Types of Rock

There are three main types of rock. These are sedimentary,  igneous and metamorphic. Rock types depend on how the rock was formed.

Igneous rocks are formed when molten rock from the Earth’s mantle cools and hardens. There are two types of Igneous rock.

The first is intrusive igneous rock. This is formed by magma, below the surface of the earth, slowly cooling and solidifying. Large, coarse crystals are formed. Large domes of cooled, igneous rock beneath the surface of the earth are known as batholiths. Where magma has flowed through vertical cracks, called joints, in bedrock it forms dykes and in horizontal gaps, it forms sills. Granite is an example of an intrusive igneous rock.

Intrusive Igneous features – Source Wikipedia

The second is extrusive igneous rock. This rock is formed when magma reaches the surface of the earth as lava, cools and solidifies. The lava cools much quicker which results in the formation of smaller crystals and therefore a fine texture. Basalt is an example of an extrusive igneous rock.

Sedimentary rock is formed when layers of sediment become compact to form solid rock. The process of the sediment becoming compact is known as lithification. Carboniferous limestone is an example of a sedimentary rock. This is formed from the remains of sea creatures and shells compacted over time on the seabed. Sedimentary rock can often contain fossils. Clays and shales are also examples of sedimentary rock.

Metamorphic rock is formed when other rocks, such as sedimentary or igneous rock, are changed under heat and pressure. This can occur when the Earth’s plates collide and the intense heat and pressure transform rock into metamorphic rock. It can also form when magma within the Earth heats the crust forming metamorphic rock. Under these processes, limestone becomes marble and clay become slate.

The Rock Cycle

Rocks have a habit of recycling themselves. The rock cycle illustrates how sedimentary, igneous and metamorphic rocks are formed and how one type can change into another. This is shown in the diagram below.

The Rock Cycle