Skoool Kerala

The Rock Record (Higher)

Types of Rocks

There are three main types of rocks.

1. Igneous rocks are formed when molten magma cools and solidifies.

If the magma cools quickly, as following a volcanic eruption, the rock formed contains small crystals.

If it cools more slowly below the Earth's surface, larger crystals are formed.

2. Sedimentary rocks are formed from small particles of other rocks, produced by the weathering and erosion of older rocks.

The particles are transported in streams to the sea where they form layers of sediments.

Over millions of years these are compressed together to form sedimentary rock.

Younger sedimentary rocks are usually found at the surface of the Earth on top of older rocks.

The sediments contain evidence about how they were deposited.

Defined layers show the particles weren't deposited continuously, and ripple marks were formed by currents or waves affecting the sedimentation.

Sedimentary rock layers are often folded, tilted or fractured (faulted) and are sometimes turned upside down. This is evidence that the Earth's crust has been subjected to very large forces, causing it to move and sometimes crack.

Large-scale movements of the crust can cause mountain ranges to form over millions of years. These replace older mountain ranges that have been worn down by weathering and erosion.

Sedimentary rocks often contain fossils of plants and animals that were alive when the sediment was laid down and their bodies got covered and trapped in the rock.

3. Metamorphic rocks are formed when other types of rock get moved deep within the Earth, where the high temperature and pressure cause them to change into hard rocks with bands of interlocking crystals.

Although they are formed deep within the Earth, they can appear at the surface due to movements of the tectonic plates which make up the Earth's crust.