piece of it around the classroom back in September. The limestone is made from the shells of creatures that lived in shallow ocean water. We can figure out when the limestone formed because it has fossils of animals that went extinct. So the rock told us that there used to be an ocean here X million years ago, before the mountains formed!

Next I try to figure out what story rocks can tell us about plate tectonics. I use my special compass to measure how the sedimentary rocks have tilted and I make a map. My map tells us which way the rocks have folded and that tells us which way they’re being pushed by plate tectonics.

Limestone is also really cool because it can be dissolved and form caves. We found and explored a cave in India! We found stalagtites that have been tilted, and my friend Chris is going to measure how old they are so we can estimate how fast they are being tilted. He’s also going to measure the age of broken stalagtites to try to figure out when ancient earthquakes happened so we can estimate the repeat time of earthquakes here. The cave was a little bit scary – I saw a HUGE spider! (that's Professor Seeber's hand. He's braver -- or perhaps more foolish? -- than I)

Luis Tapia asked how rocks are folded, which is a really great question. Sedimentary rocks are formed in horizontal (flat) layers. When plate tectonic forces push on rocks, they bend and break. When rocks bend the layers become curved and that’s what we call a fold. When the rocks break a fault is formed and that’s where earthquakes happen.

Here's a dramatic photo of folded rock I borrowed from someone else's geology blog so that you can see a really clear example. (http://www.itsasickness.com/geology/content/folded-rock)

I really enjoyed reading all your questions and comments, thanks for all the great ideas for new blog posts! Here's a brief food update:
I was served rice in a banana leaf last week, and yesterday we got to have pineapple for dessert!

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