Two places where you might be most likely to encounter granite are kitchens and cemeteries. But the material’s importance goes far beyond its uses, because it forms the foundation of our continents.
Definition
Granite is an igneous rock composed mostly of two minerals: quartz and feldspar. It is an intrusive rock, meaning that it crystallized from magma that cooled far below the earth’s surface. Its name is derived from the Latin word granum, which means grain, a reference to the easily-seen minerals in the rock.
Where is Granite Found?
Much of the earth’s continental crust is made of granite, and it forms the cores of the continents. In North America, the landscape surrounding Canada’s Hudson Bay and extending south to Minnesota consists of granite bedrock. Those rocks are part of the Canadian Shield, the oldest rocks on the continent.
Granite also is found below much of the rest of the middle of the continent, buried under hundreds of feet of sedimentary rocks and glacier-deposited sediment. In those places it makes up most of what are known as the continent’s basement rocks.
In mountain ranges like the Sierra Nevada, Appalachians and Rocky Mountains, granite is found in huge masses of rock called batholiths, which form the roots of the mountains. Half Dome and Pike’s Peak are mountains sculpted from granite batholiths.
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How Does Granite Form?
Given the abundance of granite, it is a little surprising to learn that geologists still have many questions about how it forms. Obviously it forms from molten rock, but just where all of that magma came from is still open to discussion, as is the question of exactly how far below ground the magma crystallized into the rock.
Probably the most widely-accepted idea (at least at the moment) is that granite magma originated from a mechanism called partial melting, in which rocks of a very different composition melt in stages and the initial magma is enriched in the minerals that melt first. But where that happens – whether in the mantle or in the lower lithosphere – remains unclear. Regardless of where the magma formed, it probably migrated upward before collecting in large magma chambers prior to cooling and solidifying.
Mineral Composition
Although the term granite or granitic is sometimes used as a general description for any intrusive rocks that look like granite, the name really applies to a rock with a very specific mineral composition.
Granite is composed mostly of two minerals: quartz and orthoclase feldspar (a potassium-rich variety of feldspar). Quartz must make up at least 20% of the rock and orthoclase at least 35%. If either of those criteria is not met, then the rock is not granite. In fact, those are the only two minerals that have to be in the rock!
The remaining rock (up to 45% ) can be one or more other minerals, such as plagioclase feldspar (a sodium-rich variety), hornblende, pyroxene, muscovite or biotite (the last two are kinds of mica).
Texture and Color
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There are two obvious physical properties of granite that determine what it looks like: its texture (the size of the individual mineral grains) and its color. The variability in these two properties leads to a wide range in the appearance of granite.
The individual minerals in granite grow into visiblegrains because the magma cools slowly many miles below the surface. It is the size of the grains of different minerals that imparts the speckled look to the rock.
All granite has what is called coarse-grained texture, meaning that the individual mineral grains are visible to the naked eye. The fancy term for that isphaneritic texture. Under certain conditions the mineral grains can grow very large. When that happens, the granite is called a pegmatite.
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