The large crystalline structure of granite makes it useful in decoration.
Intrusive rock is formed when magma released from deep volcanic fissures becomes trapped and cools beneath the surface of the earth, creating igneous rock formations. Rocks that form in this manner are characterized by large crystal formations that can be seen without the use of a microscope. Granite is the most common formation of this type.
Formation
Magma under high pressure is released from underground volcanoes and begins to press upward toward the surface of the crust layer. Magma that seeps into cracks or chambers beneath the Earth's surface may becomes trapped and begin to cool. The earth provides insulation, which causes the magma to cool at a slower rate than it would at the surface. It can take years, or thousands of years to fully cool and solidify. Magma contains a variety of elements that combine into silicate minerals. These minerals grow in crystalline formations, becoming large enough to see without a microscope.
Names
These type of rock formations are called intrusive igneous rock or plutonic rock, named after the Roman god Pluto, god of the underworld. Phaneritic rock is igneous rock, whereby the crystals within the rock are all large and roughly equal in size. This type of igneous rock is usually formed at a greater depth than other types of igneous formations, because the cooling process takes longer, allowing larger crystals to form.
Types
There are five types of intrusive igneous rock classified by geologists. Diorite is a white crystal with black flecks made of feldspar, pyroxene, hornblende and sometimes quartz. Gabbro is comprised of feldspar, augite and olivine, and looks dark and coarse. Granite is mainly quartz and feldspar in a light-colored structure. Pegamite is a very coarse, light-colored rock formed near the final stage of cooling and contains many elements. Peridotite is a dark rock formed almost entirely from olivine, though it may contain trace amounts of other elements.
Exposure
450,000 tons of granite were removed to carve Mount Rushmore.
Through the process of erosion, igneous rock may be exposed to the surface of the crust layer. This is usually the reason granite stone formations appear within layers of sedimentary rock. The Granite Mountains of the American Mojave desert are an example of this process. Scientists believe these peaks of pink stone were created during the Jurassic or Cretaceous period (208 to 65 million years ago), and revealed only a few million years ago. Half Dome in Yosemite National Park and Mount Rushmore are also granite formations.
Tags: igneous rock, rock formed, becomes trapped, crust layer, intrusive igneous