What Are Plates? (Lectures 5-8)

Question 1

What is the reference surface for gravity observations?

Answer 1

Sea surface

Geoid

Question 2

What is a geoid?

Answer 2

Equipotential surface

Not horizontal due to non-uniform mass distribution

Question 3

What is the geoid anomaly?

Answer 3

Difference between height of geoid and theoretical reference surface

Question 4

Where are geoid anomalies?

Answer 4

Deep in the mantle or core

Question 5

What does it mean for the geoid anomalies to be very small?

Answer 5

On a planetary scale the Earth resembles a perfect fluid

Continents and oceans are hydrostatically balanced

Question 6

What were the two theories for isostasy for mountains?

Answer 6

Airy: mountains have roots
Pratt: mountains have lower density than surrounding rocks

Question 7

Which versions of isostasy apply for continents and oceans?

Answer 7

Airy’s for continents

Pratt’s for oceans

Question 8

How do oceanic island volcanoes cause gravity anomalies?

Answer 8

Volcano added to cold, strong lithosphere

Lithosphere bends to support the load but not fully compensated

Question 9

End result of erosion produces a continent at sea level with a crustal thickness of 35km, this means what?

Answer 9

Continental root replaced over time by mantle material, requires mantle flow
Flow takes place by creep as mantle is solid

Question 10

What is secular variation for the magnetic field?

Answer 10

Non-dipole components of the field migrate on a time scale of decades
Essentially the magnetic pole moves a bit

Question 11

Magnetism is preserved as natural Remnant Magnetism in which 3 ways?

Answer 11

Thermo-remnant
Chemo-remnant
Depositional remnant

Question 12

How does thermo remnant magnetism work?

Answer 12

Rock cools from high T through critical Curie point T
Above this, mineral can’t retain a mag field
Below it the field is permanent

Question 13

How does chemo remnant magnetism work?

Answer 13

Magnetisation acquired by chemical action of phase change during formation of iron oxides at low T

Question 14

How does depositional remnant magnetism work?

Answer 14

Alignment of magnetised particles in a sediment

Question 15

What is the evidence the Earth’s mag field is not permanent using remnant magnetism?

Answer 15

Dipole component of mag field reverses roughly every 1 My

Question 16

Why is secular variation not important over long time frames?

Answer 16

It averages to zero over more than 10000 years

Question 17

Best fitting average dipole field coincides with what?

What is the equation for this field?

Answer 17

Spin axis
tan(I) = 2 tan(λ)
I is inclination
λ is paleolatitude

Question 18

What is an apparent polar wander path (APWP)?

How is it obtained?

Answer 18

Change in latitude and orientation with time for a particular continent
Sequence of paleomagnetic poles from rocks of known ages

Question 19

How can APWPs be interpreted knowing the continents are in relative motion?
(4 points)

Answer 19

Must be similar for continents moving as part of the same mass
If they differ significantly then in relative motion
Can reposition two continents moving relative by superimposing APWPs
APWPs do not give longitudes

Question 20

What were key discoveries made from bathymetry?

6 points

Answer 20

Most sea floor is flat
Mid ocean ridge system
Fracture zones offsetting mid ocean ridges
Trenches
Oldest rocks are 180 Ma
Most rocks are basalt

Question 21

What are observations obtained from magnetometer over the ridge axis?
(3 points)

Answer 21

Symmetrical about ridge axis
Linear anomalies are parallel to ridge axis
Spacing of +ve and -ve anomalies proportional to magnetic reversal timings

Question 22

Why are magnetic reversals parallel to ridges not equal widths?

Answer 22

Different spreading rates

Question 23

How is ocean floor created?

Answer 23

Sea floor spreading

Question 24

What are fracture zones?

Answer 24

Long faults that cut mid ocean ridges

Extend beyond offsets of the ridge axes

Question 25

What are transform faults?

Answer 25

The active parts of fracture zones Between offset ridge axes Earthquakes occur here

Question 26

If movement on transform faults is horizontal, what are the faults called?

