Recognizing Igneous Rocks
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Recognizing Igneous Rocks
How to recognize an igneous rock? There are two main types of igneous rocks. Plutonic rocks cool underground, and are later exposed at the surface by erosion. Plutonic rocks will have crystals that are large enough to see with your unaided eye. Granite, for example, typically looks like a bunch of pink, black, white, and clear crystals joined together.
Some igneous rocks aren't plutonic, but rather are volcanic. Volcanic rocks are ejected from below the surface, and cool rapidly at the surface. That means the crystals that form don't have time to "grow," and are too small to see without a microscope. Although they can be made of the same individual mineral grains as, for example, a granite, in this case the minerals are microscopic in size, and the rock just looks like a light grey, light pink, or dark black rock. If the rocks are hard, too hard to scratch with a nail, they are probably extrusive igneous rocks (like lava). On the other hand, the solid color rock is softer than a nail, it could be a sedimentary rock, like limestone or shale.
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