Plate Tectonics

Question 1

What is the earth organised into?

Answer 1

It is organised into ocean basins and continental highlands

Question 2

What is the hypsometric curve?

Answer 2

• The max elevation to max depth

- Based on the structure and composition of the earths outer skin-crust

Question 3

What is oceanic crust?

Answer 3

• Melted upper mantle

- dense and thin

Question 4

What is continental crust?

Answer 4

• Melted oceanic crust

- Less dense and thick

Question 5

What is isostasy?

Answer 5

The downward pressure vs buoyancy resulting from displacement of the mantle

• iso = equal
• stasy = stasis

Question 6

What causes isostasy?

Answer 6

• If density increases in the crust, it will start to sink into the mantle causing displacement of mantle
• When mountains begin to erode, less pressure and weight is on the mantle
• Crust and the mantle then rebounds upwards
• When mantle rebonds, other parts begin to sink due to loss of displaced mantle

Question 7

What are the origins of plate tectonics?

Answer 7

Alfred Wegener - continental drift

Question 8

What are the different theories of plate tectonics?

Answer 8

1) Earth was contracting due to earth cooling after formation - created mountains but does not explain shape of continents
2) Earth was expanding due to earth warming, suggested after radioactivity was discovered. Explains why continents separated

Question 9

Outline Wegeners theory

Answer 9

Continental drift

• Continents massed together - Pangea
• Evidence of fossil faunal correlation between Africa and South America
• Correlation of geological structures - glacial deposits

Question 10

What were the problems with the continental drift hypothesis?

Answer 10

• Observations revealed comparatively thin - sea floor sedimentary accumulations were inconsistent with basins in existence for 4 billion years
• What drives the motion of the plates question could not be answered

Question 11

Outline evidence for sea floor spreading

Answer 11

• Ocean surveys

- Ferromagnetic minerals in solidifying rock aligned to the current planetary magnetic field

Question 12

Who came up with the theory of mantle convection?

Answer 12

• Hypothesis proposed by Holmes in 1930s

- Refined by Hess in 1960s

Question 13

What is the mantle convection hypothesis?

Answer 13

• Mantle behaves as a fluid over geological timescales
• Heat transfer could occur by mass transfer - convection
• This occurs when forces driving fluid motion are greater than those resisting
• Driving force = density driven buoyancy
• Resisting force = viscosity

Question 14

What is Hess’ model of mantle convection?

Answer 14

• Heat source = radioactive core and residual heat
• Upwelling at ridges creates new plate material
• Plates move apart because of convection
• Plate cools, densifies and subsides at continental margins

Question 15

What are the three mechanisms to sea floor spreading?

Answer 15

1) Convective currents
2) Plate subsidence - slab pull
3) Plate uplift - ridge push

Question 16

What is a plates rheology?

Answer 16

The material of the plates response to stress

Question 17

How does the lithosphere affect plates?

Answer 17

• As mantle cools and loses heat to the atmosphere by radiation, the material properties change
• lithospheric boundary defines the depth at which the mantle behaves as a fluid or solid
• Depth of lithosphere depends on history of cooling, which depends on velocity of plate from spreading centre

Question 18

What are the different types of plate boundaries?

Answer 18

1) Divergent
2) Convergent
3) Conservative

Question 19

Describe a divergent plate boundary

Answer 19

• Plates moving away from each other

- Basaltic magma pushed up - ocean ridges

Question 20

Describe a convergent plate boundary

Answer 20

• Subduction zone - two plates colliding and one over rises the other
• Oceanic always sinks under continental
• Forms deep sea trenches, ocean volcanos and island arcs

Question 21

Describe a conservative plate boundary

Answer 21

• Plate move horizontally past each-other

Question 22

How are mountains formed?

Answer 22

• Two continental plates being pushed together upwards as one cannot be subjected
• Occurs at convergent plate boundaries
• When plates collide, the plate thickens
• This contributes to the topography or to the deepening of the plate root

Question 23

What is erosional uplift?

Answer 23

When a large amount of material is eroded away from the earths surface, uplift occurs in order to maintain isostatic equilibrium

Question 24

What are feedbacks between climate and tectonics?

Answer 24

• Over short time scales tectonic topography induces rainfall
• Air is forced to rise, cools, condenses and precipitation on windward side of orogen - a belt on the earths crust involved in the formation of mountains
• Feedback cycle causing runaway uplift

Question 25

Outline cooling in the Cenozoic

Answer 25

• Cenozoic - last 65 million years
• Ocean core evidence points to significant cooling
• Linked to major continental collisions

Question 26

What were the 3 mechanisms involved in the cooling in the Cenozoic?

Answer 26

1) Uplift enhances snow cover - increases global albedo
2) Uplift distorts global circulation of the atmosphere, lowering poleward redistribution of heat
3) Enhanced chemical weathering absorbs CO2 from the atmosphere

Question 27

Who questioned whether climate change may actually have triggered erosional uplift?

Answer 27

• Malnar and England 1990
• Evidence to link mountain building purely to plate collision is lacking
• Ocean archives of sedimentation rates highlight peaks of continental erosion, not linked to tectonic history