Answer 26

Strike-slip faults

Question 27

Describe the features of trenches in the ocean | 7 points

Answer 27

``` Long and arcing, on the edge of ocean basins Up to 11 km deep Associated with island arcs Contains andesitic active volcanoes Largest earthquakes, on thrust faults Big gravity anomalies Not in isostatic equilibrium ```

Question 28

What is subduction?

Answer 28

Where oceanic lithosphere is returned to the asthenosphere | Occurs at trenches

Question 29

What are the features of subduction zones? | 4 points

Answer 29

Subduct at around 70 mm per year Cold enough to fail in earthquakes Deepest earthquakes at 670 km Andesitic volcanoes usually above where subducting slab is 100 km deep

Question 30

How do we know that subducting slabs are colder than the surrounding rocks?

Answer 30

P waves travel faster through them

Question 31

What are the three types of plate boundaries?

Answer 31

Extensional: plates moving apart (spreading ridges) Convergent: plates move towards each other (subduction zones and mountain building) Transform: plates move past each other (strike-slip)

Question 32

Where do earthquakes mostly occur in oceans?

Answer 32

Plate boundaries

Question 33

'Horizontal' motion of the surface of a sphere is equivalent to what? Where does the axis exit?

Answer 33

Angular rotation about an axis through the sphere | Two Euler poles

Question 34

The directions of 'horizontal' motion about an axis through a sphere follow what?

Answer 34

Small circles | Defining planes that are normal to the rotation axis and don't have to cut the centre of the sphere

Question 35

What does it mean for motion on transform faults to be horizontal?

Answer 35

Euler pole describing their motion lies in the direction perpendicular to their strike

Question 36

If faults bound a rigid plate, what does this mean?

Answer 36

Same Euler pole works for all of the faults | Perpendicular directions to transform faults on same plate boundary converge at the Euler pole

Question 37

Define rigidity in plate tectonics

Answer 37

Deformation is restricted to plate boundaries and interiors don't deform at all

Question 38

What occurs when the plate boundary is drawn in a Mercator projection and using the Euler pole as the projection pole?

Answer 38

Horizontal directions of slip on the plate boundary are parallel to each other and the top and bottom of the map

Question 39

What observations are made at ridges and for the ocean floor? (5 points)

Answer 39

No significant gravity anomaly so isostatic equilibrium Oceanic crust mostly constant thickness so isostasy by density change Plot of water depth against sea floor age formation, all ridges have the same shape Depth is proportional to square root of age Beyond 70 Ma depth flattens out

Question 40

Which two effects achieve Pratt isostasy for ocean plates?

Answer 40

As new lithosphere moves away from the ridge axis it cools = thermal contraction so it's denser Greater water depth away from the ridge

Question 41

What is the lithosphere-asthenosphere boundary?

Answer 41

An isotherm

Question 42

Why is the infinite half-space cooling mode for ocean plates half right?

Answer 42

Predicts depth is proportional to square root of age This is true up to 70 Ma Older than this is flattens out

Question 43

What is the plate cooling model for ocean plates?

Answer 43

Sea floor flattens out so lithosphere columns have same density and T Isotherms are horizontal

Question 44

What is the asthenosphere temperature?

Answer 44

1300

Question 45

What is the lithosphere thickness?

Answer 45

125 km

Question 46

Why would the asthenosphere be at a constant T?

Answer 46

It convects

Question 47

What is the potential temperature for the asthenosphere?

Answer 47

If the adiabatic gradient were extrapolated to the surface | 1300

Question 48

What is the thermal thickness?

Answer 48

Plate thickness by the change from conduction to convection | Where water depth and isotherms flatten out

Question 49

What is adiabatic decompression melting?

Answer 49

Material rising follows the adiabatic gradient | Intersects the mantle solidus at 50 km depth and starts to melt

Question 50

What are the key features of adiabatic decompression melting?

Answer 50

Cools all the way up, only melts by intersecting the solidus | Solidus is onset of melting, only partial melt until liquidus hit

Question 51

Why does adding water to the mantle i.e. at subduction zones induce melting?

Answer 51

Presence of water reduces the temperature of the solidus | Subducting slab releases water