GEOL 101 Crustal Deformation
GEOL 101 Crustal Deformation GEOL 101
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This 5 page Class Notes was uploaded by Victoria Williams on Friday April 1, 2016. The Class Notes belongs to GEOL 101 at George Mason University taught by Mark Uhen in Fall 2016. Since its upload, it has received 17 views. For similar materials see Introductory Geology in Geology at George Mason University.
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Date Created: 04/01/16
Crustal Deformation India has been on a collision course with Asia Proper for millions of years, so far there has been about 2,500km of ‘crunching’ of the plates Deformation – term applied to all changes in the size, shape, orientation, or position of a rock mass. Often seen in plate tectonics. Stress o Stress is the amount of force per unit area (causing deformation) o Compression stress is the stress that shortens a rock body (like in India and Asia) o Tensional stress that lengthens a rock body and also thins it o Shear stress is stress that causes layers within a rock body to slip past one another like in a deck of cards, if you push it, the cards will slide out. It’s like that. How rocks perform with different stress o Brittle Compression – snaps and breaks in a crack in which motion occurs (a fault) Tension – fractures rock and pulls it apart Shear – Will fracture apart and slide o Ductile Compression – Layers bend in waves Tension – Rock is stretched thinner Shear – Moving in two directions but doesn’t break apart, sticks together Strain is the change in the shape of a rock body due to stress, researched in structural geology. Deformation of rocks – under stress rocks can flow, fold, or fracture. General Behavior of Rocks o When stress is applied slowly, rocks experience elastic deformation, that is, they will resume their original shape when stress is removed o When the rocks surpass the elastic limit, rocks experience ductile deformation, that is, they flow, or brittle deformation, they fracture. Temperature and Pressure o At a low temperature and pressures, rocks tend to undergo brittle deformation, while at a higher temperature and pressures, they tend to undergo ductile deformation. If you have a rock column under low pressure and low heat with high stress, the rock will fracture (mostly occurs near the surface of the earth). But if the rock is under high pressure and high heat the rock will mold in a ductile way (this mainly happens deeper into the earth) Mapping Geologic Structures o Strike is the compass direction of the line produced by the intersection of a layer with the horizontal surface o Dip is the inclination of the surface measured perpendicular to the strike Folds – diagram in lecture is kind of similar to this: Symmetrical Folds – same on both sides of the axial plane (the center). The dip angles are the same as well. When there is a slope in the middle and then it goes downward, that is called an anticline. But there is also something called a plunging anticline, the ridges are not perpendicular to the axial plane and they meet in a ‘clothes iron’ shape. There is a new factor in this, the angle of plunging anticline is the plunge line. Anticline and Synclines – The opposite of an anticline, these are the dips between ridges. These inclines can be at different angles and sometimes they are asymmetrical. Monoclines are a weirder structure and are rarer. It’s when a fault occurs in layers of rock and below the rocks fractured and the rocks above behaved in a ductile faction, so it just creates a ‘bump’ Domes and Basins o Domes expose the oldest strata at their centers. Basins expose the oldest strata at their margins. When the domes form, they form upwards and they weather away and the one in the middle is the oldest because it is the bottom most layer. The opposite happens for the basin, the rock layers bend downwards and the oldest stuff surfaces near the edges and the youngest is in the middle. Dipslip faults exhibit motion up or down along dip. In a normal fault, the hanging wall falls. In a reverse slip, the hanging wall rises (which isn’t normal) Fault Scarp – the visible part on the surface where you can see the slip Normal Faulting is associated with tensional stress, the footwall that goes ‘up’ often creates higher elevations in mountains and the hanging wall is a valley. Horsts and Grabens are results of normal faults. The ‘horsts’ are the elevated parts of the fault and the grabens ‘fall’ down to make a valley. This happens because the land is being pulled apart. But there are sometimes half grabens, they are not symmetrical like regular horsts and grabens. The half grabens only fall on one side. So it looks like \ \ \ \ Thrust faults occur with reverse faults. With a low angle reverse fault, the earth faults and the thrust fault forms by pushing the wedge of material upwards into an ‘over thrust.’ The older rocks ‘thrust’ over the younger rocks. The older rocks create higher ridges and the younger rocks on the other side make a flatter valley Strikeslip faults exhibit motion along strike (a trend along the surface of the earth in which motion has occurred). If the crack is horizontal and moves in that fashion. A slight valley is usually formed on the fault because of the movements. Transform Fault – a kind of strikeslip fault that is special because they separate two tectonic plates. The fault is not a straight line, there are pieces that are offset from others that are strikeslip faults/transform fault. Occasionally there is one large one of these, like the San Andreas Fault. Joints are fractures along which no displacement has occurred. They are made with igneous rocks that cools in cracked columns that form.