The rock cycle

BBC

For humankind, rocks and minerals have great economic value, whether it is good hard stone for building, or minerals and metals for weapons and jewellery. All the Earth's processes depend on the properties of rocks and minerals in some way.

Most of the Earth's major events, such as volcanic eruptions, mountain building, weathering, erosion and even earthquakes involve rock and minerals. So a knowledge of the Earth's basic building blocks is essential to understanding the Earth.

Every rock contains clues about the environment in which it formed. If a rock is made up of small fossil shell fragments, that tells us it was formed in a shallow marine environment.

Other rocks may contain clues that show they were formed by a volcanic eruption, or deep within the Earth during mountain building. Rocks provide a record of events that occurred during the 4.5 billion years of the Earth's life.

The rocks of the Earth are constantly being transformed into new types of rock and recycled by a number of processes which together make up the rock cycle.

Magma
Some rocks form by the hardening of molten rock or “magma”. This can happen either at the Earth's surface after a volcanic eruption, where magma might flow out onto the surface to form lava, or beneath the Earth's surface where the magma slowly crystallises to form granite. These kinds of rocks are known as igneous rocks.

Sediment
New rocks, however, can also form by the Earth's processes acting on existing rocks. Weathering and erosion by wind, water and ice produce vast quantities of rock and mineral particles called sediment, which is subsequently deposited on the Earth’s surface or in the oceans. Over time sediments become buried and compressed to form sedimentary rocks.

Sedimentary Rocks
These sedimentary rocks can themselves become eroded to form new sediments. Completely new sedimentary rocks can form from biological processes such as the accumulation of shells or the growth of coral reefs.

Metamorphic rocks
Igneous and sedimentary rocks can be changed or altered by high temperature and pressure into metamorphic rocks. This often happens at the boundaries of the Earth’s tectonic plates, where for example, two continents may collide forming new mountain ranges. Under very extreme conditions, metamorphic rocks can begin to melt and produce new magma.