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Types of Igneous Rocks: Lesson for Kids

Instructor: Lisa Hanson

Lisa is a Continuous Improvement Coach for her school district and has taught in elementary school for many years. She has a master's degree in curriculum and instruction.

Rocks come in all shapes, sizes, and colors and are found all over the world. Rocks are classified, or grouped, according to how they are formed. In this lesson, you'll learn more about one of those groups, igneous rocks. Now, let's rock and roll!

How Igneous Rocks Are Formed

We have all seen a volcano erupt on television or in a movie. They are explosive, and the lava can cause a lot of damage. What happens to all of that lava once it cools down? This is where igneous rocks come into play. Igneous is a Latin word that means fire. The rocks are formed when lava or magma, which is melted rock, cools, hardens, and forms new rocks. These rocks are often strong because the minerals are tightly packed, and they generally have no layers to them. Igneous rocks have different properties depending on where the melted rock cools and how quickly it cools.

This diagram shows the two different ways igneous rocks are formed.
Igneous Rock Diagram

Extrusive Igneous Rocks

Magma is liquid rock that flows beneath the Earth's surface. When a volcano erupts that magma shoots out, or flows down onto the Earth's surface, and then becomes known as lava. When lava cools and hardens, it is known as an extrusive igneous rock. These rocks have smaller grain crystals that you can barely see because the lava cools so quickly. They do not have time to form. These types of rocks can form anywhere from a few seconds to a few months.

The most common type of extrusive igneous rock is basalt.
Basalt

There are many different kinds of extrusive igneous rocks. Basalt is the most common type found above the Earth's surface. It is dark in color and has very fine grains. It is often crushed and used in construction. Some other types are obsidian, which is a black, glassy looking rock, and tuff, which is hardened volcanic ash. Another common type of extrusive igneous rock is pumice. This is a very light weight, light colored rock. It is covered in small holes, because gas bubbles were caught in the rocks while it was cooling. It is similar to the foam you see when you shake up and open a soda.

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