The Rock Cycle

It Totally Rocks!

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What is the Rock Cycle?

Throughout the Earth's history, the rocks on it have changed a lot. They've been spewed out be volcanoes, squeezed by geologic forces and have been broken down and eroded away by the wind and rain. This ongoing process of transforming the Earth's rocks is called the rock cycle.
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The diagram above shows the processes that causes rocks to melt into magma and cool into igneous rocks.

Magma, Crystallization & Igneous Rocks

When rocks are heated up to their melting point (naturally, this occurs when rocks are drawn into the Earth's interior) the become magma. Magma is molten rock within the Earth. When magma cools, it hardens and crystals form. After the magma goes through cooling (crystallization) becomes an igneous rock.


Igneous rocks are rocks formed from magma or lava. There are two main types of igneous rocks: extrusive igneous rocks and intrusive igneous rocks. Intrusive igneous rocks are rocks that form from cooled magma under the Earth's surface. These rocks cool slowly, which result in the formation of large crystals. Examples of this type of rock include granite and diorite. Extrusive igneous rocks are rocks that form from lava, magma that has been brought to the surface of the Earth. Since the molten rock is more exposed on the surface of the Earth, it cools more quickly. This results in smaller crystals. Examples of this type of igneous rock include basalt and obsidian.

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Due to the process of weathering and erosion, the Grand Canyon was formed.

Erosion, Sediment & Sedimentary Rocks

Once at the surface, rocks can be broken down and carried away. In the process of weathering, rocks are broken down into pieces. There are two types of weathering: chemical weathering and mechanical weathering. Mechanical weathering physically breaks down the rocks. This can occur by:


  • Water seeping into the cracks of a rock and expanding when it freezes.
  • Temperature changes, causing the rock to expand and contract.
  • The growth of salt crystals over time (the salt is left behind after saltwater evaporates).
  • Plant roots and animals breaking the rocks apart.


Chemical weathering is when chemical reactions take place in order to break down a rock. For example, when carbon dioxide and water combine, it creates carbonic acid. This weak acid can dissolve rocks. In fact, limestone caves owe their existence to carbonic acid (which is really good at dissolving limestone).


After being weathered away, bits and pieces of rock are carried away by water, ice, gravity and, or wind. This process is known as erosion. These particles settle down and are deposited as sediment.


Over time, the sediment accumulates and becomes sedimentary rock. This type of rock can be classified into three groups: clastic sedimentary rocks, chemical sedimentary rocks and organic sedimentary rock. Clastic sedimentary rocks are made up of bits and pieces of mechanically weathered debris. Sandstone and conglomerate (above right) is a good example. Chemical sedimentary rocks are made up of pieces of chemically weathered rocks. Flint is an example of this type of rock. Finally, organic sedimentary rocks are made up of accumulated plant and animal debris. Limestone is an example of an organic sedimentary rock.

Geology Meets Biology: Fossils

One cool thing about sedimentary rock is that fossils can be found in it. In fact, paleontologists (scientists who study fossils) most commonly find fossils in this type of rock.

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Heat and pressure (such as the clashing of tectonic plates that form mountain ranges) can create metamorphic rock.

Metamorphic Rocks

Within the Earth itself, geologic forces can heat up and pressure existing rock. This heat and pressure can change the chemical composition of a rock, cause additional crystal growth and it can change its physical aspects. This results in a metamorphic rock. There are two variations of metamorphic rock: foliated metamorphic rock, which has bands and layers due to heat exposure and directed pressure, and non-foliated metamorphic rock, which has no bands or layers. Marble and quartzite (above right) are non-foliated metamorphic rocks while schist and gneiss are foliated metamorphic rocks.

Rock Cycle Music Video

WE WILL ROCK YOU! (The Rock Cycle)

Conclusion

Through this amazing continuous process, the rocks at our surface have changed dramatically over time. This cycle continues constantly (and not necessarily in the order presented) and keeps recycling the rocks of the Earth's crust in new ways. This is just one of the many fascinating cycles that occurs on our wonderful planet.

Citations

"Earth Floor: Cycles." Earth Floor: Cycles. The Exploring the Environment Project. Web. 14 Feb. 2016.


"Igneous Rocks." Geology.com. Geology.com. Web. 14 Feb. 2016. <http://geology.com/rocks/igneous-rocks.shtml>.


"Metamorphic Rocks." Geology.com. Geology.com. Web. 14 Feb. 2016. <http://geology.com/rocks/metamorphic-rocks.shtml>.


"Sedimentary Rocks." Geology.com. Geology.com. Web. 14 Feb. 2016. <http://geology.com/rocks/sedimentary-rocks.shtml>.


"The Rock Cycle." The Rock Cycle. Mineralology4Kids. Web. 14 Feb. 2016. <http://www.mineralogy4kids.org/rock-cycle>.


"Weathering." National Geographic Education. National Geographic Society, 2011. Web. 14 Feb. 2016. <http://education.nationalgeographic.org/encyclopedia/weathering/>.