AS Earth Structure

Question 1

Solar System

Answer 1

consists of the Sun, planets, moons, comets and asteroids.

Question 2

Sun

Answer 2

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.

Question 3

Planet

Answer 3

a sizeable object orbiting a star, massive enough to have its own gravity.

Question 4

Moon

Answer 4

or natural satellite is a body that orbits a planet.

Question 5

Asteroids

Answer 5

rocky objects which failed to form a planet.

Question 6

Meteorites

Answer 6

rock fragments which fall to Earth from space.

Question 7

Comet

Answer 7

composed of ice and dust. The outer layer melts into water vapour as it gets closer to the Sun.

Question 8

Protoplanetary disc

Answer 8

rotating disc of dense gas and dust surrounding a newly formed star.

Question 9

Planetesimals

Answer 9

solid objects in protoplanetary discs.

Question 10

Protoplanet

Answer 10

moon-sized planetary body that formed within a protoplanetary disc.

Question 11

Stress

Answer 11

the force per unit area acting on or within a body.

Question 12

Strain

Answer 12

the change in shape of a body in response to stress.

Question 13

Seismometer

Answer 13

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

Question 14

Seismogram

Answer 14

the paper or electronic record made by a seismograph.

Question 15

Seismograph

Answer 15

a device which receives and records seismic vibrations.

Question 16

Focus

Answer 16

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.

Question 17

Epicentre

Answer 17

the point on the Earth’s surface directly above the focus.

Question 18

Shadow zone

Answer 18

an area where earthquake waves are not recorded.

Question 19

Intensity

Answer 19

a measure of the surface damage caused by an earthquake.

Question 20

Mercalli scale

Answer 20

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

Question 21

Magnitude

Answer 21

a measure of the amount of strain energy released by an earthquake.

Question 22

Richter scale

Answer 22

a logarithmic scale that measure the magnitude (energy released) of an earthquake.

Question 23

Moment magnitude scale

Answer 23

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

Question 24

Partial melting

Answer 24

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.

Question 25

Lithosphere

Answer 25

the upper rigid layer of the crust and upper mantle.

Question 26

Rheid

Answer 26

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.

Question 27

Low velocity zone (LVZ)

Answer 27

is characterised by low seismic velocities.

Question 28

Asthenosphere

Answer 28

a layer of the mantle below the lithosphere.

Question 29

Kimberlites

Answer 29

fine crystal size, ultramafic igneous rocks.

Question 30

Xenolith

Answer 30

fragment of ‘foreign’ rock included in an igneous rock, which has come from a different source.

Question 31

Ophiolite

Answer 31

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

Question 32

Peridotite

Answer 32

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

Question 33

Gravity

Answer 33

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

Question 34

Isostasy

Answer 34

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.

Question 35

Isostatic rebound

Answer 35

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

Question 36

Subduction

Answer 36

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

Question 37

Slab pull

Answer 37

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

Question 38

Hot spot

Answer 38

an area of high heat flow above a mantle plume.

Question 39

Thermal flux

Answer 39

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.

Question 40

Advection

Answer 40

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.

Question 41

Convection

Answer 41

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.

Question 42

Conduction

Answer 42

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.

Question 43

Seismic tomography

Answer 43

a technique for 3D imaging of the subsurface of the Earth using seismic waves.

Question 44

Mantle plume

Answer 44

a stationary area of high heat flow in the mantle, which rises from great depths and produces magma that feeds hot spot volcanoes.

Question 45

Curie point

Answer 45

the temperature above which magnetic materials lose their permanent magnetism. For magnetite, this is 585°C.

Question 46

Remanent magnetism

Answer 46

recorded in rocks due to the alignment of their magnetic minerals according to the Earth’s magnetic field at the time of their formation.

Question 47

Palaeomagnetism

Answer 47

ancient magnetism preserved in the rocks.

Question 48

Magnetometer

Answer 48

an instrument which detects the strength and direction of the magnetic field.

Question 49

Magnetic inclination

Answer 49

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.