Lecture 1

Question 1

Mines and Boreholes

Answer 1

Mines are less than 3km usually
Boreholes are less than 10km
Both get very hot very quickly

Question 2

Mean Radius of Earth

Answer 2

6371km

Question 3

Density of Earth’s surface rocks

Answer 3

0.002-0.003 kgm^-3

Question 4

Earth’s Average Density

Answer 4

0.00552 kgm^-3

Question 5

Moment of Inertia of uniform sphere compared to earth

Answer 5

If was a perfect sphere then the moment of inertia would be 0.4Ma^2
But it is 0.33 and so mass is concentrated to the centre of the earth

Question 6

South Africa Gold Mine

Answer 6

up to 4km deep and gets to 60-70 degrees and so pump ice slurry into mines
Every kilometre deeper you go it is roughly a 20 degree temperature increase

Question 7

Earthquake properties

Answer 7

• Occur when faults move and release elastic energy as sound waves
• Seismology is the study of these waves
• Important in creating images on earth’s interior

Question 8

Waves that propagate out from focus of earthquake

x2

Answer 8

1. Body waves which are split in p and s waves

2. Surface waves which are restricted to the free surface or to internal interface

Question 9

P waves

Answer 9

Arrive first and are longitudinal

also called pressure waves

Question 10

S waves

Answer 10

Secondary or shear waves and are transverse

Question 11

Example of how earthquakes can affect other parts of the world

Answer 11

8.2 mag in Bolivia 1994 could be felt as moving buildings in toronto 6000km away

Question 12

Different mode types and definition

Answer 12

A normal mode is the free oscillations the earthquakes have on earth
Most common=
1. Balloon mode (entire earth expanding and contracting) period=20 minutes
2. Football mode (compression along x axis while expansion along y axis before switching) period= 54 minutes
3. Twisting mode (halves of earth rotate in different directions) period= 40 minutes
Each have different periods associated with them and the modes can get a lot more complicated

Question 13

Core-mantle boundary-who discovered it

Answer 13

Richard Oldham in 1900 by looking at travel times from an earthquake in Assam in 1897

Question 14

Who studied earth magnetic field and though the earth might be hollow

Answer 14

Edmund Halley

Question 15

Why do p and s waves curve in earth

Answer 15

They change between materials and increase in velocity in the earth

Question 16

At what angles do p and s waves stop and reappear

Answer 16

P and S waves both stop at an epicentral angle (Δ) of 104 degrees
P waves reappear at Δ=142

Question 17

What is the order of wave arrival

Answer 17

1. P waves
2. S waves
3. Surface waves

Question 18

What is Δ?

Answer 18

The epicentral angle.
This is measured as the angle between the focus/epicentre, the centre of the earth, and the seismograph that recorded the wave

Question 19

Wave field

Answer 19

A wave field is all the energy released by an earthquake.

However it is easier to visualise and calculate the field as separate rays

Question 20

Epicentre angle to time before recorded

Answer 20

Graph 1

Question 21

Core Shadow zones

Answer 21

The areas in which none of the waves are recorded.

Question 22

What does the fact that there are no s waves recorded after 103 degrees tell us?

Answer 22

That the outer core is liquid as S waves cannot travel through that-Richard Oldham. We know from Inge Lenmang that the inner core is solid.

Question 23

Mantle-Core boundary

Answer 23

All waves are refracted/reflected at this interface and p waves experience a reduction in velocity and so therefore switch directions towards the core.

Question 24

The moho

Answer 24

the crust-mantle boundary discovered by Mohorovicic by using an earthquake in Croatia in 1909 when beyond 250km the p waves were earlier than expected

Question 25

What is defined as the crust and mantle

Answer 25

Anything above the moho is the crust and anything below is the mantle

Question 26

How was the moho found

Answer 26

Graph and diagram 2
- the p wave that went directly above the moho was slower than the wave that ran along the moho interface as the wave sped up from 6–>8ms^-1

Question 27

Average thickness of the continental crust

Answer 27

35km

Question 28

Average oceanic crust thickness

Answer 28

7km

Question 29

What is the jump ion velocity at the moho caused by

Answer 29

• Either a change in composition or a change in mineral structure (phase change)

Question 30

Graph of relative velocities of waves through the earth

Answer 30

graph 3

Question 31

Low velocity zone at roughly 150km depth

Answer 31

may reflect partial melting or otherwise solid mantle. Cannot be a lot as the s waves can still travel through the majority

Question 32

Increases in velocity of waves occurs at 450km and 670km why?

Answer 32

there are changes in the mineral structure—>a phase change due to denser materials
670km is the boundary between the upper and lower mantle

Question 33

What is peridotite

Answer 33

A rock composed of mainly the mineral olivine which gives it the green colour. Most likely the rock thar forms the majority of the matle

Question 34

What are the meteorites we use most often to estimate the composition of the earth

Answer 34

Carbonaceous chondrites

Question 35

These meteors have a similar composition to the sun. however, that are the differences?

Answer 35

GRAPH 4

• N, O, C, H- mostly in gas form and so not kept in meteor as much as sun
• Li more in meteorite as cosmic ray interaction causes this to form on the surface

Question 36

What is the chondritic model of earth

Answer 36

The idea that the earth’s composition resembles that of chondritic meteorites

Question 37

Iron meteorites contain roughly 90% iron. What does this show

Answer 37

represent cores of small planetary bodies that were broken by collision and so most likely the earth’s core as well

Question 38

Who discovered the earth’s magnetic field and when

Answer 38

Gauss in 1830

Question 39

Describe the earth’s magnetic field

Answer 39

90% dipolar (like a bar magnet)