The Great Rock Cycle Content from the guide to life, the universe and everything

The Great Rock Cycle

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All rocks are constantly undergoing slow, but inevitable, transformations from one state to another. Some changes in this cycle are fast and occur on the surface of the Earth where they can be observed even during a human lifespan. Others take millions of years and occur at depths of several kilometres, making them, therefore, more difficult to observe. The great rock cycle contains many shortcuts and most of the material actually never makes the whole circle.

From Magma to Solid Rock

All rock material1 began its journey as magma during the last 4.5 billion years2. If magma crystallises deep down in the Earth's crust it will form plutonic3 rocks. Granite is a typical plutonic rock. If any magma manages to make it closer to the surface before crystallising, it becomes a dyke rock - for example diabase - or a sill. Magma that makes it to the surface through volcanoes becomes lava, which then commonly forms volcanic rocks, rhyolites and basalts. If an eruption causes ash flows and other pyroclastic deposits to form, material skips one step and becomes sediment. Plutonic, dike and volcanic rocks form the family of igneous stones.

From Solid Rock to Loose Deposits

Uplifting caused by plate tectonic processes and erosion brings rocks - not just igneous, but also sedimentary and metamorphic rocks - to the surface of Earth. Once a rock mass is on the surface it will be attacked by weathering. Weathering can be chemical or mechanical. In chemical erosion atmospheric water and / or oxidation start slowly but surely to loosen minerals' chemical bonds. In mechanical weathering, water, wind or ice break rock into small solid particles.

Material weathered from rocks is transported either in solution by water or as solid particles by water, wind or ice. These transportation processes are driven by gravity. At some point the transportation stops and weathered material is deposited. The stop can be final or it can be temporary, allowing material to be transported further in the future. If the stop is final and deposits start to get buried deeper and deeper, they'll enter the next step.

From Loose Deposits Back to Solid Rock

When piles of sediments are buried deep enough, they'll start to turn back into solid rock due to increased heat and pressure. This process is known as 'diagenesis'. In other words, sediments become sedimentary rocks such as mudstone or limestone depending on the composition of the sediment. These can be uplifted and start to be eroded again, or they can be buried even deeper and face the consequences of metamorphism. In metamorphism, the rock's mineralogy starts to change to reach equilibrium with its new surroundings. For example, mudstone will become first mica schist and then mica gneiss depending on how high the temperature and pressure get. Metamorphic rocks can at any point start to uplift and end up on the surface. Plutonic, dyke and volcanic rocks can take a short cut and enter metamorphism without weathering, transportations and sedimentation.

From Solid Rock to Magma

If metamorphism occurs at a high enough temperature4, the rocks will start to melt to form magma, completing the great rock cycle. Sedimentary and volcanic rocks can, under special conditions, turn into melt without being metamorphosed - for example, large lava-flow over older rocks or large meteorite impact cause such conditions. Magmatic rocks can also be re-melted without ever being uplifted to the surface.

1Excluding the marginal amount brought to Earth by meteorites.2The Earth's approximate age.3Named after the Greek god of the underworld.4High being somewhere over 700°C, depending heavily on the water content, pressure and composition of the rock.

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