Intrusive igneous rocks form in magma chambers that are deep underground.
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Courtesy of Boise National Forest
Intrusive Igneous Rocks
Many kilometers below the Earth’s surface, molten rock called magma flows into cracks or underground chambers. There, the magma sits, cooling very slowly over thousands to millions of years. As it cools, elements combine to form common silicate minerals, the building blocks of igneous rocks. These mineral crystals can grow quite large if space allows.
Rocks that form in this way are called intrusive igneous or plutonic rocks. The mineral crystals within them are large enough to see without a microscope. There are many different types of intrusive igneous rocks but granite is the most common type.
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The Fall 2010 issue of The Earth Scientist
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