The rock cycle

The Earth’s surface is made up of different types of rock. If you take for example granite, sandstone and gneiss, you can notice that they differ in colour and structure. Granite is quite coarse and has large visible grains. Sandstone has sand-sized grains arranged in thin layers, one on top of the other. And gneiss seems to be made up of irregular bands of different colours.

This is mostly because these three rocks are formed in very different ways. How? Let’s see! Inside the Earth, below the solid rock surface, there is extremely hot molten rock - magma. When magma gets close to or above the Earth’s surface, for instance in a volcanic eruption, it cools down and solidifies.

Granite is an example of rock that is formed like this - directly from magma. It is an igneous rock. Sandstone and gneiss, on the other hand, are formed when water, wind, heat and pressure act on many types of rock. Let’s take a look at this mountain. It’s made mostly of igneous rock, and it’s been here for millions of years.

During this time, wind, changing temperatures, rain, and living organisms have had an impact on the mountain. The rock has gradually worn away, which means that tiny pieces of rock have broken off and washed down the slopes together with minerals dissolved in rainwater. Then, they join streams and rivers, and are carried away all the way to the sea. There, these tiny bits and pieces of rock sink to the bottom and become sediment. Over time, the weight of the ocean on the seabed presses layers of sediment together, and they cement into rock.

Because it’s made of sediment, this type of rock is called sedimentary rock. Sandstone is an example of this sedimentary rock. The bottom of the ocean, where sedimentary rocks form, is part of the Earth’s crust. The Earth’s crust is divided into several pieces that fit together like a jigsaw puzzle, and constantly move very slowly. Over time, these movements can squeeze and push some rock deeper down into the Earth, creating pressure and heat.

This can change the structure of the rock. It transforms or metamorphoses into a different type of rock: metamorphic rock. Gneiss is an example of such a rock - it can form either from a sedimentary rock like sandstone or an igneous rock like granite! Sometimes, when two pieces of the Earth’s crust collide, rock can be pushed back into the inside of the Earth, melt and become magma. And sometimes, instead, it can be pushed up closer to the Earth’s surface, forming new mountains!

A combination of many different factors is constantly causing rocks and landforms to form, break apart, move, build layers, and transform. This cycle is called the geological cycle or simply the rock cycle. All these processes happen very slowly, so a cycle like this can take up to hundreds of millions of years! So, even though we can’t really see it, the rock cycle never stops. The igneous, sedimentary and metamorphic rocks on the Earth’s surface have been transforming since the Earth was formed.

And they will continue to do so for billions of years to come!