Gravity, Isostasy and Paleomagnetism

Question 1

Equatorial radius of Earth

Answer 1

6378km

Question 2

flattening

Answer 2

c. 1/300

Question 3

Polar radius of Earth

Answer 3

6357km

Question 4

Gravity at equator

Answer 4

978gal

Question 5

Gravity at poles

Answer 5

983gal

Question 6

Reference gravity values

Answer 6

Assuming Earth is a fluid, use observed angular velocity to calculate the reference spheroid from which you calculate gravity.

Question 7

Geoid

Answer 7

Reference surface for gravity observations is the sea surface.
Equipotential surface

Question 8

Geoid anomaly

Answer 8

Difference in the height of the geoid and the theoretical reference surface.
Measured by satellites with an accuracy of +- 10cm
A height of 100m is equivalent to 60mgal change in gravity

Question 9

Long-wavelength gravity

Answer 9

Long-wavelength part of the geoid differs from the theoretical value by less than 100m (60mgal = 0.006%)

On the planetary scale, Earth resembles a perfect fluid
Geoid anomalies reflect deep mantle/core structure
Continents and oceans must be hydrostatically balanced

Question 10

Small-scale gravity

Answer 10

Submarine mountains that make up the mid-ocean ridge systems have no effect on the geoid.
Effect of terrestrial mountains is less than expected.
Therefore, ‘excess’ mass is compensated for

Question 11

Airy’s isostasy

Answer 11

mountains have roots

Question 12

Pratt’s isostasy

Answer 12

mountains have lower density than surrounding rocks

Question 13

Continents

Answer 13

Airy’s isostasy

Correlation between the Moho depth and the height above sea-level for the continental mountains

Question 14

Oceans

Answer 14

Oceanic crust is constant thickness, but water depth varies by several km.
Oceanic mountains are compensated by density variations in the mantle
Pratt’s isostasy as large mountains have lower density material below them

Question 15

Oceanic islands

Answer 15

Usually volcanoes that have added thickness to the normal oceanic crust
Volcano on cold, oceanic lithosphere e.g. Hawaii will cause the lithosphere to flex and support the load by elastic deformation. This will result in a gravity anomaly and a trench around the island.
Volcano on hot, weak lithosphere e.g. Iceland will not be supported by flexure and there won’t be a gravity anomaly.

Question 16

Erosion of mountains

Answer 16

Relationship between average height and depth of root is maintained. When they’re fully erroded, the crust is 35km

Continental root is replaced with mantle material that moves by solid-state creep

Question 17

Present magnetic field

Answer 17

Approximately the shape of a dipole magnet

Axis of magnetic field is inclined in relation to the axis of rotation, currently by 11.5 degrees

Question 18

Earth’s magnetic field

Answer 18

Undergoes secular variation

Visible from old naval maps

Question 19

Remnant magnetism in rocks

Answer 19

Thermo-remnant magnetism:
Cooling from high temperature through the Curie temperature, typically 500 degrees celsius

Chemo-remnant magnetism:
Magnetization acquired by chemical phase changes during formation of iron oxides e.g in haematite

Depositional remnant:
Alignment of magnetized particles in a sediment

Question 20

Magnetic reversals

Answer 20

Dipole component of the magnetic field occasionally reverses, on average every million years. But reversals are random in time
Secular variation averages to 0 over 10^5 years

Question 21

Origin of Earth’s magnetic field

Answer 21

Self-exciting dynamo
Originates in fluid portion of the core
Requires outer core to be good electrical conductor (support that it’s made of iron)

Question 22

Assumption when using paleomagnetism

Answer 22

Field averages to an axial geocentric pole

Requires many samples to average out secular variation

Question 23

Apparent polar wander path

Answer 23

APWPs must be similar for continents that were moving as part of the same mass
If APWPs for 2 continents differ significantly, the continents were in relative motion
Do not give longitudes