Igneous rocks are those that form by cooling from a melt. If they erupt from volcanoes as lava, they are called extrusive rocks. If they cool underground but near the surface, they are called intrusive and often have visible, but tiny mineral grains. If they cool very slowly deep underground, they are called plutonic and have large mineral grains.
This is just a starting point to explore the variety in igneous rocks.
Learn more:
Introducing Igneous Rocks
Introducing Sedimentary Rocks
Introducing Metamorphic Rocks
More picture galleries: Fossils | Geologic Features and Processes | Glaciers and Ice | Landforms | Minerals | Rocks | Geology and Society
 Andesite |  Anorthosite |  Basalt |  Diorite |
 Dunite |  Felsite |  Gabbro |  Granite |
 Granodiorite |  Komatiite |  Obsidian |  Pegmatite |
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