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The largest group of minerals is silicates which are igneous rocks. They originate from the centre of the earth and reach the surface via volcanoes. Minerals can be classified as intrusive (forming inside the earth) or extrusive (the result of magma cooling outside the volcano). The latter includes pyroclastics which are released into the atmosphere.
Eruptions occur when magma from the mantle (deep inside the earth) reaches the earth's surface. This occurs through gaps in tectonic plates. These are plate like blocks of earth which cover the earths surface like paving slabs. Volcanic eruptions take place when these plates move against each other.
Eruptions may be mild e.g. Hawaii or violent e.g. Krakatoa. The volcanoes shape depends on the chemical composition of the lava i.e. the amount of silica contained and this determines the mineral type.
Basalt................low in silica
Andesite.............intermediate
Rhyolite..............high in silica
Therefore texture and grain size indicate how and where rocks cool. Fast cooling results in no crystal formation which is evident in minerals such as obsidian. Slow cooling results in the formation of large crystals.
Igneous rocks tend to have a very simple chemical composition and have very fine interlocking grains. Basalt is generally the most common.
