EARTHQUAKES / EARTHS INTERIOR

Question 1

Elastic rebound theory

Answer 1

1. Undeformed crust
2. Crust deforms due to stress and is under strain
3. Strain overcomes friction and crust snaps and returns to original position. Fault occurs as rock snaps

Question 2

Earthquake recording

Answer 2

• recorded using a seismometer
• diagram in notes 1
• recording = seismograph

Question 3

Earthquake waves

Answer 3

p waves

• primary/fastest
• causes compression
• pass through all material

s waves

• secondary
• shear waves (shake)
• stopped by liquids

L waves

• slowest
• travel on ground surface
• damaging but don’t travel far

Question 4

Earthquake body wave paths

Answer 4

s waves

• stopped at Gutenberg discontinuity
• creates s wave shadow zone between 103° each side

p waves

• pass through core but get refracted
• this creates ring shaped shadow zone between 103° - 142°

Question 5

Interior of the Earth

Answer 5

• lithosphere (crust/plates)
• Moho discontinuity - 100km
• asthenosphere (upper mantle, rheid)
• lower mantle - 250km
• Gutenberg discontinuity - 2400km
• outer core
• Lehmann discontinuity - 5100km
• inner core

Question 6

Asthenosphere - low velocity zone

Answer 6

• asthenosphere is a rheid - 5% partial melt of peridotite
• p waves slow down as the rock is partially molten (faster in solids)

Question 7

Direct evidence for Earth’s interior

Answer 7

• surface rock, mines, boreholes
• ophiolite suites
• volcanoes
  
  • lava originated at depth so it’s composed of those materials
  • solid mantle (xenoliths) erupted in lavas from a kimberlite pipe

Question 8

Indirect evidence for Earth’s interior

Answer 8

Density

• whole earth density = 5.5g/cm³ but surface rocks are only 2.7g/cm³
• suggests denser rocks must be hidden in the interior

Gravity surveys

• gravitational strength varies across earth
• indicates density changes in hidden rocks

Meteorites

• asteroid are failed terrestrial planets
• stony and iron meteorites are similar to mantle and core

Body waves

• p and s waves are refracted by different densities and states
• indicates the composition of interior

Question 9

Goldschmidts classification

Answer 9

lithophile

• rock loving
• oxides (combine with oxygen)
• low density
• near surface

siderophile

• high density
• combines readily with iron over oxygen
• lower in earth

chalcophile

• ore loving (used to be copper loving)
• sulphides (combines with sulphur)
• higher density than lithophile (deeper)
• lower density than siderophile (shallower)

almophile

• gas loving
• liquid or gas
• Earth’s surface

Question 10

Elements of the earth

Answer 10

Crust

• O, Si, Al, Fe, Ca, Na, K, Mg,
• oxsilal ufecal sod pot mag

Mantle

• O, Si, Mg, Fe, Al, Ca, Na, K
• oxsimag fak nak

Core

• Fe, O, Ni
• foni

Question 11

Composition of Earth’s layers (and evidence)

Answer 11

Crust

• oceanic = basaltic (ophiolites)
• continental = granatic (boreholes)
• lithophiles

Asthenosphere (upper mantle)

• 5% molten peridotie (xenoliths,ophiolites)
• chalcophile

Lower mantle

• silicates (stony meteorites)
• chalcophile

Outer/inner core

• iron-nickel (iron meteorites)
  -siderphiles

Question 12

Earth’s magnetic field - magnetosphere

Answer 12

• magnetic field extends beyond our atmosphere - ‘magnetosphere’
• provides a barrier from ionising radiation from sun

Question 13

Self exciting dynamo

Answer 13

• iron core works like a dipole magnet
• liquid outer core flows over solid inner core which creates a current
• magnetic field is created which helps liquid to flow

Question 14

Evidence for magnetic field reversal

Answer 14

• true polar wandering
• poles slowly wander
• field strength rises and falls
• dipole collapses and restarts in opposite direction