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Rock Cycle

Did you know that rocks change over time? Nature changes them, like the ultimate recycling system. A rock can start its life as one kind of rock and then change to another.

An image of lots of small pebbles or rocks. These are part of the Rock Cycle.

The rocks under our feet have more of a story than you think.

There are three main types of rocks: igneous, sedimentary, and metamorphic. Each of these types of rocks forms in different ways. Just as well, each type of rock changes into the other types. Geologists call this process the Rock Cycle. The rock cycle is the process that both makes and recycles rocks.

The Cycle in Action

Most rocks on Earth start their life as igneous rocks. These rocks form from magma when it cools and hardens. Over time, the elements break the rock down into smaller rocks. This happens by weathering and erosion. Wind and water gather the small pieces of the igneous rock into a pile known as a sediment bed. Over time the sediment beds get buried. The pieces of rock become cemented together to form a new type of rock called a sedimentary rock.
An image of obsidian, a rock made by lava.

Old, solidified lava makes igneous rocks.

So igneous rocks are slowly turned to sedimentary rocks. If a sedimentary rock becomes exposed at the surface, it can erode and change into a new kind of sedimentary rock. But, if the sedimentary rock is buried in the Earth it undergoes metamorphosis. For rocks, this means that the heat and pressure from the Earth bake it. This new rock is a type of metamorphic rock. This process can happen to igneous rocks too.

An image looking up from the bottom of Antelope Canyon, you can see many rock layers.

Lower Antelope Canyon features red sandstone, a sedimentary rock

If a metamorphic rock moves deeper into the Earth, the rock can melt and become magma. When this magma cools it will be an igneous rock again! Metamorphic rocks can also weather and erode into sedimentary rocks.

an image of a working marble quarry, with large stones and tools.

Marble is a kind of metamorphic rock

To recap, igneous rocks form from magma that has cooled and hardened. Through a process called weathering, it becomes a sedimentary rock. When buried, sedimentary rock goes through metamorphosis. Heat and pressure make it a metamorphic rock.  If this rock moves deeper into the earth it can become magma.

Looking Closer

Let’s take the process from the beginning, and find out how it works. The rock cycle starts with new rocks formed along a divergent plate boundary. The crust pulls apart and magma pushes up from the mantle, cooling down and forming new rock. This usually takes place on the bottom of the ocean, it is often called seafloor spreading. As the new rock forms, 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. One, the plates either pile up onto each other forming giant mountain ranges. Two, one plate dives under another plate, like the Marianas Trench. Or three the plates grind past each other like they do at the San Andreas fault.

Sign for the San Andreas Fault Line

A sign on the San Andreas Fault.

When the plates crash into each other, this type of plate boundary is a convergent boundary. When one plate dives under another plate, this boundary is a subduction zone. Subduction zones are an important boundary in the rock cycle. At this boundary, all the different types of rocks recycle into new rocks. As the plate pushes deep into the mantle the rocks melt into magma. The magma created at subduction zones can form volcanoes near the boundary. It can also cool near the surface and form intrusive igneous rocks. Or it can be carried by currents deep in the mantle to the divergent boundary. There it forms igneous rock as a part of seafloor spreading, taking the process right back to the beginning.

Other Great Resources:

Bill Nye The Rock Cycle: https://www.youtube.com/watch?v=BsIHV__voMk

The Rock Cycle by Free School: https://www.youtube.com/watch?v=EGK1KkLjdQY

The Rock Cycle Song: https://www.youtube.com/watch?v=G7xFfezsJ1s

Written By: Monica Siegenthaler