What can road cuts tell us about the geologic history of an area?
Stratigraphy is the branch of geology that studies rock layers; structure includes the faults and folds that result from regional & local forces acting on the area. A hypothetical cross section is studied by going back to the beginning to study its progressive geologic history. Why study rock relations? The earthquake potential of an area can be determined by studying the geologic history of the rock strata, both locally and regionally. Faults and folds record a probable earthquake history, so by studying the age of the rocks and their deformation we interpret past earthquakes and gain an understanding about the potential for future earthquakes. In this animation, no strata are overturned, thus the following laws apply: The law of superposition simply says that for non-deformed geologic strata, sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest on the top. The principle of original horizontality states that rock layers form in the horizontal position, and any deviations from this horizontal position are due to the rocks being disturbed later (the exception being layers deposited on a slope or thick viscous lava flows). The crosscutting law states that if a rock is cut by another rock, then it is older than the rock which has cut it.
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Interpreting the geologic history includes understanding:
Over most of the last 30 million years, movement of hot mantle beneath the region caused the surface to dome up and then partially collapse under its own weight, as it pulled apart. Currently, there is very little actual stretching going on, and the small amount is concentrated on the Western and Eastern edges of the Basin and Range.
During Basin & Range extension, the plates pull apart, the mantle rises and melts due to lower pressures near the surface. The style of eruption depends on how long the magma sits in the crust and undergoes processes such as crystallization and melting and assimilation of wall rock.
As extension and uplift occur, erosion and sedimentation happen simultaneously but at slower rates. As extension slows down, erosion and sedimentation can overcome mountain building.
The Earth has 3 main layers based on chemical composition: crust, mantle, and core. Other layers are defined by physical characteristics due to pressure and temperature changes. This animation tells how the layers were discovered, what the layers are, and a bit about how the crust differs from the tectonic (lithospheric) plates, a distinction confused by many.
How can I demonstrate plate tectonic principles in the classroom?
Video lecture demonstrates the use of foam faults to demonstrate faults, and a deck of cards to demonstrate folds and fabrics in rock layers. Different types of faults include: normal (extensional) faults; reverse or thrust (compressional) faults; and strike-slip (shearing) faults.
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