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Igneous rocks

The Giant's Causeway, in Northern Ireland, is an area of about 40,000 basalt columns. They were formed during an ancient volcanic...Read More >>The Giant's Causeway, in Northern Ireland, is an area of about 40,000 basalt columns. They were formed during an ancient volcanic eruption. The lava flow cooled rapidly, causing the rock to fracture into columns. Most of the columns are hexagonal in shape. Igneous rocks are formed when magma, molten rock from beneath the Earth’s crust, rises, cools and solidifies. Intrusive igneous rock, such as granite, is formed when magma becomes solid in cracks or chambers beneath the ground. Extrusive igneous rock, such as basalt, is formed when the magma solidifies above the ground quite quickly. This happens when magma seeps out on to the ocean bed, or when runny lava spreads out from a volcano on land over a large area.


Volcanic dykes in the Wrangell Mountains, Alaska

A close-up view of granite rock.

Intrusive igneous rock

Magma bubbles up through the crust wherever one plate sinks down beneath another in subduction zones. Water from the wet sediments on the ocean floor makes the magma more fluid, so it can rise through cracks in the crust above. The magma seeps into the crustal rocks, sometimes collecting in large chambers.

When magma turns to solid rock (black) under the ground, it takes on the shape of the surrounding rock, resulting in ledges,...Read More >>When magma turns to solid rock (black) under the ground, it takes on the shape of the surrounding rock, resulting in ledges, columns, domes and other shapes.There it may stay, eventually cooling down slowly to become a mass of igneous rock. Magma that cools deep underground is called intrusive igneous rock. Granite is an example of intrusive igneous rock.

There are more than 700 types of igneous rock.

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