Rocks
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[edit] Definition
A rock may be defined as a naturally occurring solid composed of one or more minerals or mineraloids.
[edit] Identification of rocks
One key to the identification of rocks is the blend of minerals that compose them. The dividing line between different kinds of rocks is therefore somewhat arbitrary, since in nature there is no hard and fast line between, for example, granite and granodiorite: granite has more quartz and more potassium feldspar; granodiorite contains more plagioclase feldspar and hornblende, and any gradation between them can be found in nature.
Besides the composition of the rocks, we must also consider their structure. Granite and rhyolite, for example, are made up of the same minerals. However, a glance at a lump of granite will show you that in granite the different minerals have formed into large crystals visible to the naked eye, whereas in rhyolite they are too fine to be distinguished. This difference is far from trivial, since it reflects the different histories of the rocks: granite forms from molten rock which solidifies underground, where it cools slowly, thus forming large crystals; rhyolite forms from molten rock on the surface, which cools more quickly, forming smaller crystals.
To take another example, marble is usually composed of calcite, the same mineral from which limestone is formed; but such marble is limestone that has undergone metamorphosis by heating, which changes its texture. So again we have two different rocks reflecting two different histories.
[edit] Classification of rocks
This brings us onto an important point. Geology is nothing if it is not a historical science; merely being able to tell the difference between shale and schist is not in itself science any more than being able to tell a Ford from a Ferrari. The essence of geology is deducing the historical processes that account for the geology that we can observe around us.
For this reason the fundamental groupings of rocks are not according the minerals that compose them, but rather according to the way they are formed, as follows:
- Igneous rocks are those that were produced by the solidification of molten rock. These may be divided into
- Extrusive rocks are igneous rocks formed from molten rock flowing on the surface of the Earth. They are called extrusive rocks because the molten rock has been extruded onto the Earth's surface. Basalt is a typical example of an extrusive rock.
- Intrusive rocks are igneous rocks formed from molten rock beneath the surface. They are so called because they intrude into the Earth's crust. The commonest intrusive rock is granite.
- Sedimentary rocks are those which are formed by sediment which has turned into rock.
These may be further divided into the following classes:
- Detrital sedimentary rocks are those formed from sediments produced by rocks being physically broken down into small particles (detritus). Sandstone, mudstone and siltstone are examples of detrital sedimentary rocks.
- Chemical sedimentary rocks are formed from sediments produced by dissolved chemicals being precipitated out of the water they're dissolved in: for example rock salt.
- Biochemical sedimentary rocks are those formed from sediments composed of dead organisms, or parts of dead organisms; for example, some kinds of limestone prove, on microscopic examination, to be formed from tiny shells.
- Metamorphic rocks are those which have been transformed from their original state by heat and/or pressure.
[edit] Note on vocabulary
The most irritating thing about geology is the excessive number of synonyms in its terminology. One name for one thing is a good rule in science, and we eagerly look forward to the day when the International Union of Geologists will prescribe just one standard set of terms, with severe penalties for those who transgress --- stoning would seem appropriate.
In this text we shall consistently use the terms given above. However, the reader who wishes to read further elsewhere should be aware of the following alternative terms:
- Primary rocks: another name for igneous rocks.
- Volcanic rocks: another name for extrusive rocks. A slightly misleading term, since you don't need a full-blown volcano to produce "volcanic" rocks, you just need any way for the molten rock to get to the surface.
- Plutonic rocks: another name for intrusive rocks. So named after Pluto the Greek god of the underworld.
- Clastic sedimentary rocks: another term for detrital sedimentary rocks.
