About Cobbles

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Granites are usually a white, black or buff color and are mediumto coarse grained, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their chemistry and mineralogy. Outcrops of granite tend to form tors, rounded massifs, and terrains of rounded boulders cropping out of flat, sandy soils.

Granite is a hard, coarse-grained rock that makes up a large part of every continent. Granite contains three main minerals - quartz, alkali feldspar, and plagioclase feldspar. These minerals make granite white, pink, or light grey. Granite also contains small amounts of dark brown, dark-green, or black minerals, such as hornblende and biotitic mica. The grains of the minerals in granite are large enough that they can easily be distinguished.

The minerals in granite are interlocked like the pieces of a jigsaw puzzle. Consequently, granite is a strong and durable which makes it useful for construction.

Geologists classify granite as an igneous rock. The slow cooling and crystallization of molten material called magma forms most granite. Magma has the same chemical composition as granite. It forms from rocks that melt 16 to 25 miles (25 to 40 kilometers') below the surface of the continents. These rocks melt at temperatures between 1200' and 1650' F. (650' and 900' C). As the magma rises, it cools. Most granite magma cools slowly enough to form coarse crystals and it solidifies below the earth's surface.

Sometimes granitic magma erupts from volcanoes and cools too quickly to form large crystals. The resulting rock, called rhyolite, has the same mineral composition as granite but is fine grained

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