Lecture 2: Seismology & the Earth's Interior

Question 1

What is a body wave?

Answer 1

A seismic wave that travels through the body of the Earth

Question 2

What is a surface wave?

Answer 2

A seismic wave that travels on the surface of the Earth

Question 3

Which body wave is transverse?

Answer 3

S-waves

Question 4

Which body wave is longitudinal?

Answer 4

P-waves

Question 5

What 2 factors influence seismic wave velocity?

Answer 5

Density and elasticity

Question 6

What is the shear modulus in liquids and gases?

Answer 6

0

Question 7

What is a seismic discontinuity?

Answer 7

A boundary between 2 of the Earth’s layers that have different properties

Question 8

What 3 simultaneous events occur when a seismic wave hits a discontinuity?

Answer 8

Wave refraction, reflection, and transformation (P-wave -> S-wave and vice versa)

Question 9

If seismic wave velocity is constant, what happens to the motion of the seismic wave?

Answer 9

It travels in a straight path

Question 10

If seismic wave velocity is gradually increasing, what happens to the motion of the seismic wave?

Answer 10

It travels in a curved path

Question 11

When was the Moho discovered?

Answer 11

1909

Question 12

How was the Moho discovered?

Answer 12

Shallow refraction surveying

Question 13

What is the typical thickness of oceanic crust?

Answer 13

7km thick

Question 14

What is the typical thickness of continental crust?

Answer 14

35km thick

Question 15

What is the maximum thickness of continental crust?

Answer 15

80km under Himalayas

Question 16

What layers of the Earth’s interior are considered the lithosphere?

Answer 16

Crust and rigid upper mantle

Question 17

What is the typical thickness of oceanic lithosphere?

Answer 17

70km thick

Question 18

What is the typical thickness of continental lithosphere?

Answer 18

150 km thick

Question 19

Why is the asthenosphere considered a ‘weak’ layer?

Answer 19

It has low elasticity to be able to ‘flow’ and allow the vertical and horizontal movement of the lithosphere

Question 20

Why can S waves pass through the asthenosphere?

Answer 20

It is partially molten, but a lot of their energy is absorbed because of this

Question 21

What happens to the seismic velocity at the base of oceanic lithosphere?

Answer 21

It decreases

Question 22

How thick is the ‘low velocity zone’?

Answer 22

About 100km thick

Question 23

Why is the Mantle completely solid?

Answer 23

The melting point gradient is higher than the temperature gradient

Question 24

Which rock type is the Mantle completely comprised of?

Answer 24

Peridotite

Question 25

Which 4 main minerals is peridotite made from?

Answer 25

Olivine, clinopyroxene, orthopyroxene, and an aluminium rich mineral dependant on depth

Question 26

How do we know that the mantle is comprised of peridotite?

Answer 26

Ophiolite suites, xenoliths, and meteorites

Question 27

What are ophiolite suites?

Answer 27

Fragments of old oceanic crust uplifted onto continental crust

Question 28

What are xenoliths?

Answer 28

Fragments of mantle peridotite contained within explosive volcanic rocks (torn from the mantle during a volcanic eruption)

Question 29

How do meteorites suggest that the mantle is comprised of peridotite?

Answer 29

Many meteorites are comprised of peridotite and originate from asteroids which are thought to have a similar interior to Earth

Question 30

What is the boundary between the lithosphere and asthenosphere called?

Answer 30

The Lithosphere-Asthenosphere Boundary (LAB)

Question 31

How does seismic velocity change between 200-400km deep?

Answer 31

It increases steadily

Question 32

How does seismic velocity change at 400km deep?

Answer 32

Increases more rapidly

Question 33

At 400-450km deep, what happens to the main mantle mineral, olivine?

Answer 33

It changes to a higher presser form (same atoms but a more densely packed structure, mimics structure of spinel)

Question 34

At 670-700km deep what happens to the main mantle mineral, olivine?

Answer 34

Structural change from 4-fold to 6-fold, even more densely packed, mimics structure of perovskite)

Question 35

By how much does the density increase over the MTZ?

Answer 35

Around 11%

Question 36

Why does the density increase in the MTZ NOT cause a decrease in seismic velocity?

Answer 36

The elasticity increases which allows the seismic velocity to increase

Question 37

True or False? The lower mantle (below the Mantle Transition Zone) is homogeneous with a constant uniform structure and composition.

Answer 37

True

Question 38

How deep is the Core-Mantle Boundary (CMB)?

Answer 38

2900km deep

Question 39

How do P waves move?

Answer 39

Compressions and dilations

Question 40

How do S waves move?

Answer 40

Peaks and troughs

Question 41

What does seismic wave velocity depend on

Answer 41

Density and elasticity of the material its travelling through

Question 42

How thick is the transition zone of the upper mantle?

Answer 42

400km