Types of Rock

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We know in general about rocks. They are basically the stuff that makes up the ground beneath us. The are used in construction and some of them have precious jewels,metals, and other minerals. We even know that early humans used them as the first tools. However rocks come in various types and these types are largely determined by how they are formed. The three main types of rock are igneous, sedimentary, and metamorphic.

Igneous rock is the rock formed from cooling magma. Igneous rocks are further divided into plutonic and volcanic rock. Plutonic rock is igneous rock that is formed by magma cooling with in the earth. Volcanic rock is rock formed on the surface. There are several ways to differentiate the two types of igneous rock. First plutonic rocks are formed by magma that cools slowly with in the Earth’s crust this allows them to form crystals. This is why most plutonic rock is coarse grain. The most common plutonic igneous rock is granite. The volcanic type of rock is formed when magma cools quickly on the surface of the Earth’s crust. This often prevents it forming large crystals so most volcanic rocks are fine grained. A common type of volcanic rock is the basalt rock. However there are others types such as obsidian which was the first type of glass discovered by humans.

Types of Igneous Rock

There are two main types of igneous rocks: intrusive and extrusive. Each type has numerous subsets. Intrusive igneous rocks form from magma that cools and solidifies within the crust of a planet. Since it is surrounded by pre-existing rock the magma cools slowly, so these rocks are coarse grained. Extrusive igneous rocks are formed at the crust’s surface as a result of the partial melting of rocks within the mantle and crust. These rocks cool and solidify more quickly than intrusive igneous rocks, so they are fine grained. Below is a brief description of the various subsets within each type.

Andesite is an extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene and biotite.

Basalt is a dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene.

Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende and sometimes quartz.

Gabbro is a dark colored intrusive igneous rock that contains feldspar, augite and sometimes olivine.

Granite is a light colored intrusive igneous rock that contains mainly quartz and feldspar minerals.

Obsidian is a dark-colored volcanic glass(extrusive rock) that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form.

Pegmatite is a light-colored, extremely coarse-grained intrusive igneous rock. It forms near the edges of a magma chamber during the final phases of magma chamber crystallization. It often contains rare minerals that are not found in other parts of the magma chamber.

Peridotite is an intrusive igneous rock that is composed almost entirely of olivine. It may contain small amounts of amphibole, feldspar, quartz or pyroxene.

Pumice is a light-colored porous igneous rock. It forms through very rapid solidification of a melt. The porous texture is a result of gas trapped in the melt at solidification.

Rhyolite is a light-colored extrusive igneous rock that typically contains quartz and feldspar minerals.

Scoria is a dark-colored, porous, extrusive igneous rock. The porous nature is a result of gas trapped at the time of solidification. It often forms as a frothy crust on the top of a lava flow or as material ejected from a volcanic vent that solidifys in the air.

Welded Tuff is a rock that is composed of materials that were ejected from a volcano, fell to Earth, and then became a rock. It is usually composed mainly of volcanic ash and sometimes contains larger size particles such as cinders.

Sources:
http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Notes/igneous_rocks.html
http://en.wikipedia.org/wiki/Igneous_rock
http://geology.com/rocks/igneous-rocks.shtml

Sedimentary Rock

The next type of rock is sedimentary rock. This type of rock is formed by sediment accumulated with in bodies of water on the surface of the Earth’s crust. Over time rivers,lakes, and other waterways carry and accumulate dissolved soil and minerals that eventually settle on the bottom. With time these beds of sediment settle in strata and the overall weight exerts pressure turns the lose sediment into rock. The composition of sedimentary rock depends on the environment and the type of soil. For example chalk is a sedimentary rock that is often formed from the calcified remains of sea creatures. Another well known sedimentary rock is sandstone.

Finally there is metamorphic rock. This a type of rock that is made from the others types when they are exposed to a lot of pressure within the Earth’s crust. Any type of rock can become metamorphic rock. In fact even older metamorphic rock can be turned into new metamorphic rock. The process is caused by the constant renewal of the Earth’s crust caused by tectonics. This process is how old rock is recycled and turned into new rock. A well known metamorphic rock is marble which is a metamorphic rock formed organic sedimentary rock.

If you enjoyed this article there are several other articles on Universe Today that you will find interesting. There is a more detailed article on granite and another on sedimentary rock.

You can also find some great resources online. A great place to start is the Ask Geo Man page on the University of Oregon website. You can also take a look at the USGS education web page on rocks.

You should also listen to Astronomy Cast. Episode 141 talks about volcanoes.

Sources:
http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Notes/igneous_rocks.html
http://geomaps.wr.usgs.gov/parks/rxmin/rock2.html
http://geomaps.wr.usgs.gov/parks/rxmin/rock3.html

Tagged as: Earth, geology, rock types