Igneous Environment

Molten rocks on the surface and below

Eruption of Mount Merapi
Image: Robert Francis
Source: Photolibrary

Spectacular volcanic eruptions show that rocks can get hot enough to melt deep in the Earth. Molten rock is called magma; rocks formed from magma are called igneous rocks.

When magma erupts to the Earth's surface, it is known as lava. The rocks that form from cooled lava, such as basalt, are called volcanic. Volcanoes are named after Vulcan, the Roman god of fire.

Most magma solidifies slowly below the Earth’s surface. Rocks formed in this way, such as granite, are referred to as plutonic. As magma cools, crystals form within the rock. The size of crystals in rocks depends mainly on the rate of cooling, so slowly cooling plutonic rocks have larger crystals than volcanic rocks, which cool much faster.