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The breakdown of Granite

Granite, the typical example of plutonic igneous rock, is well suited for illustrating what happens when such rocks disintegrate to form soil. It represents the starting point for the vast deposits of sandstone, limestone, and shale that cover the larger part of the earth's land surface, for all the soil that was derived form these rocks and now lies as a mantle on top of them, and for all the salts and sediment in the sea.

Granite is made up of a mixture of minerals comprised of about 60% orthoclase and placioglase feldspars, about 25% quartz and about 5% darker minerals of which biotite and horneblende are examples. Each of these minerals will be considered in turn when demonstration how grante is reduced to clay.

In the end the calcium and magnesium that was released in this process was joined to the air-derived carbonic acid in sea water to form the vast deposits of calcium and magnesium carbonates that were laid down in the deep sea and later consolidated into limestone rock. Marine organisms also removed dissolved calcium carbonate from seawater to construct their shells, which later became a part of the limestone as fossils.

In the end, rocks which started life as granite and related igneous rocks, becomes a soil consisting of coarse quartz-sand particles, finely divided silicate clays, and intermediate sized silt particles, plus soluble salts.


This drawing shows how an initial granite rock becomes particles of soil.

Go to a listing of Rocks by mineral group


The Breakdown of Granite

Granite, the typical example of plutonic igneous rock, is well suited for illustrating what happens when such rocks disintegrate to form soil. It represents the starting point for the vast deposits of sandstone, limestone, and shale that cover the larger part of the earth's land surface, for all the soil that was derived from these rocks and now lies as a mantle on top of them, and for all the salts and sediment in the sea.

Granite is made up of a mixture of minerals comprised of about 60% orthoclase and placioglase feldspars, about 25% quartz and about 5% darker minerals of which biotite and horneblende are examples. Each of these minerals will be considered in turn when we explain how granite is reduced to clay.

In the end the calcium and magnesium that was released in this process was joined to the air-derived carbonic acid in sea water to form the vast deposits of calcium and magnesium carbonates that were laid down in the deep sea and later consolidated into limestone rock. Marine organisms also removed dissolved calcium carbonate from seawater to construct their shells, which later became a part of the limestone as fossils.

In the end, rocks which started life as granite and related igneous rocks, became a soil consisting of coarse quartz-sand particles, finely divided silicate clays, and intermediate sized silt particles, plus soluble salts.


This drawing shows how an initial granite rock became particles of soil.

Go to a listing of Rocks by mineral group


The major types of minerals

Not applicable at this reading level.


This drawing shows how an initial granite rock becomes particles of soil.

Go to a listing of Rocks by mineral group



Last modified January 15, 1998 by the Windows Team

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