The Different Types of Rocks

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Earth’s Natural Treasures: the Three Types of Rocks: Sedimentary, Metamorphic, & Igneous

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Sedimentary Rock Metamorphic Rock Igneous Rock

Alisha Anderson March 13, 2010 SCI 110 Introduction to Physical Science Professor Valery Shemetov


There are three types of rocks. The three types of rocks are sedimentary, metamorphic, and igneous. Sedimentary rocks are rocks that form when sediments are cemented together. Metamorphic rocks are rocks in which are exposed to high pressure and heat thus causing the material’s properties and structure to change. Metamorphic derives from the Greek language meaning to transform or to change of form. Igneous rocks are rocks that are formed by hot melted materials known as magma. Igneous derive from the Latin language meaning from fire. Igneous rocks have two types; intrusive and extrusive. Sedimentary rocks come into two forms; clastic and chemical.

Igneous Rocks: From Liquid to Solid

Igneous rocks form when hot melted materials cools and becomes a solid rock. This melted material is called magma once it is found beneath the earth’s surface. As mentioned in the introduction, there are two kinds of igneous rocks; extrusive and intrusive. The key difference between these two kinds of rocks is based on where the magma cools. The magma may cool while it’s underneath Earth’s surface or it may break through the earth’s surface at a volcano. In this case, magma becomes lava. Igneous rocks were the earth’s first rocks and made up the early crust. When geologists are searching for rocks that contain iron, igneous rocks would be the rocks they obtain.

Present to scientific understanding, the earth was formed from gas, dust, and ice that was resolving in space around the newly formed star, the sun. Once more gas and dust is accumulated; Earth probably had developed a liquid interior due to the extreme heating and melting. Elements like iron have moved toward the earth’s center and started to form a dense core. However, hot, less dense material was pushed toward the surface thus, separating into two different layers. The outermost rock was cooled to form the crust. The inner rock became the earth’s mantle. But, none of the earth’s present crust is in its original condition. Over geological time, most of the earth’s original crust as either remelted or eroded. The early crust that wasn’t remelted has been under great pressure and heat, therefore changing its original character.

The Two Types of Igneous Rocks: Extrusive & Intrusive

Extrusive Igneous Rocks

Extrusive Igneous rocks are formed when lava cools quickly at the earth’s surface. Magma, which is formed as heat melts rocks of the lower crust which is less dense than surrounding rocks. The surrounding rock pushes the less dense magma up towards the surface. Extrusive Igneous rocks are rarely used as sources of minerals to build automobiles. But, they are mostly processed into road gravel on which automobiles travel on. They also provide decorative stones and abrasives that can clean and polish. In some hand soaps, they contain pieces of certain type of extrusive igneous rock known as pumice. All extrusive igneous rocks share at least one characteristic-a small crystal size. The rate of cooling affects the crystals’ size. Just as with plants, crystals take time to grow. The longer the time they have to grow, the bigger the crystals will become. However, an extrusive igneous rock cools so rapidly that the crystals have less time to enlarge. As a consequence, the crystals are small. Actually, the crystals may be so tiny they can...
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