What are Igneous Rocks?

How Do Igneous Rocks Form?

Igneous rocks form when hot molten rock (either magma or lava) cools and solidifies. They're one of the three main rock types, with the other two being sedimentary and metamorphic.

Igneous rocks can form above or below Earth's surface.

When they form deep below the surface as magma cools, the process takes thousands to millions of years. This type of rock is known as an intrusive igneous rock.

Intrusive rocks often have large crystals because they have plenty of time to grow. An example of an intrusive igneous rock is granite.

Igneous rocks that form above Earth's surface, often from lava expelled from a volcano, are known as extrusive igneous rocks. An example is basalt.

When lava cools too fast for crystals to grow, the rock that forms will be non-crystalline. Examples of non-crystalline, extrusive igneous rocks include obsidian and pumice.

The cooling rate of the lava and its chemical composition play a significant role in the final texture and appearance of an extrusive igneous rock. A rock that cools quickly is usually fine-grained or has a glassy texture.

Igneous rocks are widely used in construction and industry. Basalt is commonly crushed for road stone and concrete aggregate, while granite is valued as a durable building and decorative stone.

Obsidian has historically been used for cutting tools due to its sharp edges, and pumice is used as an abrasive and lightweight building material.

Article Picture

The extrusive igneous rock in our photo at the top of this article is vesicular basalt. This formed from gas-rich lava that cooled quickly, trapping gas bubbles as small holes in the rock. Photo: Courtesy of Stan Celestian. 

Pop-up photos
Pumice: Courtesy of James St. John. Granite, obsidian: Courtesy of Stan Celestian.