5.2: The Theory of Plate Tectonics

Question 1

What is the core?

Answer 1

Made up of dense ricks containing iron and nickel alloys. Divided into inner and outer core.

Question 2

What’s the difference between the inner and outer core?

Answer 2

The inner core is solid. The outer core is molten and has a temperature of over 5000*C.

Question 3

How is the heat in the outer core produced?

Answer 3

Due to primordial heat (left over from the Earth’s formation).
And radiogenic heat (produced from radioactive decay of isotopes).

Question 4

What is the mantle?

Answer 4

Made up of molten and semi-molten rocks containing lighter elements eg oxygen.

Question 5

How is the crust lighter than the mantle?

Answer 5

Because of the elements that are present eg potassium.

Question 6

How thick is the crust beneath oceans?

Answer 6

6-10km.

Question 7

How thick is the crust beneath continents?

Answer 7

30-40km.

Question 8

How thick is the crust beneath mountain ranges?

Answer 8

70km

Question 9

What is the aesthenosphere?

Answer 9

Lies beneath the lithosphere. Is semi-molten on which the plates float and move.

Question 10

What is the lithosphere?

Answer 10

Consists of the crust and the rigid upper section of the mantle. Around 80-90km thick. This is divided into 7 v large plates and a number of smaller ones.

Question 11

What did Alfred Wegener propose?

Answer 11

That a supercontinent existed about 300m years ago (Pangea), which later split into Gondwanaland and Laurasia. Today’s continents were formed from further splitting of these land masses.

Question 12

What is the geological evidence of Wegener’s theory?

Answer 12

• fit of South America and west Africa.
• rock sequences in northern Scotland closely matching those found in eastern Canada, indicating that they were laid down under the same conditions in one location.
• evidence of a late Carboniferous glaciation, deposits from which are found in Antarctica, America and India.

Question 13

What is the biological evidence for Wegener’s theory?

Answer 13

• fossil remains of the reptile: mesosaurus found in both South America and Southern Africa. It’s unlikely that the same reptile could have developed in both areas or could have migrated across the Atlantic.
• the fossilised remains of a plant which existed when coals was being formed have been located only in India and Antarctica.

Question 14

What did Harry Hess find in 1962?

Answer 14

The youngest rocks were in the middle of the floor of the Atlantic Ocean, and oldest near the USA and Caribbean.

This provides evidence for seafloor spreading (the Atlantic sea floor was spreading outwards from the centre).

Question 15

What conforms Hess’ idea of sea floor spreading?

Answer 15

Paleomagnetism.

Whereby every 400,000 years, the Earth’s magnetic field switches polarity, causing the magnetic north and south poles to swap.

Sea floor spreading is shown from mirror imaged patterns of switches of magnetic north and south.

Question 16

How is the paleomagnetism that provides evidence for sea floor spreading recorded?

Answer 16

Iron oxide in lava erupted onto the ocean floor records the Earth’s magnetic orientation of that time.

Question 17

Seafloor spreading and palaeomagnetism would suggest that the Earth is getting bigger. So what evidence is used to explain this?

Answer 17

The discovery of deep ocean trenches - where the ocean floor is being pulled downwards (subducted), and destroyed.

Question 18

What is the main difference in structure between continental and oceanic trenches?

Answer 18

Continental - mainly sial.

Oceanic - mainly sima.

Question 19

Which is older: continental or oceanic?

Answer 19

Continental (1500m years old) - oceanic is only 200m years old.

Question 20

Why don’t continental plates sink into the aesthenosphere?

Answer 20

Due to their relatively low density.

Question 21

Why don’t oceanic plates last as long as continental plates?

Answer 21

Because they’re denser - so continually being formed at mid-ocean ridges and destroyed and deep ocean trenches.

Question 22

How are convection currents created?

Answer 22

Hotspots around the core heat the lower mantle, creating convection currents which rise towards the surface before spreading in the aesthenosphere, then cooling and sinking again.

