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Material Type: Notes; Class: Physical Geology; Subject: Geosciences; University: Penn State - Main Campus; Term: Unknown 1989;
Typology: Study notes
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Stress and Strain Stress is the force acting per unit area. The same force acting over a smaller area will give a far higher stress. (a stiletto heal dents flooring whereas a normal sole of a shoe will not) Materials respond to stresses rather than forces There are different kinds of stress.
Strain is the change in shape of an object in response to a stress. The rate of straining in the Earth is very slow in general with deformation rates slower than 1mm per year. On a continental scale:
Question: What controls deformation? Elastic and brittle deformation are largely independently of temperature and the composition of the rock. Brittle deformation is affected more by the pressure due to the overlying rock mass, so that the deeper a rock is buried, the more difficult it is to crack and fracture and the higher its brittle strength is. Plastic deformation is very temperature sensitive (the higher the temperature, the weaker the rock) and is quite insensitive to pressure (depth). Because rocks have very low brittle strengths near the Earth's surface and the temperatures are too low for plastic deformation to occur, the rocks near the Earth's surface will deform in a brittle way. Below a certain depth (about 15 km) the brittle strength gets so high and the temperature gets warm enough so that plastic deformation processes can occur. In a very simple minded model of deformation:
Field Structural Geology An outcrop is an exposure of rock at the Earth's surface that is not covered by vegetation or soil Measurements made by a geologist at a series of outcrops in an area are put together to form a geological map. The information that the geologist takes from the outcrop includes the attitude of the bedding at the outcrop, given by Strike - the compass direction of a line formed by the intersection of an inclined plane (such as a bedding plane) with a horizontal plane. Dip - a vertical angle measured downward from the horizontal plane to an inclined plane
Reverse faults are compressional faults Thrust faults are special cases of reverse faults where the dip of the fault plane is at a low angle to the horizontal. Strike-slip faults are shear faults, where the relative motion of the two sides of the fault may be left- lateral (looking across the fault the block opposite moves to the left) or right-lateral.
Transform faults : Occur at the boundaries between plates. Horst and Graben Commonly, you find two or more normal faults with parallel strikes, but opposite dips enclosing an upthrust or down-dropped segment of the crust. The down-dropped block is a graben or rift if it is bounded by two normal faults. The mid-Atlantic rift is a graben, as is the East Africa Rift and the Rio Grande rift. An upthrust block is called a horst.
