North American craton
|
|
The North American craton, like all craton land, was created as continents move about the surface of the Earth, bumping into other continents and drifting away. This process takes millions of years to happen, but the results can be seen today in the rocks of the continents.
The middle of most continents have not been crunched or squished up the way the edges have been. That is because the middle of the continents are made of very strong old rock. This part of the continent is called the stable craton. Indiana is located near the middle of the North American craton.
Interior Platform
While much of the stable craton is exposed at the surface north of Indiana as the Canadian Shield, the middle of the craton, located in the United States, is covered with sedimentary rocks of the Interior Platform. The sequence of rocks varies from approximately 3,500 in excess of 20,000 feet in thickness. The cratonic rocks are metamorphic and igneous while the overlying sedimentary rocks are composed mostly of limestones, sandstones, and shales. These sedimentary rocks were deposited from 650 to 290 million years ago.
Tectonic Setting
The metamorphic and igneous rocks of the "basement complex" were created 1.5 to 1.0 billion years ago in a tectonically active setting. It was a setting of great pressure and temperature. The younger sedimentary rocks that were deposited on top of this basement complex were formed in a setting of quiet marine and river waters. Much of this time the craton was covered by a large shallow sea, a so-called "epicratonic sea" (meaning literally "on" the craton). Sometimes land masses or mountain chains rose up on the distant edges of the craton and then eroded down, shedding their sand across the landscape.
External link
- USGS Interior Plains Region web site (http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Notes/interior_plains_region.html)