Geology And Geophysics

Understanding the three Basic Types of Rock



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Rocks work in cycles, going through three basic stages:

SEDIMENTARY: As shellfish and other sea creatures die, their bodies are laid down on the sea bed, and over many years compressed together along with sand and mud and so on into layers on the sea bed. Examples of this are sandstone, mudstone and limestone - they often form sea cliffs, as they are formed by the sea, and (as rocks go) are quite soft and crumbly, so are susceptible to erosion by wind and waves.

METAMORPHIC: Over time, the sedimentary rock is pressed deeper and deeper into the earth's crust by the layers building up on top of it. Over thousands of years, as a result of the intense crushing pressure from above, and the intense heat from the earth's core, the rock is compressed into very hard, smooth rocks. Examples of this are slate, marble and schist. The rocks derive their name from the fact that they "metamorphose" from their sedimentary (or igneous) form into new rocks - limestone becomes marble, and so on.

IGNEOUS: Eventually the rock is pressed so far into the earth's crust that the heat melts it, and it may flow into the magma chamber of a volcano. Then one of two outcomes may follow - either it will be erupted onto the sides of the volcano and cool quickly, or it will remain in the magma chamber and cool very slowly. Either way, the cooling process produces crystalline igneous rock, but rapid cooling will produce small crystals (such as those of basalt), and slow cooling will produce larger crystals (like those of granite).

Igneous rocks may be compressed again into metamorphic rocks, or lie scattered about in the countryside for a few million years. Erosion from rivers may return them in sand form to the sea, where they may become sedimentary rocks again...and repeat the process.

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