Rocks are generally divided into three groups and classified by mineral and chemical composition, by the texture of the constituent particles and by the processes that formed them. These indicators separate rocks into igneous, sedimentary, and metamorphic rocks.
Igneous rocks are formed when molten magma cools and i divided into two main categories: Plutonic rock and volcanic. Plutonic, or intrusive, rocks result when magma cools and crystallizes slowly within the Earth's crust (e.g. Granite), while volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental 'ejecta' (e.g pumice).
Sedimentary rocks are formed by deposition of clastic sediments, organic matter, or chemical precipitates (evaporites), followed by compaction of the particulate matter and cementation during diagenesis. Sedimentary rocks form at or near the Earth's surface. Mud rocks comprise 65 per cent (mudstone, shale and siltstone); sandstones 20 to 25 per cent and carbonate rocks 10 to 15 per cent (limestone and dolostone).
Metamorphic rocks are formed by subjecting any rock type (including previously formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the Earth's surface and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals (e.g. By recrystallisation).
These three classes of rocks are subdivided into many groups. There are, however, no hard and fast boundaries between allied rocks. By an increase or decrease in the proportions of their constituent minerals they pass by every gradation into one another. The distinctive structures of one kind of rock may often be traced gradually merging into those of another. Hence the definitions adopted in establishing rock nomenclature merely correspond to, arguably arbitrary, selected points in a continuously graduated series.
Igneous rocks are formed when molten magma cools and i divided into two main categories: Plutonic rock and volcanic. Plutonic, or intrusive, rocks result when magma cools and crystallizes slowly within the Earth's crust (e.g. Granite), while volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental 'ejecta' (e.g pumice).
Sedimentary rocks are formed by deposition of clastic sediments, organic matter, or chemical precipitates (evaporites), followed by compaction of the particulate matter and cementation during diagenesis. Sedimentary rocks form at or near the Earth's surface. Mud rocks comprise 65 per cent (mudstone, shale and siltstone); sandstones 20 to 25 per cent and carbonate rocks 10 to 15 per cent (limestone and dolostone).
Metamorphic rocks are formed by subjecting any rock type (including previously formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the Earth's surface and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals (e.g. By recrystallisation).
These three classes of rocks are subdivided into many groups. There are, however, no hard and fast boundaries between allied rocks. By an increase or decrease in the proportions of their constituent minerals they pass by every gradation into one another. The distinctive structures of one kind of rock may often be traced gradually merging into those of another. Hence the definitions adopted in establishing rock nomenclature merely correspond to, arguably arbitrary, selected points in a continuously graduated series.
