What is a rock?
A rock is a solid collection of minerals. There are three main types of rock, classified by how they are sourced and formed – sedimentary, igneous and metamorphic.
Sedimentary rocks are recycled rocks formed by the deposition of fragments of material which have been eroded and weathered from other parent rocks. They often consist of sand, pebbles, minerals and mud either removed from the land by erosion, carried by rivers or blown by the wind and then eventually deposited. Sediments are usually deposited in seas and lakes but they can also accumulate in desert environments. They are often deposited in layers, known as strata. As the layers accumulate one on top of another they become buried underneath younger sediments and they become compressed into solid rock.
Some sedimentary rocks are made from deposits on the seafloor that consist almost entirely of the shell fragments of dead sea creatures. The shells are made largely of calcium carbonate and when they are compressed they form a rock called limestone.
In other cases, the accumulation of large amounts of dead plant material may, over millions of years, turn into coal which is another type of sedimentary rock.
Igneous rocks are formed by the cooling of very hot molten material called magma which originates deep inside the Earth. There are two categories of igneous rock – extrusive and intrusive.
Magma rises towards the surface of the Earth because it is hotter and lighter than the surrounding rock. It can reach the Earth’s surface through a vent or fissure at which stage it becomes known as lava. Such an event is known as an extrusion. Due to exposure to air and water it cools quickly and because of this the crystals which make it up are usually very small and this is described as fine grained.
Alternatively, the magma may not reach the surface of the earth and this leads to it cooling very slowly deep below the surface—maybe over thousands or millions of years. This type of event is known as an intrusion. The slow cooling process allows the crystals to become quite large and easily seen. One such rock is granite, it is described as being coarse grained is usually light in colour with clearly visible mineral crystals.
While granite intrusions did originally cool slowly at depth, uplift, erosion and weathering over millions of years mean that they can be seen on the surface of the Earth today.
Metamorphic rocks were originally either a sedimentary rock, an igneous rock or even a metamorphic rock. Metamorphism means literally ‘change in form’. Rocks that are deeply buried in the ground can become subject to very high temperatures and intense pressures. Such environments, throughout processes that last millions of years, change the rocks through physical or chemical changes. Sometimes, already buried rocks may come in contact with hot igneous material and without becoming molten themselves, they become ‘baked’ or ‘cooked’; this is known as ‘contact metamorphism’.
When limestone is subjected to metamorphism, it becomes harder and more crystalline and changes into a marble. When a shale rock is subjected to metamorphism, it may change into a slate.
What are minerals?
A mineral is a naturally occurring substance with distinctive chemical and physical properties, composition and atomic structure.
Rocks are in general made up of two of more minerals, mixed up through geological processes. For example granite is an igneous rock mostly made from different proportions of the minerals quartz, feldspar and mica, as interlocked crystals. A sandstone is a sedimentary rock that can also contain quartz, feldspar and mica minerals, but as grains compacted and cemented into each other.
The definition of an economic mineral is broader, and includes minerals, metals, rocks (i.e. building stone and aggregates) and hydrocarbons (solid like coal and liquid like petroleum) that are extracted from the Earth by quarrying, mining and pumping. Economic minerals are used in a wide range of applications related to construction, manufacturing, agriculture and energy supply. Visit the BGS MineralsUK website for further information about economic minerals.