Published on December 18, 2007
Rocks : Rocks Mike Scisney Slide2: Granite is an igneous rock that is composed of four minerals. These minerals are quartz, feldspar, mica, and usually hornblende. Granite forms as magma cools far under the earth's surface. Because it hardens deep underground it cools very slowly. This allows crystals of the four minerals to grow large enough to be easily seen by the naked eye. Look at the photo of granite above, notice the different crystals in the rock. Slide3: Rocks Tell the Story of the Earth The Earth is made of rock, from the tallest mountains to the floor of the deepest ocean. Thousands of different types of rocks and minerals have been found on Earth. Most rocks at the Earth's surface are formed from only eight elements (oxygen, silicon, aluminum, iron, magnesium, calcium, potassium, and sodium), but these elements are combined in a number of ways to make rocks that are very different. Rocks are continually changing. Wind and water wear them down and carry bits of rock away; the tiny particles accumulate in a lake or ocean and harden into rock again. The oldest rock that has ever been found is more than 3.9 billion years old. The Earth itself is at least 4.5 billion years old, but rocks from the beginning of Earth's history have changed so much from their original form that they have become new kinds of rock. By studying how rocks form and change, scientists have built a solid understanding of the Earth we live on and its long history. Igneous Rocks : Igneous Rocks Igneous rocks are called fire rocks and are formed either underground or above ground. Underground, they are formed when the melted rock, called magma, deep within the earth becomes trapped in small pockets. As these pockets of magma cool slowly underground, the magma becomes igneous rocks. magma is produced at a subduction zone : magma is produced at a subduction zone When most people think about igneous rocks they envision a volcano erupting pumice and lava. The term igneous comes to us from the Latin word "Ignis" which means fire. Igneous rocks are produced this way but most igneous rocks are produced deep underground by the cooling and hardening of magma. Magma is molten (melted) rock under the surface of the Earth. It is produced in the upper reaches of the mantle or in the lowest areas of the crust usually at a depth of 50 to 200 kilometers. Minerals : Minerals A mineral is any naturally-occurring, homogeneous solid that has a definite chemical composition and a distinctive internal crystal structure.