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Types of Igneous Rocks

Igneous rocks are born in fire. There are two types of igneous rocks.The first type and most common is the intrusive igneous rocks. These rocks form when a pocket of magma slowly cools down enough to form into solid rock.

The first thing that we need to know about rocks is: rocks are created from minerals.  When a liquid cools to a solid, the substance is said to have crystallized. This means as magma cools, the elements in the magma form solid bonds with their neighbors in a repetitive pattern. The minerals form into bigger and bigger crystals until they smash into other crystals. Once enough mineral crystals smash together so hard that they can’t be taken apart, the minerals have formed into an intrusive igneous rock. A great example of this type of rock is granite.

Granite is a type of igneous rock
Granite rocks.

Intrusive igneous rock has coarse grained crystals.That means, if you look closely at the rock you can see the mineral crystals.If you look closely at a piece of granite you will see little flecks of white, black, gray, and sometimes pink.These flecks of color are crystals.If you can seethe crystals that form an igneous rock, you’re looking at an intrusive igneous rock.

The second type of igneous rock is called extrusive igneous rock.When magma reaches the surface of the earth people call it lava. Extrusive igneous rocks are formed when lava cools and forms into solid rock.This cooling is much faster than the slow cooling that forms intrusive igneous rocks. Extrusive igneous rocks have small grained crystals.Yes,they have crystals; they’re just so small you can’t see them. There are many different types of extrusive igneous rocks.The type of rock you get depends on what kind of lava the rock was formed from, and how fast the rock cooled.

Magma cools to become igneous rock

Magma erupts to the surface becoming lava.The place that magma erupts to the surface is called a volcano.There are three main kinds of volcanoes and each produces their own kind of extrusive igneous rock.If the lava was formed at a high temperature and has a lower amount of silicon (the main ingredient of the mineral quartz), the lava will flow relatively smoothly and cool to form the rock basalt.The type of volcano that produces this kind of lava is called a shield volcano.The volcanoes in Hawaii are shield volcanoes and most of the rock they produce is basalt. When shield volcanoes erupt they normally don’t explode.The lava flows up from inside the planet and pours down the side of the volcano like a thick hot syrup.These lava flows cover a large area and are very destructive to property.However, these volcanoes are less dangerous to people because the lava flows are usually very slow.

The Hawaiian volcanoes are an example of shield volcanoes
A Hawaiian shield volcano.

Thus, scientists divide igneous rocks into two different categories. Those which cooled below the surface, and those that cooled on the surface. Those which began their lives below the surface are called intrusive rocks, while those which cooled on the surface are referred to as extrusive rocks.

A second type of volcano is called a stratovolcano. Stratovolcanoes are created by alternating layers of volcanic ash and other broken up bits of igneous rock covered by layers of solid igneous rock formed from lava flows.The lava from stratovolcanoes is thicker and cooler than the lava from shield volcanoes.But don’t let that fool you; stratovolcanoes are far more dangerous and more common than shield volcanoes. The reason stratovolcanoes are so dangerous is: when they erupt, they erupt with a violent explosion that hurls thousands of tons of hot igneous rock, ash, and gas into the air.Sometimes these eruptions form columns of a hot gas and rock mixture.When the column cools enough that it can no longer support its own weight it falls back down, flowing down the sides of the volcano in a scorching hot flow called a pyroclastic flow.These flows are usually 1000ºC (1832ºF) and travel at about 700 kph (435mph).It was a pyroclastic flow that destroyed the ancient cities of Pompeii and Herculaneum in 79CE.

When Mt. Vesuvius eropted, it destroyed the city of Pompeii
Mt. Vesuvius was the name of the volcano that destroyed Pompeii and Herculaneum.

Igneous rocks formed by stratovolcanoes have a lot more silica (mineralquartz) in them.An example of this kind of rock is called rhyolite. Rhyolite is almost the same rock as granite, the only difference is the magma that formed granite was able to cool down slow enough to form large crystals: because rhyolite was exposed to the air, it cooled faster so large mineral crystals were not able to form.

The third and last kind of volcano is called a cinder cone. (This is the kind of volcano you would make for your science fair.)  Cinder cone volcanoes are essentially big piles of ash and rocks formed by splattering lava. Pumice and rhyolite are common rocks made by this type of volcano.

Igneous rocks are the most common of the rocks that make up the crust. About 95% of the rocks that make up the crust of the planet are igneous rocks. Igneous rocks are the beginning of the rock cycle. The rock cycle is how the crust of our planet is formed and recycled.

Did You Know?

The same reason a 2L bottle of shaken soda explodes when it is opened is the same reason a stratovolcano explodes. Dissolved gas, usually CO2, builds in pressure until the pressure is so great that the rock above the magma chamber can no longer hold.The rapid release of pressure pulls gas and liquid magma out of the magma chamber and onto the surface. Just like opening a bottle of shaken soda pulls most of the soda out of the bottle and on to the floor.

Written for the KidsKnowIt Network by:  Brandon Guymon