Key Definitions - Earth Structure: Direct and Indirect Evidence

Kimberlites

Fine crystal size, ultramafic igneous rocks

Gravity

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

Mercalli scale

Measures the intensity of an earthquake and is based on the effects that are felt in the area

Moment magnitude scale

Measures the magnitude and leverage on two sides of the fault in an earthquake

Lithosphere

The upper rigid layer of the crust and upper mantle

The Gutenberg Discontinuity

*Depth* - 2900km *Features* - A very distinct and clear boundary marking a change of material from metallic iron nickel to stony silicate material. There is also a change in state between the liquid outer core and the solid mantle *Evidence* - The P wave velocity suddenly decreases and the S waves stop altogether at this boundary

The outer core

*Depth* - 2900km to 5100km *Features* - The outer core is iron-nickel. The outer core is liquid with reduced rigidity. The pressure is less than that in the inner core, allowing a liquid to exist *Evidence* - S waves do not travel through the outer core which shows that it must be liquid. P waves slow down due to lower rigidity

The lower mantle

*Depth* - 700km to 2900km *Features* - The lower mantle is solid due to the increasing pressure, causing the rocks to become more rigid and more incompressible. The mantle is made of the same type of silicate material as the stony meteorites *Evidence* - S waves can travel through the mantle. P and S waves steadily increase in velocity with depth

The Lehmann Discontinuity

*Depth* - Approx. 5100km *Features* - A phase boundary; a zone of about 100km where the rocks change from all liquid in the outer core through a liquid-solid transition to all solid in the inner core *Evidence* - The P wave velocity increases at the boundary

The inner core

*Depth* - Approx. 5100km to the centre of the Earth (6371km) *Features* - The inner core is solid because of the extreme pressure, estimated at 3,600,000 atmospheres. The composition of the core is a mixture of iron and some nickel. Density of more than 12g/cm *Evidence* - P and S waves travel through the inner core. S waves are generated by the P waves at the Lehmann Discontinuity. Iron meteorites. The Earth has a magnetic field

The Moho Discontinuity

*Depth* - Average 35km *Features* - Distinct and clear boundary marking a change of rock type and density *Evidence* - The P and S wave velocities suddenly decrease

The upper mantle

*Depth* - Average of 35km to 700km *Features* - The upper mantle is solid. Consists of solid silicates, less dense than the lower mantle. The main rock type is peridotite *Evidence* - S waves can travel through the mantle

Seismograph

A device which receives and records seismic vibrations

Seismometer

A device which receives seismic vibrations and converts them into a signal which can be transmitted and recorded

Richter scale

A logarithmic scale that measures the magnitude (energy released) of an earthquake

Magnitude

A measure of the amount of strain energy released by an earthquake

Intensity

A measure of the surface damage caused by an earthquake

Rheid

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

Asthenosphere

A rheid layer of the mantle below the lithosphere

Shadow zone

An area where earthquake waves are not recorded

Peridotite

An ultramafic igneous rock composed of the minerals olivine and pyroxene

Xenolith

Fragment of 'foreign' rock included in an igneous rock, which has come from a different source

Low velocity zone (LVZ)

Is characterised by low seismic velocities

Partial melting

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

Ophiolite

Section of oceanic crust and upper mantle broken off and attached to the edge of a continent during plate movement

Strain

The change in shape of a body in response to stress

L waves

Take their name from: *L*ong - The wavelength of the wave is longer than for the P and S waves *L*ast - They are the slowest of the three main waves and arrive last as they also only travel through the surface layers of the Earth

P waves

Take their name from: *P*rimary - Travel fastest and arrive first *P*ush - Lonitudinal/compression waves so the vibration of the rock particles is back and forth. They can travel through any material *P*ressure - The particles alternately move together (compression) and apart (rarefaction) in the direction of travel of the wave

S waves

Take their name from: *S*econdary - Travel at 60% of the speed of P waves so arrive after P waves *S*hear - The movement of the particles is sideways, in a shearing motion, perpendicular to direction of travel of the wave *S*everal time larger in amplitude than P waves

Stress

The force per unit area acting on or within a body

Seismogram

The paper or electronic record made by a seismograph

Epicentre

The point on the Earth's surface directly above the focus

Focus

The point within the Earth at which the earthquake originates as movement occurs along a fault plane. Seismic waves radiate out from the epicentre in all directions