AS Earth Structure Flashcards
Solar System
consists of the Sun, planets, moons, comets and asteroids.
Sun
a star composed of hydrogen and helium. It is the largest object made up of more than 99.8% of the total mass of the solar system.
Planet
a sizeable object orbiting a star, massive enough to have its own gravity.
Moon
or natural satellite is a body that orbits a planet.
Asteroids
rocky objects which failed to form a planet.
Meteorites
rock fragments which fall to Earth from space.
Comet
composed of ice and dust. The outer layer melts into water vapour as it gets closer to the Sun.
Protoplanetary disc
rotating disc of dense gas and dust surrounding a newly formed star.
Planetesimals
solid objects in protoplanetary discs.
Protoplanet
moon-sized planetary body that formed within a protoplanetary disc.
Stress
the force per unit area acting on or within a body.
Strain
the change in shape of a body in response to stress.
Seismometer
a device which receives seismic vibrations and converts them into a signal which can be transmitted and recorded.
Seismogram
the paper or electronic record made by a seismograph.
Seismograph
a device which receives and records seismic vibrations.
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.
Epicentre
the point on the Earth’s surface directly above the focus.
Shadow zone
an area where earthquake waves are not recorded.
Intensity
a measure of the surface damage caused by an earthquake.
Mercalli scale
measures the intensity of an earthquake and is based on the effects that are felt in the area.
Magnitude
a measure of the amount of strain energy released by an earthquake.
Richter scale
a logarithmic scale that measure the magnitude (energy released) of an earthquake.
Moment magnitude scale
measures the magnitude and leverage on two sides of the fault in an earthquake.
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.
Lithosphere
the upper rigid layer of the crust and upper mantle.
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.
Low velocity zone (LVZ)
is characterised by low seismic velocities.
Asthenosphere
a layer of the mantle below the lithosphere.
Kimberlites
fine crystal size, ultramafic igneous rocks.
Xenolith
fragment of ‘foreign’ rock included in an igneous rock, which has come from a different source.
Ophiolite
section of oceanic crust and upper mantle broken off and attached to the edge of a continent during plate movement.
Peridotite
an ultramafic igneous rock composed of the minerals olivine and pyroxene.
Gravity
measured in Gals (after Galileo).
1 Gal = acceleration of 1 cm 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
Isostasy
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 floors, although the pressure exerted on the underlying asthenosphere is the same.
Isostatic rebound
the rising up of land masses that were once depressed by ice sheets.
Subduction
a process whereby one section of the lithosphere is forced down beneath another.
Slab pull
a process where a cold dense section of lithosphere sinks into the mantle.
Hot spot
an area of high heat flow above a mantle plume.
Thermal flux
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.
Advection
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 into the crust and replaces rising hydrothermal fluids.
Convection
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.
Conduction
the process by which thermal energy is transferred through a substance with no overall movement of that substance. Energy is transferred from atom to atom down a thermal gradient. The process of conduction is slower than advection or convection.
Seismic tomography
a technique for 3D imaging of the subsurface of the Earth using seismic waves.
Mantle plume
a stationary area of high heat flow in the mantle, which rises from great depths and produces magma that feeds hot spot volcanoes.
Curie point
the temperature above which magnetic materials lose their permanent magnetism. For magnetite, this is 585°C.
Remanent magnetism
recorded in rocks due to the alignment of their magnetic minerals according to the Earth’s magnetic field at the time of their formation.
Palaeomagnetism
ancient magnetism preserved in the rocks.
Magnetometer
an instrument which detects the strength and direction of the magnetic field.
Magnetic inclination
the angle of dip of the lines of a magnetic field. It is the dip angle made with the horizontal and the Earth’s magnetic field lines, measured with a compass.
Moho discontinuity
The distinct boundary between the crust and the mantle
Gutenberg discontinuity
The distinct boundary between the solid mantle and the liquid outer core
Lehmann discontinuity
The phase boundary between the liquid outer core and the solid inner core
Solar nebular disc theory
nebula (cloud) - shockwave - cloud collapse - rotation increases - hot, flat, protoplanetary disc - gravity draws material to the centre, - hot and dense - nuclear fusion begins - continued accretion of gas to star - small bodies of rock grains form planetesimals - accrete to from protoplanets - protoplanets collide to form terrestrial planets - gas giants form further from the star - leftover planetesimals form asteroids (belt)
