Theory Of Plate Techtonics

Question 1

What is sea floor spreading?

Answer 1

Hypothesises that as continents split apart, magma wells up along the line of the rift and cools to form new crust when extruded. Ocean floors grow outwards from the central rift.

Question 2

Name the 5 evidences of the theory of plate tectonics.

Answer 2

Rock Magnetism 
Geothermal heatflow
Seismic activity distribution 
Dating of Volcanic activity
Patterns of sedimentation

Question 3

Describe rock magnetism as an evidence of sea floor spreading.

Answer 3

When magma is extruded at the Rift Valley and cools to form rocks, those rocks would be magnetised in the direction of the magnetic field of the earth at that time. As the earth’s polarity reverses over time, there will be alternating and symmetrical bands of rock of normal and reverse polarity on either flank of the rift. Eg. Reykjanes ridge, Iceland

Question 4

Explain geothermal heat flow as an evidence of seafoor spreading

Answer 4

Geothermal heat is greatest over mid-oceanic ridges. Measured by a thermistor probe. This shows upwelling of magma and sea floor spreading.

Question 5

Explain seismic activity distribution as an evidence of sea floor spreading

Answer 5

Seismic studies show that mid ocean ridges are centres of seismic activity, suggesting hat sea floor at the ridges are moving.

Question 6

Explain dating of volcanic activity as an evidence of sea floor spreading.

Answer 6

Volcanic islands further away from the mid oceanic ridge found to be older

Question 7

Explain patterns of sedimentation as an evidence of sea floor spreading

Answer 7

Younger sediment near the rift which is thinner. Eg. Deep sea drilling project shown samples of thickness of sediment increasing with increased distance from the rift.

Question 8

What are the 3 evidences of subduction?

Answer 8

1) seismic activity distribution
2) geothermal heatflow
3) distribution of earthquake foci

Question 9

Explain seismic activity distribution as an evidence of subduction

Answer 9

Ocean trenches are centres of seismic activity, showing crystal movement, supporting the hypothesis that old crust is thrust downwards and reabsorbed into the mantle.

Question 10

Explain the distribution of earthquake foci as an evidence of subduction zone

Answer 10

The Benioff zone supports the subduction theory as it shows the line of disturbance that results from the movement of the oceanic plate back into the mantle. (45degrees inclination)

Question 11

How does the mantle convection currents work?

Answer 11

1) rocks will undergo thermal expansion as they gain heat
2) density decreases and they become more buoyant than the surrounding rocks
3) rise up to the base of the lithosphere and move laterally and release heat
4) move apart due to frictional drag
5) thermal contraction
6) density increases and become less buoyant than surrounding rock
7) sink back down
8) cycle repeats as hot rock rises slowly again to balance downward flow

Question 12

What are the 2 sub theories that can explain the theory of place tectonics?

Answer 12

1) sea floor spreading

2) subduction