This circulation of heat (magma) causes crustal plates to move.

Question 23

What are the two types of plate boundaries?

Answer 23

Constructive (plates moving away from each other)

Destructive (plates moving towards each other)

Question 24

Bearing in mind that no gaps can occur between plates, what happens if plates are moving apart?

Answer 24

New oceanic crusts must be formed.

Question 25

Name the landforms associated with plate movement.

Answer 25

• ocean ridges
• rift valleys
• deep sea trenches
• island arcs
• young fold mountains
• volcanoes

Question 26

Outline ocean ridges as a landform associated with plate movements.

Answer 26

Formed when plates move apart in oceanic areas.

So the space between these plates is filled with basaltic lava upwelling from below, to form a ridge.

Question 27

Outline rift valleys as landforms, associated with plate movements.

Answer 27

Plates move apart on continental areas because the lithosphere has stretched, causing it to fracture into sets of parallel faults. The land between these faults then collapse into deep, wide valleys that are separated by HORSTS (the area between two parallel rift valleys, forming an upstanding block).

Question 28

What landform is a Horst associated with and why?

Answer 28

Rift valleys - they form an upstanding block between two parallel rift valleys

Question 29

Outline deep sea trenches as a landform associated with plate movement.

Answer 29

Where oceanic and continental plates meet, the denser oceanic plate is forced beneath the lighter continental one (subduction) - forming a trench. Eg Peru-Chile trench.

Question 30

Outline how fold mountains can form from an oceanic and continental plate meeting.

Answer 30

As the two plates converge, the continental land mass is uplifted, compressed and folded into chains of fold mountains eg the Andes.

As compression continues, simple folding can become asymmetrical, then overfolded (making a recumbent fold). Increasing the compression further would make the middle section so thin that it might break, creasing a nappe.

Question 31

What is a nappe?

Answer 31

When fold mountains are overfolded (further from recumbent fold), making the middle section so thin that it might break and creating a nappe.

Question 32

How can continental and oceanic plates meeting cause an earthquake?

Answer 32

The descending oceanic plate starts to melt at depths beyond 100km (entering the Benioff zone) - and the melting is caused by friction. This friction may lead to tension (stresses) building up, which may suddenly be released as intermediate or deep-focus earthquakes.

Question 33

How can a continental and oceanic plate converging cause a volcanic eruption?

Answer 33

The melted oceanic plate creates magma, which is less dense than the surrounding asthenosphere and as a result, rises in great plumes. Then, passing through cracks in the buckled continental plate, the magma may eventually reach the surface to form explosive volcanic eruptions.

Question 34

What happens when two oceanic plates meet?

Answer 34

When they collide, the denser plate subducts beneath the other, leading to the formation of a deep ocean trench.

Question 35

What happens when two continental plates meet?

Answer 35

Because continental plates are of lower density than the asthenosphere beneath them, subduction doesn’t occur. The colliding plates simply become uplifted and buckle to form high fold mountains.

Question 36

Why does volcanic activity occur when two continental plates meet?

Answer 36

Because there is no subduction (as continental plates are of lower density than the asthenosphere beneath them).

Question 37

It’s now believed that plate movement is too complex to be explained by convection currents, so has been replaced by the idea of ‘gravitational sliding’. Explain this in constructive boundaries.

Answer 37

The upwelling of hot material at ocean ridges generates a buoyancy effect that produces the ocean ridge which stands 3km above the ocean floor. Here, oceanic plates experience a force that acts away from the ridge (aka gravitational sliding), which is the result of gravity acting down on the slope of the ridge.

Question 38

Outline gravitational sliding at destructive plate boundaries.

Answer 38

There is a downward gravitational force acting on the cold and dense descending plate as it sinks into the mantle. This gravity generated force pulls the whole oceanic plate down as a result of the negative buoyancy of the plate - slab pull.