Igneous and Metamorphic Rocks

The three main types of rock are:

  • Sedimentary
  • Igneous
  • Metamorphic

Igneous rocks

Igneous rocks are formed from the solidification of magma or lava. These rocks are made up of interlocking crystals that are randomly arranged. They are also non-porous, meaning that water cannot penetrate the rock.

Some examples of igneous rocks are granite, basalt and obsidian.

Forming igneous rock

Igneous rocks are formed from volcanic activity. Molten rock can come in two forms:

  • Magma is molten rock that is underground
  • Lava is molten rock that has erupted onto the Earth’s surface

Igneous rocks can form from either magma underground or lava above ground.

Forming igneous rocks from magma

Igneous rocks can form from the cooling and solidification of magma underground. These rocks are known as intrusive igneous rocks.

They typically have larger crystals due to the slow cooling process of magma deep below the Earth’s surface. As magma cools at a slower rate, crystals have more time to grow and develop, resulting in larger crystals forming.

Granite is an example of an igneous rock that has large crystals. It is used for construction and also as a decorative stone, as the visible crystals give it a sparkling effect.

Forming igneous rocks from lava

Igneous rocks can form outside a volcano when lava cools, which are called extrusive igneous rocks. Lava that falls in water or on the ground outside the volcano cools quickly to form igneous rock. As the lava cools quickly, there isn’t much time for crystals to form, so the crystals are quite small.

Basalt is an example of an igneous rock that forms from quickly cooling lava, resulting in very small crystals. It is a strong material used in construction (e.g. building blocks or groundwork).

Remember!

Many factors cause igneous rocks to form in different ways. We can categorise igneous rocks as extrusive and intrusive.

IntrusiveExtrusive
How fast the magma or lava coolsSlowFast
Size of crystals formedBigSmall

Metamorphic rocks

Metamorphic rocks form when existing rocks undergo changes due to high heat or pressure. These conditions are present deep underground. Some examples of metamorphic rocks are gneiss, schist and slate.

When sedimentary or igneous rocks are exposed to extreme heat and pressure, their mineral structures may change. This forms metamorphic rock.

For example, when heat and pressure are applied to limestone (a sedimentary rock), it transforms into marble (a metamorphic rock).

This process can take place deep within the Earth. It changes the physical and chemical properties of the rock, which gives it a unique appearance and texture.

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