Rock Cycle

By Matt Rosenberg, About.com


Rocks are composed primarily of minerals and can be an amalgam of different minerals or can be composed of one mineral. Over 3500 minerals have been identified; most of these can be found in the earth's crust. Some of the earth's minerals are exceedingly popular - fewer than 20 minerals compose more than 95% of the earth's crust.
There are three different ways rock can be created on earth and thus there are three main classifications of rock, based on the three processes - igneous, sedimentary, and metamorphic.
Igneous Rock
Igneous rocks are formed from the molten liquid minerals that lie below the earth's crust. They're formed from magma that cools beneath the earth's surface or from lava that cools upon the earth's surface. These two methods of igneous rock formation are known as intrusive and extrusive, respectively.
Intrusive igneous formations can be forced to the surface of the earth where they can exist as masses of rock known as plutons. The largest types of exposed plutons are called batholiths. The Sierra Nevada mountains are a large batholith of igneous granite rock.
Slowly cooling igneous rock will usually contain larger mineral crystals than igneous rock that cools more quickly. The magma that forms igneous rock beneath the surface of the earth can take thousands of years to cool. Quickly cooling rock, often extrusive lava that comes from volcanoes or fissures in the earth's surface has small crystals and may be quite smooth, such as the volcanic obsidian rock.
All rocks on earth were originally igneous as that's the only method entirely new rock can be formed. Igneous rocks continue to form today under and above the earth's surface as magma and lava cool to form new rock. The word igneous comes from Latin and means "fire formed."
Most of the rocks of the earth's crust are igneous although sedimentary rocks usually cover them. Basalt is the most common type of igneous rock and it covers the ocean floor and thus, exists over two-thirds of the earth's surface.
Sedimentary Rock
Sedimentary rocks are formed by the lithification (cementing, compacting, and hardening) of existing rock or the bones, shells, and pieces of formerly living things. Rocks are weathered and eroded into tiny particles which are then transported and deposited along with other pieces of rock called sediments.
Sediments are cemented together and compacted and hardened over time by the weight and pressure of up to thousands of feet of additional sediments above them. Eventually, the sediments are lithified and become solid sedimentary rock. These sediments that come together are known as clastic sediments. Sediments usually sort themselves by the size of the particles during the deposition process so sedimentary rocks tend to contain similarly sized sedimentary particles.
An alternative to clastic sediments are chemical sediments which are minerals in solution that harden. The most common chemical sedimentary rock is limestone, which is a biochemical product of calcium carbonate created by the parts of dead creatures.
Approximately three-quarters of the earth's bedrock on the continents is sedimentary.
Metamorphic Rock
Metamorphic rock, which comes from the Greek to "change form," is formed by applying great pressure and temperature to existing rock converting it into a new distinct type of rock. Igneous rocks, sedimentary rocks, and even other metamorphic rocks and be modified into metamorphic rocks.
Metamorphic rocks are usually created when they come under extreme pressure such as under many thousands of feet of bedrock or through being crushed at the junction of tectonic plates. Sedimentary rocks can become metamorphic rocks if the thousands of feet of sediments above them apply enough heat and pressure to further change the structure of the sedimentary rock.
Metamorphic rocks are harder than other types of rock so they're more resistant to weathering and erosion. Rock always converts into the same type of metamorphic rock. For example, the sedimentary rocks limestone and shale become marble and slate, respectively, when metamorphosed.
The Rock Cycle
We know that all three rock types can be turned into metamorphic rocks but all three types can also be changed through the rock cycle. All rocks can be weathered and eroded into sediments, which can then form sedimentary rock. Rocks can also be completely melted into magma and become reincarnated as igneous rock.
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The way rocks change depends on various processes that are always taking place on and under the earth's surface. Now let's take a closer look at each of these processes.
Heat & Pressure
What happens to cookie dough when you put it in the oven? The heat of the oven produces changes in the ingredients that make them interact and combine. Without melting the dough, the heat changes it into a whole new product — a cookie. A similar process happens to rocks beneath the earth's surface. Due to movements in the crust, rocks are frequently pulled under the surface of the earth, where temperatures increase dramatically the farther they descend. Between 100 and 200 kilometers (62 and 124 miles) below the earth's surface, temperatures are hot enough to melt most rocks. However, before the melting point is reached, a rock can undergo fundamental changes while in a solid state — morphing from one type to another without melting.An additional factor that can transform rocks is the pressure caused by tons of other rocks pressing down on it from above; heat and pressure usually work together to alter the rocks under the earth's surface. This kind of change, which results from both rising temperature and pressure, is called metamorphism, and the resulting rock is a metamorphic rock.
IGNEOUS ROCK + HEAT&PRESSURE = METAMORPHIC ROCK

Melting
What happens to a chocolate bar when it gets very hot? It melts.The same thing happens to a rock when it is heated enough. Of course, it takes a lot of heat to melt a rock. The high temperatures required are generally found only deep within the earth. The rock is pulled down by movements in the earth's crust and gets hotter and hotter as it goes deeper. It takes temperatures between 600 and 1,300 degrees Celsius (1,100 and 2,400 degrees Fahrenheit) to melt a rock, turning it into a substance called magma (molten rock).
METAMORPHIC ROCK + MELTING = MAGMA

Cooling
What would you do to turn a melted chocolate bar back into a solid? You'd cool it by putting it into the refrigerator until it hardens.Similarly, liquid magma also turns into a solid — a rock — when it is cooled. Any rock that forms from the cooling of magma is an igneous rock. Magma that cools quickly forms one kind of igneous rock, and magma that cools slowly forms another kind. When magma rises from deep within the earth and explodes out of a volcano, it is called lava, and it cools quickly on the surface. Rock formed in this way is called extrusive igneous rock. It is extruded, or pushed, out of the earth's interior and cools outside of or very near the earth's surface. What if the magma doesn't erupt out of a volcano, but instead gets pushed slowly upward toward the earth's surface over hundreds, thousands, or even millions of years? This magma will also cool, but at a much slower rate than lava erupting from a volcano. The kind of rock formed in this way is called intrusive igneous rock. It intrudes, or pushes, into the earth's interior and cools beneath the surface.
MAGMA + COOLING = IGNEOUS ROCK

Weathering & Erosion
What do dandelions rely on to separate their seeds, carry them, and deposit them elsewhere? The wind. All objects on the earth's surface are exposed to the wind, along with many other elements — water, the sun, temperature changes. Over time, these factors wear objects down and break them apart. The resulting bits and pieces of material are called sediment. Sediment is then transported by wind and water, often ending up far from where it started. These processes of breakdown and transport due to exposure to the environment are called weathering and erosion. Weathering and erosion affect all rocks on the earth's surface.
IGNEOUS ROCK + Weathering & Erosion = SEDIMENT

Compacting & Cementing
What happens to a loose pile of garbage when it's put into a compactor? The squeezing of the machine produces a solid cube of compacted garbage.The same thing happens to sediment formed from the weathering and erosion of rock. Over time, sediment accumulates in oceans, lakes, and valleys, eventually building up in layers and weighing down the material underneath. This weight presses the sediment particles together, compacting them. Water passing through the spaces in between the particles helps to cement them together even more. This process of compacting and cementing sediment forms sedimentary rock.
SEDIMENT + Compacting & Cementing = SEDIMENTARY ROCK