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Igneous Rocks

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While there are over 700 different kinds of igneous rocks, they all started as molten rock called magma. If they all started as the same stuff, why are there so many different kinds?

  1. Magma underground cools much slower than lava does on the surface. Magma that cools slowly tends to form large crystals. Lava that cools quickly tends to form tiny crystals or glass with no crystals at all.
  2. Different magmas form by the melting of different rocks, so they contain different chemicals. Melted granite will have different chemicals from melted sandstone. Those chemicals determine which minerals will be in the new igneous rock when the magma cools.
  3. Magmas vary in temperature. Some magmas are hotter than 1300°C/2400°F, while others may be cooler than 700°C/1300°F. Hotter magmas may melt some of the rock around them, adding to their chemical content. With cooler magmas, some of the minerals may have already crystallized and settled out.