1.2 - Earths Structure And Theoretical Model

Question 1

Crust

Answer 1

• 400 degrees at base, 22 degrees at top
• There are two types of crust:
• Continental - a thicker (45-50km), less dense layer (mostly granite)
• Oceanic - a thinner (6-10km), denser layer (mostly basalt)

Question 2

Upper and lower mantle

Answer 2

Upper mantle
- 1400- 3000 degrees
- Asthenosphere – lower part of the upper mantle, solid but so hot that it flows (molten)
- Lithosphere – top part of the upper mantle and crust, solid because it is cooler

Lower mantle
- 3000 degrees
- Hot enough to melt but is solid because of the pressure pushing down on it

Question 3

Inner and outer core

Answer 3

Outer core
- 88% iron
- Semi-molten, mostly liquid
- 4400 - 6100 degrees

Inner core
- Solid centre
- 80% iron
- Very dense
- 7000 degrees

Question 4

What is the lithosphere

Answer 4

the rigid outer part of the earth consisting of the crust and upper mantle

Question 5

What is the asthenosphere

Answer 5

the upper layer of the earth’s mantle, below the lithosphere, where convection is thought to occur

Question 6

What is convection

Answer 6

the hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat

Question 7

How does mantle convection affect mid ocean ridges (ridge push)

Answer 7

• The heat from radioactive decay in the core moves upwards into the mantle
• This creates convection currents which push into the spreading mid ocean ridges forcing them further apart. This is called ridge push.

Question 8

How does mantle convection happen in magma

Answer 8

• Heat from inner core travels through mantle into asthenosphere.
• Hot magma rises because it becomes less dense with heat (as particles spread out)
• Magma is cooler at the top as it is further away from the heat source. It becomes more dense and sinks to the bottom.
• Cooler magma is reheated, so it starts to rise again. Creating a loop called a convection current

Question 9

What is sea floor spreading

Answer 9

This is the formation of new areas of oceanic crust, which occurs through the upwelling of magma at mid- ocean ridges and its subsequent outward movement on either side

Question 10

What is palaeomagnetism

Answer 10

• A record of changes in the magnetic field of the Earth, this is shown in the orientation of the magnetic held in the rocks
• It provides evidence that the sea floor has gradually moved apart at a mid ocean ridge because the minerals in the new igneous rock align themselves with a polarity after solidifying.

Question 11

Subduction zones and slab pull

Answer 11

• A subduction zone is formed when two plates meet
• This is when an oceanic plate is forced below continental plate, due to the oceanic plate being denser than the continental plate. This occurs at destructive margins.
• Slab pull is the pulling force exerted by a cold, dense oceanic plate plunging into the mantle due a to its own weight. This also occurs at destructive margins.

Question 12

What happens at a convergent boundary (oceanic and continental)

Answer 12

When an oceanic and a continental plate meet:
- Oceanic plate and continental plates meet and the denser, heavier oceanic plate subducts under the lighter, less dense continental plate
- This forms deep ocean trenches (Deep sea trenches are long, narrow depressions in the ocean floor with depths of over 6km and up to 11km) in the subduction zone
- Both violent volcanic eruptions (often rhyolitic magma) and earthquakes occur at this type of plate boundary
- The narrow area where earthquakes tend to occur in the subduction is known as the Benioff Zone
- Oceanic and continental convergent plate boundaries are also responsible for fold mountains (long narrow belts of mountains)

Question 13

What happens at a convergent boundary (oceanic and oceanic)

Answer 13

• The heavier of the two oceanic plates subduct, forming deep ocean trenches and island arcs
• Island arcs are a series of volcanic islands, formed in an arc shape, e.g. the Caribbean
• Submarine volcanic eruptions, lead to crust building up and rising above sea level

Question 14

What is the Benioff zone

Answer 14

an area where friction is created between colliding tectonic plates, resulting in intermediate and deep earthquakes

Question 15

What happens at a divergent boundary

Answer 15

• The Mid-Atlantic Ridge is an example of a constructive plate boundary
• Both volcanic eruptions and earthquakes can occur at this type of plate boundary, however the eruptions are small and effusive, as the erupted basalt lava has a low gas content and high viscosity
• When two oceanic plates meet there are minor, shallow earthquakes
• When an oceanic and a continental plate meet there are basaltic volcanoes and minor earthquakes

Question 16

What happens at a collisional boundary

Answer 16

At a collision boundary two plates of similar density move towards each other
- When two continental plate boundaries meet, both may fold and deform; e.g. the Himalayas are formed by the collision of the Eurasian and Indian plates
- As neither plate can sink into the denser rocks below, they are crushed, crumpled and forced upwards, usually folding in the process
- This creates collision fold mountains such as the Himalayas, which are still being formed upwards, at a rate of 1cm/annum
- As there is no subduction, there is no volcanic activity
- Earthquakes are the main hazard at this type of plate bound

Question 17

What happens at a conservative/transform boundary

Answer 17

Plates move slowly past each other – they do not have to be in different directions but at a different rate of travel in the same direction
- Transform margins are offset at angles, creating zigzag patterns to accommodate movement
- They become stuck and pressure builds, the plates eventually ‘snap’ past each other
- These can be called ‘strike-slip’ faults as they strike/stick and then slip/release past each other
- The friction causes earthquakes but not volcanoes because no subduction occurs, there is no melting of the crust and so no volcanic activity
- Land is neither made or destroyed

Question 18

What kind of earthquakes occur at each plate boundary

Answer 18

Constructive: Shallow focus, usually low magnitude (5-6), This is the same for oceanic + oceanic and continental + continental.
Destructive:
Oceanic + Continental: Range of focal depths from shallow to 700km along the Benioff zone. Often high in magnitude (8-9).
Oceanic + Oceanic: Range of focal depths from shallow to 700km along the Benioff zone. Moderate to high magnitude (7-9).
Collisional: Shallow to intermediate focal depth. Moderate magnitude (6-8).
Transform: Shallow focus, moderate magnitude. (6-8)

Question 19

What is the focus and epicentre of an earthquake

Answer 19

The focus is the point underground where the earthquake originates from.
The epicentres the area above ground directly above the focus.

Question 20

What kind of volcanoes occur at each plate boundary

Answer 20

Constructive: Effusive eruptions, low VEl scale (1-3).
Destructive:
Oceanic + Continental: Explosive, moderate to high VEl scale (5-6).
Oceanic + Oceanic: Explosive, moderate to high VEl scale (5-6).
Collisional: Usually none.
Transform: Usually none.

Question 21

What are the two types of volcanoes

Answer 21

Composite volcanoes:
- Wide base and a narrow top
- Have ash clouds and pyroclastic flows
- Occurs at subducting plate boundaries

Shield Volcanoes:
- Form at a hotspot where plates are pulling apart.
- Wider than they are tall.
- Not explosive.
- Much gentler slope on sides

Question 22

Types of magma

Answer 22

Basaltic
- low silica content
- high temperature
- low viscosity meaning easy gas escape causing gentle eruptions

Andesitic
- medium silica content
- medium temperature
- medium viscosity meaning medium gas escape causing medium strength eruptions

Rhyolitic
- high silica content
- low temperature
- high viscosity meaning difficult gas escape causing explosive eruptions