The rock cycle describes the process of the formation, breakdown and reformation of rocks.
- Weathering is the breakdown of rocks at the Earth’s surface, by the action of rainwater, extremes of temperature, and biological activity. It does not involve the removal of rock material
- There are three types of weathering, physical, chemical and biological.
Erosion & Transport
- Erosion is the process by which soil and rock particles are worn away and moved elsewhere by wind, water or ice. Weathering involves no moving agent of transport.
- Transport refers to the cycle processes by which the sediment is moved along – for example, pebbles rolled along a river-bed or sea shore, sand grains whipped up by the wind, salts carried in solution.
- Deposition is the laying down of sediment carried by wind, water, or ice. Sediment can be transported as pebbles, sand & mud, or as salts dissolved in water.
- Salts may later be deposited by organic activity (e.g. as sea-shells) or by evaporation.
Burial and Compaction
- As layers are piled one upon another, the sediments beneath are buried, sometimes by hundreds of metres of sediment above. The weight of these layers compacts (squashes down) the sediment grains.
- Minerals deposited from water in the spaces between the sediment grains gradually cements them together.
Deformation and Metamorphism
- The Earth’s crust is slowly moving. Did you know that the Atlantic Ocean is getting wider at about the rate your fingernails grow, or that India is barging its way slowly northward into the continent of Asia?
- The huge forces that move continents stretch and squash parts of the Earth’s crust., generating earthquakes and building mountains. They cause rocks near the surface to be fractured and faulted.
- At greater depth, the heat and pressure involved can cause folding and/or metamorphism.
- Uplift is the key to the rock cycle, as it allows us to see rocks that were once deeply buried beneath the surface.
- If rocks did not get uplifted to form hills and mountains, then the processes of weathering and erosion would long ago have reduced much of the world’s land-masses to low-lying, flat plains.
- Weathering and erosion, transport and deposition would all effectively stop.
- Mount Everest is made of limestone that must have originally formed on an ancient sea floor because it contains fossils of marine creatures.
- Rocks are made up of different minerals that have different melting points. When hot rocks begin to melt deep down beneath the surface, some of the minerals start to melt but others stay solid.
- The rock of the Earth’s mantle layer only begins to melt in a few zones where it is disturbed in some way
- Rocks melt because of the heat beneath the Earth’s surface, as well as other factors such as changes in pressure, or the presence of water within the rock.
- Magma reaches the surface because of pressure squeezes the rock like a sponge, forcing the melted material (magma) to rise toward the surface.
Crystallisation of Magma
- Magma can have different compositions depending on how it was formed.
- Magma also varies in temperature, and in how much dissolved gas it contains. All of these factors control the viscosity of the magma (whether it is relatively runny or very thick and sticky).
- Viscosity, in turn, controls how magma behaves as it rises through the crust, and erupts at the surface.
- The rate at which magma cools controls the grain size of the igneous rock that is formed. More rapid cooling produces finer-grained rocks.
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