Hazardous Earth

Question 1

Describe the earth’s layered structured (including the asthenosphere), with different composition and physical properties (temp, density, composition, physical state)

Answer 1

3 main layers: crust, mantle and core.

• crust (lithosphere) is solid + rigid - tectonic plates
• tectonic plates move on top of the asthenosphere - a solid but ‘plastic’ layer under such high pressure that the rock flows
• lower layer of mantle is liquid magma at 3000°C
• the outer core is liquid iron and nickel, temps are 4000°C -6000°C
• inner core is iron at temperatures of 5000°C -6000°C - the pressure is so high that this iron is solid

Question 2

Explain how convection currents contribute to plate movement

Answer 2

1 - The core heats the molten rock in the mantle to create a convection current
2 - Heated rock from the mantle rises to the Earth’s surface
3 - At the surface the convection current moves the tectonic plates in the crust
4 - Molten rock cools and flows back to the core to be reheated

Question 3

Describe the density of the 2 types of crust

Answer 3

Continental crust:

• mainly granite
• less dense than oceanic crust
• thicker than oceanic crust

Oceanic crust:

• mainly basalt
• more dense than continental crust
• thinner than continental crust

Both continental crust and oceanic crust are less dense than the rocks of the asthenosphere

Question 4

Explain why radioactive heat is important to tectonics (2 marks)

Answer 4

• It used to be thought that heat of Earth’s core was ‘left over’ from when Earth formed - in other words, the Earth is still cooling down from when the crust first started to solidify, 3.8 billion years ago
• now research suggests that around 1/2 the heat of the interior of Earth is generated by radioactive decay - and also that it’s this heat from radioactive decay (uranium + thorium) that rises up through the mantle to power the convection process
• the other half of Earth’s heat, which is heat left over from the Earth’s formation, stays close to the core and isn’t involved in convection

Question 5

Describe the distribution and characteristics of the 3 plate boundary types (and collision plate boundaries as an extra)

Answer 5

Convergent

• example: Nazca Plate and South American plate
• 2 plates collide, one plate flows beneath another (subduction)
• many earthquakes and volcanoes

Divergent

• example: Eurasian and North American plates
• rising convection currents pull crust apart forming volcanic ridge e.g. Mid-Atlantic Ridge

Conservative

• example: San Andreas Fault, California
• 2 plates slide past each other
• earthquakes

Collision

• example: Indo-Australian and Eurasian plates
• 2 continental plated collide and the 2 plates buckle
• many earthquakes

Plate movements are complex and not yet fully understood

Question 6

Describe the distribution and characteristics of hotspots

Answer 6

• points on the Earth’s crust with very high heat flow, which is linked to increased volcanic activity. Some are on plate boundaries but strangely many aren’t

Question 7

Explain

Answer 7

• volcanoes form over hotspots where magma is rising towards the surface due to mantle plumes
• at the surface, the magma erupts through the thin crust
• these volcanoes often rise above the ocean surface to form islands