F4 key idea 1

Question 1

p waves (Primary)

Answer 1

travel faster and drive first
they are longitudinal (back and forth) compressional waves
can travel through any type of material, even liquid as a liquid can still be compressed

Question 2

S waves (secondary)

Answer 2

slower than p waves so arrive after
movement is shear, at right angles to direction of travel (transverse)
larger amplitude than p waves
cannot travel through fluids or gases has these have no rigidity (can’t support shear stress)

Question 3

properties affecting waves

Answer 3

density - velocity increases with depth

rigidity - higher the rigidity, faster the waves travel

Question 4

detecting seismic waves

Answer 4

seismometer detects and records ground motion made of two parts

one is attached to a large inertial mass and does not vibrate with the earth
while the other is allowed to move freely with the vibrations
relative movement between the two is called a seismogram

Question 5

meteorite evidence

Answer 5

iron - composed of an alloy of iron and nickel
thought to represent the core of planet-like object

stony - composed of silicate minerals including olivine, pyroxene and plagioclase
though to represent mantle of planet-like object

Question 6

the crust (lithosphere)

Answer 6

0 to average of 35km deep

Question 7

continental and oceanic

Answer 7

continental - density of 2.7
oceanic - density of 2.9

Question 8

the Moho

Answer 8

average of 35km deep

distinct and clear boundary marking a change in rock type and density

P and S wave velocities suddenly decrease (increase in density)

Question 9

upper mantle (asthenosphere - weaker upper mantle)

Answer 9

35km to 700km deep

rheid, can flow due to partial melting, consisting of solid silicates and main rock type is peridotite, less dense than lower mantle

S waves can travel through (mostly solid)

Question 10

lower mantle

Answer 10

700km to 2900km deep

solid due to increasing pressure causing rocks to become more rigid
made of the same silicate material as stony meteorites

S waves can travel through (solid)

Question 11

Gutenberg discontinuity

Answer 11

2900km deep

distinct and clear boundary marking change in state between solid mantle and liquid outer core

p wave velocity suddenly decreases (liquid outer core)
S waves stop altogether (absorbed by liquid)

Question 12

the outer core

Answer 12

2900km to ~5100km deep

iron core is iron nickel, liquid with reduced rigidity, pressure is less than inner core allowing liquid to exist

S waves are absorbed
p waves slow down due to lower rigidity

Question 13

Lehmann discontinuity

Answer 13

~5100km deep

zone of about 100km where the rocks change from all liquid trough a transition to all solid

p wave velocity increases with rigidity

Question 14

inner core

Answer 14

~5100km to centre of earth 6371km

solid due to extreme pressure
mixture of mostly iron with some nickel

p and S waves can travel through
S waves generated by p waves at the Lehmann
same composition as iron meteorites

Question 15

gravity surveys

Answer 15

value of gravity measured over and area of dense rock, such as gabbro, is slightly different from the value over a less dense rock, such as granite

used to plot the variations in the values from place to place
values different from the average (9.8) are gravity anomalies