This Planet Really 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.
Below is a summary of the major characteristics of igneous rocks.
Normally contains no fossils
Rarely reacts with acid
Usually has no layering
Usually made of two or more minerals
May be light or dark colored
Usually made of mineral crystals of different sizes
Sometimes has openings or glass fibers
May be fine-grained or glassy (extrusive)
To learn more about rocks and minerals, click on next below:
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