Further Reading

Darwin, Charles. The Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. 6th ed., vii, edited by C. and W. Irvine. New York: Frederick Ungar Co., 1956.

-. Voyage of a Naturalist, or Journal of Researches into the Natural History and Geology of the Countries Visited during the Voyage of HMS Beagle Round the World, under the Command of Capt. Fitz Roy, R. N. New York: Harper and Brothers, 1846. -. The Variation of Animals and Plants under Domestication. London: John Murray, 1868.

-. The Descent of Man, and Selection in Relation to

Sex. London: John Murray, 1871. -. The Expression of the Emotions in Man and Animals. London: John Murray, 1872.

deformation Of rocks Deformation of rocks is measured by three components: strain, rotation, and translation. Strain measures the change in shape and size of a rock, rotation measures the change in orientation of a reference frame in the rock, and translation measures how far the reference frame has moved between the initial and final states of deformation.

The movement of the lithospheric plates causes rocks to deform, creating mountain belts and great fault systems like the San Andreas. The terms strain and stress describe how rocks are deformed. Stress, a measure of force per unit area, is a property that has directions of maximum, minimum, and intermediate values. Strain describes the changes in the shape and size of an object, and it is a result of stress.

There are three basic ways by which a solid can deform. The first is by elastic deformation, which is a reversible deformation exemplified by a stretching rubber band or the rocks next to a fault that bend and then suddenly snap back to place during an earthquake. Most rocks can undergo only a small amount of elastic deformation before they suffer permanent irreversible, nonelastic strain. Elastic deformation obeys Hooke's law, which simply states that for elastic deformation, a plot of stress versus strain yields a straight line. In other words strain is linearly proportional to the applied stress. So for elastic deformation, the stressed solid returns to its original size and shape after removal of the stress.

Solids may deform through fracturing and grinding processes during brittle failure or by flowing

San Andreas Fault crossing Carrizo Plain in California

(Bernhard Edmaier/Photo Researchers, Inc.)

during ductile deformation processes. Fractures form when solids are strained beyond the elastic limit and the rock breaks; they are permanent, or irreversible, strains. Ductile deformation is also irreversible, but the rock changes shape by flowing, much like toothpaste squeezed out of a tube.

When compressed, rocks first experience elastic deformation, then as the stress increases they hit the yield point, at which ductile flow begins, and eventually the rock may rupture. Many variables determine why some rocks deform by brittle failure and others by ductile deformation. These variables include temperature, pressure, time, strain rate, and composition. The higher the temperature of the rock during deformation, the weaker and less brittle the rock will be. High temperature therefore favors ductile deformation mechanisms. high pressures increase the strength of the rock, leading to a loss of brittle-ness, and therefore hinder fracture formation. Time is also an important factor determining which type of deformation mechanism may operate. Fast deformation favors the formation of brittle structures, whereas slow deformation favors ductile deformation mechanisms. strain rate is a measure of how much deformation (strain) occurs over a given time. slow deformation rates favor ductile deformation, whereas fast deformation rates favor brittle deformation. Finally, the composition of the rock is also important in determining what type of deformation will occur. some minerals (like quartz) are relatively strong, whereas others (such as calcite) are weak. strong minerals or rocks may deform by brittle mechanisms under the same (pressure, temperature) conditions that weak minerals or rocks deform by ductile flow. Water reduces the strength of virtually all minerals and rocks; therefore the presence of even a small amount of water can significantly affect the type of deformation that occurs.

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The Basic Survival Guide

The Basic Survival Guide

Disasters: Why No ones Really 100 Safe. This is common knowledgethat disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

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