Igneous Rocks

Igneous rocks are rocks that are formed by magma.

When magma cools down, it becomes a solid.  This can happen below the surface of the Earth or on the surface of the Earth.

Igneous Rocks Facts for Kids:

• A lot of mountains are made from igneous rocks.
• Mountains with a lot of igneous rocks around it might be a volcano.
• The Earth’s moon is made up of igneous rocks.
• Pumice rock can float if put in water.
• Pumice is not minerals that have been put together, but it is a type of glass.
• Pumice is sometimes used in different soaps.
• Obsidian does not just look like glass; it is actually a type of glass.
• Basalt can be found on different planets such as Venus and Mars.
• Some roads are paved with basalt because it is so strong.
• Granite is called granite because it has small bits of quartz and feldspar that is stuck in it.
• Granite can be different colors.
• Egyptians used to use granite to carve sculptures of the pharaohs.
• Many counter tops in our kitchens uses granite.
• Extrusive rocks are also called volcanic rocks.

Where Does the Word Igneous Come From?

The word igneous comes from a word in Latin that means fire.

This word is “ignis.”  The reason that the word ignis means fire is because igneous rocks are formed by lava that gets hard and cools down and turns into magma.

Magma

Magma is lava that comes out of a volcano.  When the lava is in a volcano, it is called lava, but as soon as it leaves the opening of the volcano, it is called magma.

Magma can go into rocks and can cover the surface of an area.  Magma has temperatures that get as hot as 700 degrees Celsius to 1300 degrees Celsius.

What is Magma Made Of?

Magma is made up of different molecules and different atoms that of minerals that have been melted.

When the magma cools down, the atoms and molecules change from a liquid state to a mineral and the mineral grows together forming the magma rock.

The Earth’s Crust

The Earth’s crust is made up of a lot of igneous rock.  Some scientists believe that the crust is 95% igneous rock.

Since lava can form above or below the surface of the Earth, the crust is made of igneous rock from lava forming below the surface.

Intrusive Rocks

When rocks start below the surface, these are called intrusive rocks.

Extrusive Rocks

When a rock becomes cooled on the surface of the Earth, it is called an extrusive rock.

How Do Igneous Rocks Form?

When lava is a liquid and it cools into a solid, it is also known as crystallization.  The minerals of these rocks get bigger and the crystals grow until they hit into other crystals.

When there is enough crystals that are formed together, they cannot be removed form one another and they form intrusive igneous rocks.

These rocks are coarse-grained crystals which means that they have mineral crystals that help them to form.  Sometimes, when people look at intrusive igneous rocks, they can see crystals that have formed.  Some of these are different colors such as:

• Pink
• White
• Black
• Grey

If you look at a rock and you see crystals on it, you are studying a rock that is an intrusive igneous rock.

How Do Extrusive Igneous Rocks Form?

When lava is hot and it cools down, we know it turns into a solid.  When an intrusive igneous rock is formed, the lava cools down very slowly, but with an extrusive igneous rock, the cooling happens very fast.

When you look at an extrusive igneous rock, you do not see any crystals because they are very small.  The type of igneous rock that is formed depends on how fast or how slow the lava cooled down.

Minerals of Igneous Rocks

There are many minerals that help to make up igneous rocks.  Some of them include:

• Oxides
• Quartz
• Feldspars
• Micas
• Amphiboles
• Olivine’s
• Pyroxenes

How Many Different Igneous Rocks Are There?

There are more than 700 different igneous rocks.  Some of these include:

• Gabbro
• Andesite
• Pumice
• Obsidian
• Tuff
• Diorite
• Granite
• Basalt
• Kimberlite
• Olivine Basalt
• Rhyolite
• Gabbro

Basalt

Basalt is a type of igneous rock.  When basalt has a lot of heat and a lot of pressure on it, it changes from an igneous rock to a metamorphic rock.  When this happens, it is called metamorphism.

Tuff

Tuff is another form of igneous rock.  Tuff is formed when volcanic ash turns to rock.

Granite

Granite is an igneous rock that has quartz in it.  This kind of rock is very strong, and people use it to help build things.

Pumice

Pumice is a type of igneous rock that is not very strong.  It is formed when molten rock is blown quickly out of a volcano and bubbles are formed.

These bubbles cool down quickly and have hardly any pressure.  When this happens, it forms pumice.  When you look at pumice, it looks like it has bubbles in it, and it is a rock that is very easy to break.

Obsidian

Obsidian is an igneous rock that is made from volcanic rock that forms quickly and crystals grow on it making it very sharp.

This is also called volcanic glass because it is shiny and looks like glass.  This type of rock can be used to make weapons because of how sharp it is.

Chemical Types of Igneous Rocks

There are two classifications for igneous rocks that are based on what the rocks are made up of:

• Felsic-this is igneous rocks that are lighter.  Some examples of felsic igneous rocks are silicon, sodium and potassium.
• Mafic-these types of igneous rocks contain a lot of iron and magnesium.

Why Are Igneous Rocks Important?

Igneous rocks are important because they help to tell the age of the world.  Also, many of the tectonic plates that move are made up of igneous rocks.

Another reason that igneous rocks are important is because many of them have important mineral ores.  Some of the mineral ores that are found in igneous rocks include:

• Platinum
• Chromium
• Tungsten
• Tin
• Uranium

What Did You Learn?

• What is an igneous rock?
  An igneous rock is a rock that is formed when magma cools down and forms a solid object.
• What is magma?
  Magma is lava that has come out of the volcano.
• How do igneous rocks form?
  Igneous rocks form when magma cools on or below the surface of the Earth.
• What are the two types of igneous rocks?
  The two types of igneous rocks are intrusive igneous rocks and extrusive igneous rocks.  Intrusive igneous rocks cool down slower and extrusive igneous rocks cool down faster.
• Why does it matter if magma cooled down fast or slow?
  When magma cools down slow and forms intrusive igneous rocks, these rocks form crystals that can be seen.  This is how scientists are able to classify rocks.  When magma cools down fast, the crystals are no longer able to be seen in the rock or they are seen very little.  This is extrusive igneous rocks.

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