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Sedimentary Rock Classification

Sedimentary rocks are classified by how they formed and by grain size.

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Blue Eye of Africa

Blue Eye of Africa

Credit: NASA
Source: http://en.wikipedia.org/wiki/File:ASTER_Richat.jpg
License: CC BY-NC 3.0

The Richat Structure from space looks like a giant blue eye in the Sahara Desert. This image has a 6-fold vertical exaggeration and some false color to better show the structure.

Amazing But True!

  • Scientists first thought that the Richat structure was a meteorite impact crater.
  • Scientists now say that the structure is a dome, like a circular anticline.
  • Sedimentary rock layers with ages from late Precambrian to Ordovician make up the structure.
  • The rocks dip outward at 10o to 20o.
  • The rock layers that stick up are quartzite. The center is siliceous breccia.
  • Igneous intrusive and extrusive rocks are found in the interior of the structure. A large igneous intrusion is thought to have uplifted the rocks.

Explore More

With the links below, learn more about the Richat Structure. Then answer the following questions.

  1. How would scientists rule out the possibility that the Richat structure is a meteorite impact crater?
  2. What evidence would geologists look for that suggests that the feature is a dome?
  3. How is it possible to tell a circular anticline from a circular syncline?
  4. From the outside to the inside of the circle, where are the oldest rocks and where are the youngest rocks?
  5. How easy or hard is it to erode quartzite? How about the other types of sedimentary rocks found in the structure?

Image Attributions

  1. [1]^ Credit: NASA; Source: http://en.wikipedia.org/wiki/File:ASTER_Richat.jpg; License: CC BY-NC 3.0


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