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Hot Rocks = Igneous Rocks
Igneous - Ignite - Volcano rocks

How do igneous rocks form?

Play the volcano song!

Igneous rocks are volcano rocks - rocks made from the liquid magma that comes out of volcanoes. When the magma cools, igneous rocks are made. All of the rocks on Earth start out as igneous rocks.

The crystal size of the igneous rock depends on where the magma is when it cools down. If the magma is deep within the Earth’s crust, it can take thousands of years to cool down ePu'u O'o is the currently active vent on Kilauea volcano in Hawaiinough to turn into a solid rock. When it takes a long time to cool down, the crystals in the rock are bigger.

Sometimes the magma is spewed out from a volcano. When the magma reaches Earth's surface, it is called lava. When this happens, because it is at the surface of the Earth, the lava can cool to solid rock in a matter of hours. The crystals in the rock are small and sometimes even too small to see!

So there are two categories of igneous rocks:

- Intrusive, which form INside the Earth and have large crystals because they cool slowly

- Extrusive, which EXit the earth and have small crystals because they cool quickly

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Igneous rock examples

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Obsidian is a glassy textured igneous rock. because the magma has cooled instantly. Native Americans for arrow heads.


Basalt cools quickly enough that the crystals can’t be seen unless you put the rock under a microscope. basalt  

Granite has crystals that are big enough to see without a microscope. This is because the magma cooled slowly.


Pumice is a very lightweight rock - it even floats! The magma had a lot of gas bubbles in it. When it hardened, the gas bubbles made holes in the rock.


The minerals in igneous rocks determine how light or dark they are. Light colored igneous rocks such as rhyolite have the minerals feldspar and quartz in them.

Dark igneous rocks like basalt (see above) contain the minerals olivine and pyroxene.


How are igneous rocks used?

Since igneous rocks are so hard, some of them, like granite, are cut into sheets and made into counter tops and floors.  
Glassy rocks like obsidian can be used as jewelry, like this bracelet, or even just for landscaping decorations. Chunky Obsidian Bracelet (Crystal Jewelry )  
Other igneous rocks have a very scratchy texture, like pumice. Pumice is used as an abrasive. A lot of people use it to smooth the rough skin on their feet!  

Quiz time - assessing what you've learned

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Hot Rocks Quiz




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