Rocks are linked by the Rock Cycle

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Processes at the Earth's Surface

Igneous Processes

Magma, a hot melt from within the Earth's crust or upper mantle, may be erupted onto the surface as lava flow. If the eruption is more explosive, solid fragments called pyroclasts are produced. This process forms extrusive igneous rocks. The lava and solid fragments begin to cool at the surface. The lava becomes rock when crystals form from the magma and it becomes solid. This process is called crystallisation.

Sedimentary Processes

Weathering takes place when rock is exposed to the Earth's atmosphere and water. Rain, temperature changes and plants among other things, start to break it down in situ by mechanical, chemical and biological processes. Fragments and isoluble compounds that are produced by weathering are removed by erosion, which wears down the Earth's surface by the mechanical action of the weathered fragments. The fragments and soluble compounds are transported from one place to another by gravity , running water, ice, wind and the sea. The fragments are often reduced in size by collisions during transport.

Transporting agents lose energy sooner or later. The velocity of the wind may be reduced or a river may flow into a calm sea or lake. When this happens, deposition takes place.

Processes Below the Earth's Surface

Sedimentary Processes

Deposition of sediment usually occurs in lakes, oceans and seas. Burial occurs as sediment is covered by more and more layers of sediment deposited on top:

as the sediments are buried more deeply they become compacted. This means that the grains of sediment are moved into closer contact, due to the pressure of the sediment above them.

Water that circulates between the grains of sediment deposits minerals in the spaces between the grains. These and other changes that occur in buried sediments are part of the process of diagenesis

The growth of minerals in pore spaces and the compaction of grains result in the formation of sedimentary rocks

Many sedimentary rocks are buried even more deeply as more material is deposited above them. Pressure and temperature increase as they move to greater depths inside the Earth.

Earth Movements

Rocks can be exposed at the Earth's surface if they are uplifted by tectonic processes. Earth movements push rocks up by folding or faulting, so that rocks formed deep below the surface are now at the surface. Erosion over millions of years will expose rocks. In Cornwall, the granite that forms Dartmoor crystallised at least 10km below the surface and erosion over 200 Ma has removed all the overlying rocks.

Metamorphic Processes

Metamorphism is the process by which rocks in the Earth's crust asre changed by the effects of heat and pressure. This process is described as isochemical. This means that the chemical element in the metamorphic rock are the same as those in the parent sedimentary rock. Metamorphism involves recrystallisation, a solid-state process by which existing minerals are changed into new crystalline metamorphic minerals.

No melting takes place during metamorphism.

Igneous Processes

Melting is caused at depths where temperatures are high enough, but different minerals have different melting points so that part of the rock may melt whilst some is still solid. This is known as partial melting and occurs in the lower crust and upper mantle. It is the process by which magma forms.

Because it is hot and expanded, magma has a lower density than the rocks surrounding it and this causes it to rise and join with other rising magma. This magma accumulation creates underground magma chambers.

As the magma rises it forces its way into pre-existing rocks, either along bedding planes and joints or by cutting across the existing rocks. This is intrusion. The magma cools, because temperatures within the Earth decrease with decreasing depth below the surface. Crystallisation may take place below the surface rather than above it and igneous rocks formed below the Earth's surface are also described as intrusions, or as intrusive igneous rocks. The rock cycle is completed when lava and pyroclasts are extruded at the Earth's surface to form extrusive igneous rocks.

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