Download Igneous Rocks

Igneous Rocks
The oldest type of all rocks is the igneous rock (IG
nee us). The word "igneous" comes from a Greek
word for fire. Deep inside the earth, the
temperature is very high and the minerals there are
in liquid form called magma. As the magma pushes
towards the earth's surface, it starts to cool and
turns into solid igneous rock.
All igneous rocks do not cool the same way. That is
why they do not look all the same. Some cool slowly,
deep under the earth's surface. These are called
intrusive igneous rocks. The slow cooling formed
rocks with large crystals. Granite is an example of a
rock that cooled slowly and has large crystals.
Other rocks formed when the magma erupted from a volcano or
reached the earth's surface through long cracks. Magma is called
lava when it reaches the earth's surface. Lava cools quickly and
forms rocks with small crystals. They are called extrusive igneous
rocks. Basalt is an example of this type of rock. Obsidian is an
example of another extrusive igneous rock that cooled so fast that
it has no crystals and looks like shiny, black glass.