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How are rocks formed?

Bare rock in the mountainsRocks are the hard materials that make up the Earth’s crust. They lie beneath the soil, beneath city streets and beneath ocean waters. You can see them in cliffs along the seashore, mountaintops or in quarries. Rocks are, themselves, made up from a solid mixture of chemical substances called minerals. There are three major types of rock: igneous rock, metamorphic rock and sedimentary rock.



The Giant's Causeway, in Northern Ireland, is made up of about 40,000 columns of basalt, an igneous rock. They were formed during...Read More >>The Giant's Causeway, in Northern Ireland, is made up of about 40,000 columns of basalt, an igneous rock. They were formed during an ancient volcanic eruption. The lava cooled rapidly, causing the rock to form columns. Most of them are hexagonal (six-sided) in shape.

Igneous rocks

Igneous (or volcanic) rocks, such as granite and basalt, result from the cooling of magma, molten rock from beneath the Earth’s crust. Magma may rise through the crust but not erupt through volcanoes. Instead, it cools to form solid rock under ground. This is how granite is formed. When magma does reach the Earth's surface, it erupts as lava. This cools and turns hard, forming basalt.
Volcanic dykes in the Wrangell Mountains, Alaska. Dykes are made of magma (molten rock) that never erupted. Instead, it filled...Read More >>Volcanic dykes in the Wrangell Mountains, Alaska. Dykes are made of magma (molten rock) that never erupted. Instead, it filled cracks in the rocks under the ground, cooled and turned into wedges of solid igneous rock: granite. Over millions of years, the other rocks around the granite became worn away, leaving shards of granite sticking up above ground.

Metamorphic rocks

The oldest rocks on Earth are found in northern Canada. There, gneiss, a metamorphic rock, has been dated to more than 4 billion years old.

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