Lecture 5

Question 1

What shape is the earth

Answer 1

Obligate spheroid flattened at the poles

Question 2

Flattening quantified

Answer 2

(Re - Rp)/ Re =1/298

Where R is the radius
E is the equatorial one and the P is the polar radius

Question 3

Gravity of the Earth

Answer 3

At equator = 978 gal

Poles = 983 gal

gal is 10-2 ms-2

Question 4

Ideal gravity field

Answer 4

Assume the earth is fluid and then use the plausible density-depth variation and angular velocity to give a reference spheroid shape—>provides reference gravity values

Question 5

What is the geoid

Answer 5

The reference surface for gravity observations using the sea surface

Question 6

What is the geoid anomaly

Answer 6

This is the difference in height between the geoid and the theoretical reference surface

Question 7

There is no correlation with oceasns and continents trying to explain the geoid anomaly so what does that mean
x 3

Answer 7

1. on a planetary scale the Earth closely resembles a perfect fluid
2. Geoid anomalies must reflect deep structure
3. Continents and oceans must be hydrostatically balanced otherwise that would show up as anomalies

Question 8

What is isostasy?

Answer 8

The effect where the ‘excess mass’ of mountains must be compensated by the mass deficiency beneath them

Question 9

What are the two compensation mechanisms of isostacy

Answer 9

1. Airy’s Hypothesis : Mountains have roots like icebergs

2. Pratt’s Hypothesis : Mountains have lower density than surrounding rocks

Question 10

What is flexure and why is it not an example of isostacy

Answer 10

• big gravity anomaly
• not isostatic equilibrium
• topography supported by elastic bending of the plate
• not lateral density change

Question 11

Where does Airy’s Hypothesis generally work

Answer 11

Continents as continental mountains resemble icebergs ‘floating’ on denser mantle as Moho is much further from crust the thicker that crust is

Question 12

where does Pratt’s compensation generally apply

Answer 12

Oceanic crust where there is a relatively constant thickness but water-depth varies so oceanic mountains must be compensated by density variations in the mantle and that the high oceanic mountains are underlain by less dense mantle that the lower areas

Question 13

With oceanic islands, they can be supported by either flexure or isostacy. Where does isostacy work?

Answer 13

Where volcano added to hot, weak lithosphere and there is no gravity anomaly which means Pratt and Airy mechanisms working together

Question 14

The relationship between average height and crustal thickness is preserved in mountain belt erosion

Answer 14

So at all times in approximate isostatic balance. The end results of this erosion is a continent at sea level so the continental root is replaced through time by mantle material.
The mantle flow required to do this occurs by creep

Question 15

Present-day magnetic field

Answer 15

Approximates to a dipole and is presently at 11.5 degrees spin axis.
Removing the best fitting dipole from the observed field to get non-dipole field

Question 16

What is secular variation

Answer 16

non-dipole components of the field migrate on a time-scale of tens of years which is very rapid

Question 17

What is paleomagnetism

Answer 17

Using rocks to study the history of the earth’s magnetic field

Question 18

What are the three ways in which natural Remnant Magnetism (RM) is preserved in rocks

Answer 18

1. Thermo-remnant (TRM)
2. Chemo-remnant (CRM)
3. Depositional or detrital remnant (DRM)

Question 19

What is TRM

Answer 19

Thermo-remnant is the cooling from high temperatures through the Curie point and when below this temperature there is a ‘frozen’ magnetism in the rock

Question 20

What is CRM

Answer 20

Chemo-remnant
- Magnetisation acquired by chemical action of phase changing during formation of iron oxides at low temperatures (below Curie point)

Question 21

What is DRM

Answer 21

Depositional or detrital remnant
- alignment of magnetised particles in a sediment, falling through water or rotating in water-filled interstitial holes in wet sediment

Question 22

What is the model for the interior magnetism mechanism

Answer 22

A solid core surrounded by tubular convective cells which are partially controlled by thermal convection

Question 23

Inclination angle

Answer 23

the angle between the horizontal and the Earth’s magnetic field lines

Question 24

Declination

Answer 24

Bearing so the angle between the geographical north and geomagnetic north

Question 25

Geocentric axial dipole

Answer 25

Assume that secular variation averages to zero
dipole axis = rotation axis

roughly true over very long geological time

Question 26

Magnetic reversal

Answer 26

Where the dipole compenent completely reverses and the geographical south pole becomes the magnetic north pole and vice versa

Question 27

Average time between pole reversals

Answer 27

1 million years but they are random and not regular

- this is evidence for the fact the earth is not a perminent ‘bar’ magnet

Question 28

Origin of the agnetic field

Answer 28

• likely from a part of the earth that is fluid (to account for rapidity of secular variation and reversals)
• Magnetism cannot be ferromagnetism as temperatures too high
• Arises from self-exciting dynamo- so core has to be a good electrical conductor

Question 29

Long term average behaviour of magnetic field

Answer 29

the axial dipole field where

tan(i) = 2tan(λ)

I is inclination and λ is paleoaltitude

Question 30

How to find paleolatitude?

Answer 30

The inclination of the fields in the sample

- the declination gives the direction of the paleo-north

Question 31

What is the assumption when using paleomagnetism to determine distance and direction to paleo-north pole

Answer 31

That the field averages to an axial geocentric dipole, aligned to the Earth’s rotation axis

Question 32

What is an apparent polar wander path?

Answer 32

APWP is the sequence of paleomagnetic poles from rocks of known ages that give the latitude and orientation in time for a continent

Question 33

Ways to interpret APWP’s

x 4

Answer 33

• similar APWPs then continents moving as part of same mass
• IF differ significantly then continents were in relative motion
• if two continents were fixed relative to each other during some past time interval and then have moved relative to each other then can be repositioned by superimposing the APWPs for the interval during which they were not in relative motion
• APWPs dont give longitudes but can work out relative longitudes as long as the two continents were in relative motion

Question 34

Example

- Phanerozoic APWPs for North America and Europe

Answer 34

Obervation-
differ but show no obvious patten when plotted to present-day coordinates
— so NA and WE have been in relative motion at some time in past 570 myr

Observation-
When plotted on best-fit reconstruction of continental edges then the paths converge in silurian and diverge in Jurassic
— consistent with techtonic model in NA and western E collided around 400ma forming one continent until 200ma

Question 35

Pangea

Answer 35

The supercontinent where all the land fits together into one land mass