Geosphere

terrestrial impacts, gravitational contractions, decay of radioactive elements

Convection occurs in both the mantle and the core. This is a circulation pattern in which warmer low-density material rises and cooler high-density material sinks. This transfers heat to the Earth’s surface and drives plate tectonics.

• Convection currents in upper mantle diverge • If below a continent, rift valleys form • Mostly found between two oceanic plates • As plates move apart, underlying mantle melts, magma is formed and rises, erupting through crush, filling gap and creating new oceanic crust.

• A chain of underwater volcanoes is created along the spreading plate margins. • Hydrothermal vents are often located near constructive plate boundaries. • Deep faults form when the oceanic plates move apart. Cold seawater percolates down through the faults and is superheated through contact with the magma, before returning to the seafloor via hydrothermal vents.

They occur where convection currents in the upper mantle are converging, forcing the plates above to move towards each other.

• Subduction zones occur where continental plate and oceanic plates converge. • The oceanic plate moves below the continental plate because it is denser, carrying down oceanic sediments and seawater. • The sediments and seawater lower the melting point of the plate materials. • The magma generated from subduction zones has high levels of gas present and is very explosive. • This molten material forces its way up through faults to the surface of the continental plate, erupting as a volcano. • Molten material reaching the surface is known as lava, and cools to form extrusive rock. Magma that cools and solidifies before reaching the surface forms intrusive rock. • Oceanic plate moves below the continental plate, a deep ocean trench forms at the junction. • Subduction causes crumpling along the edge of the continental plate, forming fold mountain chains. • Earthquakes at this boundary can be of high magnitude due to the release of frictional pressure

Bauxite is an ore rich in aluminium oxide and is found in extractable quantities in areas associated with subduction zones.

• Forms in tropical areas around the equator, where high temperatures and abundant rainfall result in extreme chemical weathering of rock. • As water percolates through the soil in the hot, humid climate, nutrients dissolve and leach downwards. • The soil that remains, known as laterite, can be rich in aluminium oxides and form rich ore deposits near the surface.

Through surface mines, extracted by open cast mining.

• landscape changes through removal of timber, vegetation, rock and soil, with associated impacts on ecosystems and biodiversity • control of erosion and runoff from the mine • mining disturbs the movement, quality and distribution of water in the area • disposal of the overburden, tailings and other waste • dust, noise and emissions caused by mining operations and transport

• smelting and processing require enormous inputs of electricity, water and resources • releases high levels of greenhouse gases, particulates, and other harmful substances • bauxite is often transported long distances to countries with processing plants

On average, the temperature increases by about 25°C for every kilometre of depth.

• groundwater: steam and hot water • ground source: ‘hot rocks’

• steam or hot water can be extracted and used to generate electricity or piped directly to district heating schemes • ground source heat pumps

• High efficiency • Low cost (in suitable areas) • Low environmental impact

• Few suitable sites • Land subsidence due to extracted water • Depleted easily if not managed