×

More help, more resources, more learning.

KidsGeo.com will be joining the Education.com family!

We're so excited to continue to grow and support the parents and teachers championing children's education.
Read press release
KidsKnowIt Network is now part of Education.com!

Igneous Rocks

Igneous rocks are rocks that form when liquid magma cools, creating crystal structured rocks. The chemical composition of these rocks can help us gain more knowledge on the environment beneath Earth’s surface.An image of an example of igneous rocks.
 

How are they formed?

When tectonic plates drift apart, a gap appears in between them. Because the temperature is so high at that level, the layer underneath the plates is liquid and seeps through the gap.
 

This melted rock is magma. Because the temperature decreases as you travel closer towards the crust, the magma hardens and crystallizes as it ascends. The result of this crystallization is igneous rock.

An image of lava in Hawaii dripping into water.

When magma hardens and crystallizes, igneous rock forms.

Types of Igneous Rocks

Igneous rocks can either be intrusive or extrusive. In basic terms, intrusive rock forms within Earth’s crust. However, igneous rock can also form on land, above the crust; some examples include lava from volcanoes or the ocean floor.
 
Intrusive rocks might take thousands of years to cool down, which causes the crystals to be much larger. Igneous rocks formed on the surface cool down in just a matter of a few hours. The crystals in these rocks can be microscopically small.

Classification of Igneous Rocks

Since igneous rocks are extremely varied in appearance and other properties, geologists use a system of classification to help them know what specific rock they’re dealing with. Besides intrusive and extrusive, rocks are classified by occurrence, shape, chemical composition, texture, and crystal structure.
 

These classifications help geologists know more about the igneous rock, such as where it formed. Minerals like feldspar and quartz are extremely common in igneous rocks, so rocks without those minerals have separate classifications. The size and shape of these rocks will provide insight into the conditions of where the rock formed.

A macro image of tiny quartz crystals.

The minerals and crystals inside igneous rocks provide geologists with a lot of information about how and where they formed.