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The idea of the rock cycle is used by geologists to show that different types of rock are related by geological processes. It is an abstraction rather than a description of the way any particular rock will change. It is a more powerful concept than a classification as a way of discussing differences between rock types precisely because there is an element of process explanation.
Igneous rocks are those which form by the cooling of magma: molten rock originating in the mantle. The nature of igneous rocks depends on their chemical composition and on the speed at which they cool. Chemical composition varies from acid to basic, and the rocks from granitic, through intermediate to basaltic. Some igneous rocks reach the surface, where they form volcanoes, others do not reach the surface but solidify at depth. Slow cooling at depth allows large crystals to form and may give time for selective crystallization and/or mobility, which means that different parts of the cooled mass will have different compositions. Granite bathyliths are the extreme form of this. Where molten magma reaches the surface it cools quickly and cannot be chemically separated, but the form of the volcano depends on the chemical composition: basalt is extremely runny and forms wide horizontal lava flows, while acid lava is more sticky and forms more conical volcanoes. Acid lavas are more prone to explosive eruption, and can throw large boulders and ash high in the air and across substantial distances. So volcanoes may vary in shape and in the degree of consolidation of the deposits.
When any rock is exposed at the surface, it will be subject to heating and cooling and to wetting and drying, and so to physical and chemical weathering processes which weaken it and sometimes wash away some components. Weathered rock may then be eroded, or broken down into particles or into solution, and can then be transported down slopes or by rivers or glaciers. At some point, usually when slopes lessen or streams flow into lakes or sea, rock particles are deposited, commonly as sand or clay, more rarely as a chemical precipitate. In some circumstances, especially in a shallow sea in a geosyncline, sediments can build up to depths of thousands of metres. Its own weight then consolidates it into sedimentary rocks. Often, the processes of mountain building will raise sedimentary rocks to high altitudes and expose them to weathering and erosion, so making another loop around that part of the cycle.
In other cases, sedimentary rocks can be deeply buried, subjecting them to further heating and pressure. Sometimes, they can be exposed to further heating by igneous intrusions and be greatly altered in physical form. So sandstone can be turned into quartzite or limestone into marble. Rocks whose character has been changed in this way are called metamorphic rocks.
If heating is extreme, igneous, sedimentary or metamorphic rocks may be remelted, only to cool and solidify as igneous rocks. So the cycle is closed, though the new igneous rock may be different from those which first formed. As explained under composition and structure of the Earth, the first igneous rocks were basaltic while the continents were built up of intermediate, andesitic rock and eventually formed acid granitic rocks. This process required the operation of plate tectonics over thousands of millions of years. PS |
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