The rock cycle is a simple breakdown of how physical material is recycled on the Earth’s surface. This ultimately leads to the formation of the major rock types and is driven primarily by two processes:
1. Convection within the Earth’s interior which leads to slow but significant changes on the Earth’s crust.
2. The hydrologic cycle, which is the recycling and movement of water, air, and ice at the surface of the Earth.
Without either of these, the rock cycle would not exist on Earth. This is proven when we study other planets, or satellites (like our moon). Because convection does not occur within the interior of our moon, and a water based atmosphere does not exists, the recycling of rock does not occur. This means there are no volcanoes, oceans, mountains, or otherwise ‘active’ geologic features.
To begin we will start at the top of this diagram and explain the steps involved to transition from an exposed outcrop to any rock type within the cycle. Since this is a cycle, you could start anywhere along the digram and you would end right back where you began.
Weathering, Erosion, Transportation, and Deposition:
There are two types of weathering, physical and chemical. Physical weathering is the mechanical wear and tear that physically changes a rocks appearance, while chemical weathering occurs as a result of a chemical reaction leading to change, such as limestone effervescing in acidic rain water.
Transportation and deposition is the movement of materials from one location to another, and plays a major role in weathering. Typically the further a material has been transported from it’s source, the more it has been reworked. For example, if you are close to a source (like the head of a river) you’ll likely find coarse grained, angular sediment that doesn’t appear to be changed very much from it’s original state. As you move further away sediment will become finer and more rounded.
Burial, Compaction, and Cementation:
Once an outcrop has been turned to loose sediment, there has to be a way in which it is transformed into a sturdy sedimentary rock. This occurs through the processes of burial compaction and cementation. Burial is simply the covering of materials with more material. As the material on top increases pressure increases (known as lithospheric pressure) compacting the once loose sediment below it. However, you cannot simply stack loose sediment on top of each other for millions of years and expect it to hold together without a process known as cementation. Cementation occurs as minerals precipitate between the grains of sediments, acting as a ‘glue’ which holds the sediments together.
Heat and Pressure:
If a rock continues to be buried beneath the surface of the Earth, it will slowly become introduced to higher temperatures and pressures. Once the pressure or heat is high enough metamorphism occurs. Metamorphism is the transformation of a parent rock into a new rock due to the application of heat and pressure, which leads to the formation of new minerals, or recystillization, without melting. A typical example is that of a granite (the parent rock) undergoing metamorphism to become a gneiss. It is important to note that metamorphism of different materials occurs at different temperatures and pressures, and really depends on the mineralogy of the parent rock.
Uplift is a process which brings rock back to the surface of the Earth. This usually occurs at reverse fault boundaries where compressional forces create a vertical change on the Earth’s crust. Uplift is the same process which results in mountain building and escarpments.
Melting does not normally occur, geologically, on the Earth’s surface, and in order for a rock to melt must be subducted deep beneath the Earth’s surface. This subjects the rock to much higher pressures and temperatures than what could be experienced at or near the surface of the Earth, and completely liquefying the chemical makeup of any rock.
Crystallization and Cooling:
Once a rock is molten (between 800°C and 1300°C) it can either slowly cool just below the surface of the earth (for millions to thousands of years) forming intrusive igneous rocks, or erupt onto the surface, cooling much faster (seconds to years), forming extrusive igneous rocks.
Intrusive igneous rocks cool at depths of kilometers beneath the surface. Once cooled they must be uplifted to once again be exposed, or buried even deeper to change it’s position in the rock cycle.
Adapted from: Physical Geology by Steven Earle used under a CC-BY 4.0 international license.