3.1.1 Evidence (Gravity, EM, density) for the structure of the earth

Occurs when a small proportion, usually between 1% and 5%, of a rock melts. The liquid fraction surrounds crystals which are still solid. This reduces the rigidity of the rock and enables it to behave as a rheid.

The upper rigid layer of the crust and upper mantle.

A non-molten solid that deforms by viscous or plastic flow, in response to applied force. Common examples include the rocks of the mantle and rock salt (halite) which, when under pressure from overlying pressure, forms salt domes which serve as hydrocarbon traps.

Is characterised by low seismic velocities.

A layer of the mantle below the lithosphere.

An ultramafic igneous rock composed of the minerals olivine and pyroxene.

Measured in Gals (after Galileo). 1 Gal = acceleration of 1 cm s⁻². As variations in gravity are very small, the unit used is milligal, or mgal. The average value for gravity on Earth is 981 000 mgal, equivalent to the well-known 9.81 m s⁻².

The theoretical state of equilibrium between Earth's lithosphere and asthenosphere such that the lithosphere 'floats' at an elevation that depends on its thickness and density. The less dense continents rise to a higher elevation than the denser ocean floor, although the pressure exerted on the underlying asthenosphere is the same.

The rising up of land masses that were once depressed by ice sheets.

A process whereby one section of the lithosphere is forced down beneath another.

A process where a cold dense section of lithosphere sinks into the mantle.

An area of high heat flow above a mantle plume.

Measured in Watts per square metre or W m⁻². As the heat loss is small, the units used are milliwatts/m² or m W m⁻².

A process by which thermal energy is transferred through a medium by a fluid. It is an important process close to mid-ocean ridges where sea water is drawn down in to the crust and replaces rising hydrothermal fluids.

The process by which thermal energy is transferred by a substance (fluids or rheids) due to buoyancy differences within the substance. Hot material expands, density reduces, and the material rises, while colder, denser material sinks under the influence of gravity. Convection is more efficient than advection or conduction, and is an important process in the mantle, atmosphere, and ocean.