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Types of Igneous Rocks

All igneous rocks form from molten rock. But, they can take on many different shapes! Factors such as where they cool and what type of volcano they come from can affect the types of igneous rocks that form.An image of lava sitting in a depression, slowly cooling.

Types of Igneous Rocks: Intrusive, Extrusive

Extrusive Igneous Rocks

Extrusive igneous rocks are the result of lava that cools on the earth’s surface. These rocks can have a wide array of characteristics. These vary by how the rock cooled, quick or slow. Obsidian is a very smooth mineraloid that forms when lava cools too fast to form crystals.One of the types of igneous rock is obsidian. Here you can see it embedded in soil.
On the other hand, scoria is a very rough extrusive igneous rock. As it cools, air bubbles form in the rock, creating a structure with tiny holes all the way through it. Scoria forms in the crust of lava or as chunks of lava fly through the air in more violent eruptions.An image of Scoria a rough type of igneous rock.

Intrusive Igneous Rocks

Intrusive, or plutonic, igneous rocks are those that form under the earth’s surface. As magma still inside the earth slowly cools, it begins to crystallize to form rocks. This results in rocks that can be smooth and sometimes even glass-like. A common intrusive igneous rock is granite.


Granite is a type of igneous rock

Granite’s texture makes it widely used by humans. From curling to decorating kitchens, granite is used everywhere!

Intrusive igneous rocks are very tough! As a volcano dies, the intrusive rock underneath eventually forms a volcanic neck. The surface of the volcano is pretty vulnerable to erosion. But, the intrusive rock underneath is tough enough to last on its own!

Crystal Formation and Cooling

Crystal formation is an important part of the formation of intrusive igneous rocks. As the magma crystallizes, the cooling process takes a long time. This is because the inside of volcanoes is still a whopping 700 degrees Celsius! It’s hard to call that “cooling”. As the cooling process progresses, large crystals form and get erupted by the volcano.

An illustration showing parts of a volcano.

The underground of volcanoes provides space for crystallization to occur.

Other Great Resources:

Debris Flow Hazards in the US – Geology.com: https://geology.com/usgs/debris-flow-hazards/

Mudflow Facts – Kidzsearch wiki: https://wiki.kidzsearch.com/wiki/mudflow

Landslides – National Geographic: http://www.onegeology.org/extra/kids/earthprocesses/deltas.html

Written by: Jesus Cervantes

Granite rocks.