Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock.
Igneous rocks are rocks formed from molten magma. The material is made liquid by the heat inside the Earth’s mantle.
When magma comes out onto the surface of the Earth, it is called lava. Lava cools down to form rocks such as basalt. Also, the magma may slowly cool down to form rocks under the surface. An example is granite.
The upper 16 km (10 miles) of the Earth’s crust is composed of about 95% igneous rocks with only a thin, widespread covering of sedimentary and metamorphic rocks.
Igneous rocks are geologically important because they can be used in dating, to understand changes under the earth’s surface, and can have special minerals found inside of them like tin and uranium and platinum.
Igneous rock occurs in two ways.
- Igneous rock formed from magma that has become solid inside the earth is called intrusive igneous rock. An example is granite.
- Igneous rock formed from lava that has become solid on the outside of the Earth is called extrusive igneous rock. This can happen because of the eruption of a volcano. Some extrusive igneous rock types are basalt and pumice.
Forming of igneous rock
Close-up of granite (an intrusive igneous rock) exposed in Chennai, India.
Extrusive igneous rock is made from lava released by volcanoes
Sample of basalt (an extrusive igneous rock), found in Massachusetts