Many types of rocks can be found on earth, but all types of rocks can be divided into three groups. These are igneous rocks, sedimentary rocks and metamorphic rocks. These all have their own specific properties and formation conditions. For more information about these types of rocks, see the article below.
Three groups of rocks
In geography, rocks are generally divided into three groups. The first group are the igneous rocks, these are formed during the cooling and solidification of magma. There are also the sedimentary rocks, these arose because weathering material or remains of plants and animals have accumulated and subsided in the ground. Finally, there is the third group, the metamorphic rocks. These rocks can form when igneous or sedimentary rocks sink very deep into the earth’s crust. The details of the specific rock groups will be discussed below.
Igneous rock
As mentioned earlier, igneous rocks were formed during the cooling and solidification of magma. Igneous rocks can be further divided into volcanic rocks and depth rocks.
Volcanic rocks
Volcanic rock, also called effluent rock, is formed on the surface of a volcano. During the formation of valcanic rocks, the possibilities for crystal formation are very limited, so little or no crystals will occur in this type of rock.
Deep rocks
Deep rocks form when magma solidifies at great depth. In this type of rock crystals do have the chance to form. This is because the rate of solidification is much slower than with outgrowth rock. Crystals can therefore be found in deep rock.
Examples of igneous rock
- Volcanic glass (pumice stone)
- Volcanic ash (tuff)
- Andesite
- Basalt
- Granite
Sedimentary rocks
Sedimentary rocks form when weathering material or organic residues accumulate and sink into the ground. These will come under great pressure from the earth’s crust, and thus be compressed into sedimentary rock. Sediment will usually be transported by water, so the composition of the sedimentary rock will be relatively homogeneous.
Examples of sedimentary rocks
- Sandstone
- Shale
- Lignite
- Limestone
- Conglomerate
Metamorphic rock
Metamorphic rocks are rocks that form under rare circumstances. These rocks are only formed when igneous or sedimentary rocks sink very deep into the earth’s crust. Due to the high pressure and temperature, the igneous and sedimentary rocks will transform into a new type of rock, without melting. These processes are difficult to investigate, as they cannot or can hardly be simulated in the laboratory. This is because the “transformation processes” in reality often take millions of years.
Examples of metamorphic rocks
- Coal
- Quartzite
- Slate
- Schist
- Gneiss