Now that we know that rocks are made from minerals, and that minerals are made from elements, we can start to learn about how igneous rocks are formed. Everybody knows that boiling water is very hot. Some rocks deep in the Earth get ten times hotter than boiling water! When rocks get this hot they start to melt and the liquid rock that forms is called magma. Because heat rises, and magma is very hot, it begins to rise up through the crust towards the surface.
Magma starts its journey in an area of the crust that is very hot and moves upward towards the surface where the crust is much colder. Magma is like water, as it gets colder it starts to harden. Water freezes and turns to ice. Magma "freezes" and turns to rock!
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Basalt, a volcanic rock, forms columnar joints
Granite, a plutonic rock, forms very large bodies of rock called plutons |
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Different types of magma contain different amounts and different kinds of elements. This controls the kinds of minerals that can grow and therefore the types of rocks that can form. Grain size, or the size of the minerals in a rock, is another factor used in the naming of igneous rocks. If magma cools very slowly, the elements have time to join together and the minerals that form have time to grow larger. Coarse-grained rocks contain lots of large minerals. If the magma cools quickly, it hardens before the minerals can grow, and forms a fine-grained rock. Coarse-grained and fine-grained rocks often have different names even though both rocks are formed from the same minerals. Granite, a coarse-grained rock, is made from the same minerals as rhyolite, a fine-grained rock that cooled quickly at the Earth's surface.
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