The rock cycle

The Earth is an active planet. Earthquakes shake and volcanoes erupt. Sections of the crust are on the move. Mountains push up and wear down. These and many other processes contribute to the rock cycle, which makes and changes rocks on or below the Earth’s surface. The Earth is 4.6 billion years old, but you won’t find rocks that old because they have been recycled into younger rocks.

Rocks are made of minerals, which are made of specific chemical elements. For example, the mineral quartz is made of the elements silicon and oxygen. The rock called granite consists mostly of the minerals quartz, feldspar and mica.

There are three main types of rock – igneous, sedimentary and metamorphic.

Igneous rocks

Magma is a hot mix of melted minerals, and igneous rocks are formed when it cools. Cooling can happen quickly above ground to form volcanic rocks such as basaltrhyolite and andesite. Cooling can also happen slowly below the surface to form what are called plutonic rocks, such as granite.

Sedimentary rocks

On the surface of the Earth, the processes of weathering and erosion can break down any rock into small pieces called sediments. These sediments are carried away by water or wind and dumped somewhere else. The pebbles, sand and mud you find at the beach are sediments from broken down rock, transported there from somewhere else. Over a very long time, layers of sediment build up at the bottom of seas and lakes, squeezing water out of the layers beneath. Chemicals then cement the sediments into sedimentary rocks such as sandstone and mudstone. The remains of dead animals and plants often get trapped in sediments and can get turned into fossils by the same processes that turn sediments into rock.

Not all sedimentary rocks are made from broken pieces of other rocks. For example, limestone can be formed from the build-up of dead marine organisms on the sea floor or by chemical processes causing minerals to precipitate out of water.

Metamorphic rocks

The Earth’s crust is constantly, but slowly, on the move. Enormous forces push up, tilt, fold and break rocks. These processes can heat and squeeze any type of rock enough to change their structures. Such changes are called metamorphoses, and the resulting rocks are called metamorphic rocks. Marble is an example of a metamorphic rock – it is squashed and heated limestone. You may have come across the word metamorphosis before, because it is used to describe other types of change, such as a caterpillar changing into a pupa and then a butterfly.

There are parts of the Earth, called subduction zones, where the crust is carried deep inside. There is a subduction zone beneath the North Island of New Zealand. Deep down, any type of rock can melt into magma, which later cools to form igneous rock, continuing the rock cycle.

Interacting cycles

As part of the rock cycle, any rock can be changed into one of the other rock types or into a different form of themselves. For example, sedimentary rock can be broken down to form new sedimentary rock, or it may be changed into metamorphic rock. Material does not always go round in a neat sequence, but takes part in a whole network of processes.

The rock cycle interacts with other Earth cycles in many ways. For example, rivers of the water cycle transport sediments, and seas and lakes are where these sediments are dumped. Many sedimentary rocks formed beneath the ocean trap carbon as part of the carbon cycle.