The Rock Cycle Written for the KidsKnowIt Network by:
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.