Written for the KidsKnowIt Network by: Brandon Guymon
There are three main types of rocks: igneous,sedimentary, and metamorphic.Each of these types of rocks are formed in different ways and each type of rock can be changed into each of the other types of rock.Geologists call this process the Rock Cycle. Essentially the rock cycle is the process that makes and recycles rocks.
Most rocks on earth began as igneous rocks.Let’s trace a possible rock cycle for newly formed igneous rocks. Igneous rocks are formed from magma.Magma cools and solidifies into rock.When igneous rocks are exposed on the surface, time and weather break the rock down into smaller and smaller pieces.This process is called weathering and erosion.Wind and water carry the smaller pieces of igneous rocks into piles called sediment beds.Over time the sediment beds get buried and the pieces of rock become cemented together to form a new type of rock called a sedimentary rock.
Roll your mouse over the image to highlight the parts of the rock cycle.
Our igneous rock has turned into a sedimentary rock.If our sedimentary rock is exposed at the surface, it can be eroded away and eventually changed into new sedimentary rock. However, if our sedimentary rock gets buried deep in the Earth, heat and pressure essentially bake the rock, changing it into something new.This process is called metamorphosis, and the new rock is called a metamorphic rock. Metamorphosis can happen to igneous rocks as well.
Metamorphic rocks can also be weathered and eroded and eventually changed into sedimentary rocks.Or, if metamorphic rock is forced deeper into the Earth, the rock can melt and become magma.If the magma cools and hardens it will form into igneous rock.Igneous rocks and sedimentary rocks can also be forced deep into the earth and melt into magma.Once magma cools it forms igneous rocks.
An example of metamorphic rock.
So let’s take the process from the beginning, and find out how it works.Typically the rock cycle starts with new rocks formed along what scientists call divergent plate boundaries.“Divergent plate boundaries” is science speak for big cracks in the crust of the earth where the plates are pulling apart.The crust pulls apart and magma pushes up from the mantle, cooling down and forming new rock.Since this type of activity usually takes place on the bottom of the ocean, it is often called sea floor spreading.As the new rock is formed it pushes the old rock to the side like a giant conveyor belt with the oldest rock on the edges and the newer rocks in the middle. On the outside edge of where the plates crash into each other, one of three things happens:The plates either pile up onto each other forming giant mountain ranges, like the Himalayas; one plate dives under another plate, like the Marianas trench (in the western Pacific Ocean); or the plates grind past each other, like they do at the San Andres fault.
When the plates crash into each other, geologists call this type of plate boundary a convergent boundary.When one plate dives under another plate, geologists call this convergent boundary a subduction zone. Subduction zones are an important boundary in the rock cycle.At this boundary, all of the different type of rocks are recycled into new rocks.As the plate is pushed deep into the mantle the rocks are melted into magma.The magma created at subduction zones can form volcanoes near the boundary, cool near the surface and form intrusive igneous rocks, or potentially be carried by currents deep in the mantle to the divergent plate boundary and eventually form igneous rock as a part of sea floor spreading, taking the process right back to the beginning.
In short, the rock cycle is the name for the processes that forms and recycles the different types or rocks on our planet.Rocks begin the cycle as igneous rocks, erode into sedimentary rocks then change into metamorphic rocks, finally the rocks melt back to magma and start the process again.
Igneous rock forms when magma cools and makes crystals. Magma is a hot liquid made of melted minerals. The minerals can form crystals when they cool. Igneous rock can form underground, where the magma cools slowly. Or, igneous rock can form above ground, where the magma cools quickly.
When it pours out on Earth's surface, magma is called lava. Yes, the same liquid rock matter that you see coming out of volcanoes.
On Earth's surface, wind and water can break rock into pieces. They can also carry rock pieces to another place. Usually, the rock pieces, called sediments, drop from the wind or water to make a layer. The layer can be buried under other layers of sediments. After a long time the sediments can be cemented together to make sedimentary rock. In this way, igneous rock can become sedimentary rock.
All rock can be heated. But where does the heat come from? Inside Earth there is heat from pressure (push your hands together very hard and feel the heat). There is heat from friction (rub your hands together and feel the heat). There is also heat from radioactive decay (the process that gives us nuclear power plants that make electricity).
So, what does the heat do to the rock? It bakes the rock.
Baked rock does not melt, but it does change. It forms crystals. If it has crystals already, it forms larger crystals. Because this rock changes, it is called metamorphic. Remember that a caterpillar changes to become a butterfly. That change is called metamorphosis. Metamorphosis can occur in rock when they are heated to 300 to 700 degrees Celsius.
When Earth's tectonic plates move around, they produce heat. When they collide, they build mountains and metamorphose (met-ah-MORE-foes) the rock.
The rock cycle continues. Mountains made of metamorphic rocks can be broken up and washed away by streams. New sediments from these mountains can make new sedimentary rock.
The rock cycle never stops.
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