The lithosphere is the solid outer part of the Earth. It consists of the crust and the solid part of the upper mantle. It has an average thickness of about 35 km and is constantly changing due to processes such as volcanic eruptions, running water and wave action. The lithosphere is composed mainly of rock and hence rocks are of great interest to Geographers.
A rock is any hard, naturally occurring substance which is composed of minerals and is formed by geological processes. A mineral is a substance which is normally crystalline and is formed by geological processes (click here to see some minerals). A geological process is any natural process which modifies (changes) geological features. People who study rocks have found it useful to classify them according to the processes by which they were formed. There are three major ways in which rocks are formed and hence there are three major classes of rock; igneous rocks, sedimentary rocks and metamorphic rocks.
These rocks are formed when molten rock from deep within the Earth’s crust (magma) finds its way into or onto the Earth’s crust where it cools and hardens. When magma cools and hardens within the Earth’s crust the resulting rocks are called intrusive igneous rocks. Because these rocks were formed by the slow cooling of magma, they often contain large crystals, making them coarse grained. Examples of intrusive igneous rocks are granite and gabbro, which can be seen in the photos below (click to enlarge images). Notice that they are made up of large crystals which can be seen with the naked eye. The purpose of the coin in the photos is to give an idea of the actual size of the rock sample.
When magma reaches the Earth’s surface it is known as lava. This lava cools and hardens quickly, forming what are known as extrusive igneous rocks. These rocks are usually fine grained as the lava cools too quickly to allow large crystals to develop. Basalt and obsidian are examples of extrusive igneous rocks. As you look at the photos below you will realize that you are unable to see any crystals with the naked eye. Obsidian (also known as volcanic glass) is formed when lava cools so quickly that crystals are unable to form.
These rocks are formed when material which has been deposited by agents such as rivers is compacted over time until it hardens into rock. This usually happens in bodies of water such as the sea. The material is deposited in layers and these layers are often clearly visible in the resulting rock. Examples of sedimentary rocks include limestone and sandstone. Notice that the layers in the sandstone are clearly visible to the naked eye.
When igneous or sedimentary rocks are subjected to intense pressure and heat over a long period of time, their structure and mineral composition may change a great deal. These rocks metamorphose into a completely different type of rock. The heat and pressure required to bring about this change in the rock exists far below the earth’s surface. Examples of metamorphic rocks are marble (which is metamorphosed limestone) and diamonds (which are metamorphosed carbon).
The Rock Cycle
After rocks have formed, processes such as weathering, erosion, plate tectonics and other geological processes begin to act upon them. As a result of this the rocks begin to undergo changes. We already know that extreme heat and pressure may cause sedimentary and igneous rocks to become metamorphic rock. However, this is only one possible change that rocks can go through.
Metamorphic and igneous rocks may be broken down into small fragments by a process called weathering. These fragments may be carried by a river and deposited in the sea. Over time, layers of these fragments may build up and become compacted forming sedimentary rock.
Sedimentary and metamorphic rock may be forced down into the mantle where they melt. They may eventually find their way into the crust and cool forming intrusive igneous rocks. Alternatively, they may erupt out of a volcano as lava and cool forming extrusive igneous rocks.
Rocks are constantly being formed, broken down and reformed in an endless cycle known as the rock cycle. A simple diagram showing the rock cycle can be seen below.
Watch the video below about rock types and the rock cycle.
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