By:Peter Russell
In this view of the rock cycle the starting materials are in the centre of the diagram these form the sedimentary and igneous rocks in the first circle which in turn form the metamorphic rocks in the outer circle.
Igneous rocks
Igneous rocks all come from magma - molten mixtures of minerals often rich in gases, deep below the surface of the earth. If they cool while still underground they are called intrusive. When they come to the surface, as in volcanoes they are extrusive. Igneous rocks can also be divided roughly by colour. If the rocks are light both in colour and weight they are called acid rocks and are composed primarily of quartz and potash feldspar minerals and others in small quantities. If the rocks are dark they are called basic rocks and are primarily composed of ferrous (iron) and magnesium minerals and others in small quantities.
Granite is the best known of the intrusive, acidic rocks. It is used in buildings, monuments and gravestones.
Pegmatite is an intrusive, acidic rock with very coarse grain. It is mined for mica, feldspar or for gems.
Diorite is an intrusive, basic rock that looks like granite, but darker in colour.
Rhyolite is an extrusive, acidic rock, with much the same composition as granite. It is a fine grain rock.
Basalt is also an extrusive, basic rock which forms as a heavy, fine-grained, dark lava.
Sedimentary rocks
Sedimentary rocks result from erosion of original rock material - this can be igneous, metamorphic or even other sedimentary rocks. Running water, wind, ice and gravity can move material on the Earth's surface and are key erosive forces. Loose mud or sand is usually referred to as sediment, while hardened materials are called sedimentary rocks.
Sandstone consists of sand that has been cemented to form a rock.
Shale is a clay that has been hardened.
Limestone consists mainly of calcite which has been concentrated by plants, animals or simply chemically.
Coal and Oil are produced by biologic activity or organic decay.
Metamorphic rocks
Metamorphic rocks are also made from other rocks which, due to heat or pressure, have been folded, squeezed and hardened.
Marble - metamorphosed limestone
Slate - metamorphosed clay
Quartzite - metamorphosed sandstone
Two key factors in making metamorphic rocks are pressure and temperature. With increasing depth, pressure and temperature increase and the amount of metamorphism increases.
The forces developed in the earth can even bend rock. This also tends to heat the rocks up, producing new minerals, and as long as the rock doesn't melt it will bend and fold into interesting shapes. Such forces are active at boundaries of tectonic plates and the sites of new mountains in the making.