The rock cycle | The geosphere | Middle school Earth and space science | Khan Academy

Have you ever tried to hold a staring contest with a rock? If you did, you might not have expected that all that time you were staring at one of the sneakiest shapeshifters in the world. No, rocks don't shapeshift into unicorns, but they do change shape and composition. Sometimes they change quickly, like when an erupting volcano launches molten rock into the air, and sometimes they change slowly, like when a rock is heated and compressed in the earth for thousands to millions of years.

When geologists stare at a rock, they can figure out the rock's composition and tell the story of how it was created. It turns out rocks can be made in many different ways. When put in the right combination of heat and pressure, rocks can become molten and liquid. When this molten rock cools and becomes solid, we call the rock it makes igneous rock. This term comes from the Latin word "igneous," which means fiery or burning hot.

Igneous rocks make up more than 90% of the earth's crust. One common kind of igneous rock is granite, which you've probably seen in bridges, buildings, and countertops. Another kind of igneous rock is obsidian, which is smooth and glassy and is sometimes used to make knife blades. Both of these rocks were made of molten rock. So why do they look so different?

Well, granite was made from magma, which is molten rock that exists below the surface of the earth. Magma tends to cool slowly underground, which gives time for the elements in it to form large crystals. This obsidian rock was made from lava, which is molten rock that flows above the ground. Lava tends to cool quickly, which causes the rocks it forms, like obsidian, to have smaller crystals.

I always remember the difference between magma and lava like this: I keep my lava lamp above the ground, just like lava is above the ground. If I buried my lava lamp in the ground, then I can call it a magma lamp.

Once an igneous rock is created, its journey isn't over. The rock might begin to change through a process called weathering. This is when water, weather, wind, and other physical forces chip away at a rock and cause little pieces to break off. These little particles are called sediments. The sediment from our igneous rock can be moved around by wind and water. This is called erosion.

Eventually, the sediment will settle down somewhere, maybe at the bottom of an ocean or a lake, and it might join other sediment from other rocks, crushed-up shells, and plant matter. Over time, this sediment will become a rock. Rocks that are formed from sediments are called, you guessed it, sedimentary rocks.

But how does sedimentary goop at the bottom of a lake become a rock? Well, it goes through a process called lithification. "Lith" comes from the Greek word for "stone," so you can think of this process as stonification. The first step of lithification is called deposition. This is when sediment is deposited in a new location, and it spreads out to form a layer.

As more and more layers are created, the layers underneath them become squished together. This is called compaction. When water moves to the layers of sediment, it can carry dissolved minerals with it. The water can leave these minerals in between the sediment particles, which makes everything stick together. This cementing of sediment is called cementation. Limestone is a sedimentary rock that is made when sediment that contains a lot of calcium carbonate in it goes through lithification.

But once a sedimentary rock has been created, it doesn't mean that it is done changing. Sometimes a rock is squeezed or heated so much that the minerals inside it actually change composition. It is now a metamorphic rock. You might have heard of the word "metamorphosis," which describes when something changes from one form and structure to another, like when a caterpillar turns into a butterfly.

Metamorphic rocks are made when igneous, sedimentary, or even other metamorphic rocks change form in structure because of heat and pressure. For example, when the sedimentary rock limestone gets put under a lot of heat and pressure, it can turn into marble. But metamorphic rocks still might not be done changing. Rocks can actually keep on changing forever.

Igneous and metamorphic rocks can become sedimentary rocks if they are broken apart into sediments and go through lithification. Metamorphic rocks and sedimentary rocks can become igneous rocks when they are melted and then cool. Igneous rocks and sedimentary rocks can become metamorphic rocks when they're exposed to high heat and pressure. This is called the rock cycle.

So maybe next time you get the chance to stare at a rock, see if you can figure out whether it's igneous, metamorphic, or sedimentary. And when rocks change so much, who knows what kind of rock it'll be next?