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IGNEOUS ROCKS
Lesson 5 - Page 3

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The phaneritic appearance of granite is a clue to where granite forms. It takes hundreds of thousands of years for silicate minerals to grow to the sizes they are in granite. This means that the minerals cooled slowly. This can only take place deep inside the crust of the Earth. Here, the rocks surrounding the magma act like a blanket, keeping it warm and keeping the magma chamber hot, allowing the magma to cool slowly. This means that if you see granite exposed on the Earth’s surface, a lot of erosion must have taken place. All of the rock which once covered the granite has been removed.

The minerals in granite can be many different sizes. Some crystals can be quite large. This variety is caused by differences in how fast crystals grow, and in the availability of the elements to make certain minerals. For example, if the magma contains very little iron, then few iron-rich minerals will form.

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