|MadSci Network: Earth Sciences|
Mike: Granite is an igneous rock, which means it forms when hot liquid rock cools. You are probably familiar with volcanic rocks (rocks that exit volcanoes in a liquid state and cool to solidity at the Earth's surface). These rocks are extrusive; they come out of the earth, or are extruded. There is another kind of igneous rock called an intrusive rock (because it forms where liquid rock pushes its way in somewhere in the subsurface, or intrudes). Granite is an intrusive rock: it forms far underground. If you have ever looked at a lava rock, you probably did not see any crystals. That is because the crystals are tiny; the rock cooled too quickly for the crystals to grow large. But the rock is made of nothing but crystals (or glass, which forms when even the tiniest crystals do not have time to grow). Granite is made out of big crystals: you can easily see them with the naked eye. This is because the granite cooled slowly underground, shielded by the rock surrounding it so that its great heat could escape only slowly. Granite is made out of feldspar and quartz, plus minor amounts of other minerals. Feldspar and quartz are light minerals. They are light in color and they stay liquid until relatively low temperatures (less than 1,000 degrees!). Therefore, when hot molten rock is coming up from the mantle, some minerals grow crystals and then later others form. If the magma (liquid rock) keeps moving, those early crystals are left behind, until finally what's left is mostly the ingredients for feldspar and quartz. The result: granite. If a melt like one that could make a granite cools EXTREMELY slowly, then the crystals get very large. Single crystals can be meters long. This kind of rock is called a pegmatite, and it is a good place to collect beautiful mineral specimens. Hope this helps. David Kopaska-Merkel Geological Survey of Alabama PO Box 869999 Tuscaloosa AL 35486-6999 (205) 349-2852 FAX (205) 349-2861 www.gsa.state.al.us
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