How the Rock Cycle Recycles Earth's Crust

If you could speed up time, you would see rocks continuously cycling from within the planet.

First, rocks would sink deep down into the interior. Then, they would rise back up again as magma. All thanks to the mechanism of plate tectonics.

Like a well-oiled machine, rocks transform from igneous, sedimentary to metamorphic types. This process where rocks are created, altered and formed again is the rock cycle.

If you look at the Earth’s crust, 80% is igneous, 15% is metamorphic and 5% is sedimentary rocks. But 75% of land surface contains sedimentary rocks.

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Sedimentary rocks form by compaction and cementation

Igneous rocks are forged from volcanoes

Metamorphic rocks build from heat and pressure

Rock cycle diagram

Sedimentary rocks form by compaction and cementation

Sediment Size

Sedimentary rocks like shale, limestone and sandstone form from pre-existing rocks. If you start with sand, mud or organic material, these sediment deposits get eroded and transported over time.

The two main processes that form sedimentary rocks are compaction and cementing. While compaction squeezes material together, cementing glues the squeezed material together.

For example, the Grand Canyon has sedimentary rock layers dating back to 2 billion years ago. They include sandstone, shale (or mudstone), and limestone all stacked on top of one another.

For sedimentary rocks, younger layers of rock pile up on top of the older layers on the bottom. Like glue, each successive layer cements over tens of millions of years.

Igneous rocks are forged from volcanoes

Volcanoes

Volcanoes are the foundation for igneous rocks. Like a bottle of champagne with a cork in it, liquid rock pools in the magma chamber.

Once you pull the cork, the volcano erupts from the mouth spewing lava. Like wax dripping down the side of a candle, igneous rocks like feldspars form.

So hot lava drips on the side of volcanoes then cools, hardens and become igneous rocks. For example, granite, andesite and obsidian are types of igneous rocks.

Without igneous rocks, there wouldn’t be the rock cycle. Most believe the rock cycle begins with igneous rocks because you wouldn’t have metamorphic or sedimentary rocks.

Metamorphic rocks build from heat and pressure

Metamorphic Rock

Metamorphic rocks change in form from pre-existing rocks. For example, a rock that was once made of sand is changed or “metamorphosed” into another type of rock from heat and pressure.

This intense pressure for metamorphic rocks comes from inside Earth.

Earth’s major mountain chains contain metamorphic rocks because it’s at plate tectonics boundaries where this intense pressure exists.

When it’s metamorphosed, it’s soft and pliable like cookie dough. For example, metamorphic rock types include marble, slate, gneiss and schist.