The symbols used to represent rocks has been somewhat standardized through time. Three different symbols are used to represent igneous rocks. Some texts use these symbols to differentiate between intrusive and extrusive igneous bodies; however, no such distinction will be made here. Remember that a rock is a mappable unit of the earth's crust, one similar unit composing the earth's crust. So, these three different symbols can be used to represent different igneous BODIES or different igneous rocks.

The symbols for sedimentary rocks are related to the textures of the clastic rocks and properties of the chemical/organic divistion rocks. Limestone typically breaks or has JOINTS, and coal is dark in color.

The symbol for metamorphic rock represents the foliated texture, although this symbol is used for all metamorphic rocks. Two types of metamorphism are differentiated. These are CONTACT metamorphism and REGIONAL metamorphism. Contact metamorphism occurs when hot, molten rock penetrates into a crack or break in the rock and BAKES the surrounding rock as the heat penetrates through the surrounding rock material. This heat alters the rock to make it metamorphic. Remember that metamorphism occurs WITHOUT melting. The second form of metamorphism is REGIONAL metamorphism. As rock becomes buried under overlying rock, the weight and pressure from the overlying rock as well as the heat from within the earth act to change these deeply buried rock. Regional metamorphism then occurs deep in the earth and involves large masses of rock material. Although one symbol is usually used to represent metamorphic rocks, individual rock types are often evident from the relationship between the unchanged rock and the changed rock. You might want to review metamorphic rocks and what the type of rock from which each was derived.

The illustration below shows the most commonly used symbols to represent rocks on geologic maps and diagrams. You will need to know these symbols.

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