[ROCK-5] Now that you are somewhat acquainted with all three of the major rock groups and plate tectonics, it is important to consider how these rocks types are distributed at the Earth's surface. As you can see, the bulk of the crust and mantle consist of igneous rocks. However, at the Earth's surface, these are largely covered by the broad (but very thin) layers of sedimentary rocks (or sediments) of the continental platforms and margins, as well as in the ocean basins (see diagram on following page). Accordingly, what we see on the land surface is mainly (75%) sedimentary rock. In some areas though, especially ancient mountain belts and the continental shields, time and erosion have removed the thin veneer of sedimentary rocks, exposing the ancient cores of igneous and metamorphic rocks. Why are there metamorphic rocks in the orogens and shield you ask? Because, as you'll learn later, both mountain building and continent construction produce lots of heat and pressure.


Clay Harris cdharris@mtsu.edu

Published by Lotus® Freelance Graphics®