1.b. There are distinctive features and processes at plate boundaries.

Question 1

Even when continental drift theories were growing in popularity in the 1950s, there was still little understanding of what?

(The global pattern of plates and plate boundaries)

Answer 1

There was still little understanding of the tectonic plates.

Question 2

What was picked up by seismometers in the 1960s? What did this help produce? What occurred in between the bands?

(The global pattern of plates and plate boundaries)

Answer 2

Nuclear tests in the 1960s were picked up by seismometers, helping produce detailed maps from seismic data worldwide.

These maps showed the most earthquakes were spatially concentrated in narrow bands.

In between the bands of mapped earthquakes were large areas that generated few earthquakes (aseismic areas).

This pattern suggested that the rigid lithosphere and crust were broken up into tectonic plates.

Question 3

When 1960s seismometer maps suggested that the rigid lithosphere and crust were broken up into tectonic plates. Were these static?

(The global pattern of plates and plate boundaries)

Answer 3

These giants slabs of lithosphere and crust were found to not be static: in some places they were diverging, in others they were converging.

Question 4

What does the current global pattern of tectonic plates show?

(The global pattern of plates and plate boundaries)

Answer 4

7 major plates and numerous minor ones.

Question 5

Are tectonic plate boundaries still being researched? Why?

(The global pattern of plates and plate boundaries)

Answer 5

Yes, the subject of plate boundaries is still being heavily researched.

In some regions, such as the Mediterranean, the plates and their boundaries are very complex and still unclear.

Question 6

State all 7 tectonic plates.

(The global pattern of plates and plate boundaries)

Answer 6

• African.
• Eurasian.
• Pacific.
• Indo-Australian.
• North American.
• South American.
• Antarctic.

Question 7

What are divergent plate boundaries?

(Divergent Plate Boundaries)

Answer 7

Locations where plates are diverging are where magma is rising through the asthenosphere and forcing its way to the surface.

The two plates diverge (move away from each other).

Question 8

What are mid-ocean ridges?

(Mid-ocean ridges and transform faults)
(Divergent Plate Boundaries)

Answer 8

Mid-ocean ridges are spectacular relief features, but remain hidden at an average depth of 2.5 km below the ocean surface.

Question 9

What is especially notable at mid-ocean ridges?

(Mid-ocean ridges and transform faults)
(Divergent Plate Boundaries)

Answer 9

Plumes of rising material.

Question 10

Mid-ocean ridges consist of what?

(Mid-ocean ridges and transform faults)
(Divergent Plate Boundaries)

Answer 10

Consist of very long chains of mountains, in places rising 3000m above the sea bed; with submarine mountain chains of 60,000 km.

Question 11

Mid-ocean ridges are not continuous. What frequently breaks them up? By how much?

(Mid-ocean ridges and transform faults)
(Divergent Plate Boundaries)

Answer 11

They are frequently broken into segments by transform faults.

These transform faults displace the ridge laterally by 10s-100s of kilometres.

Question 12

Volcanic activity is absent along transform faults. What is seen instead?

(Mid-ocean ridges and transform faults)
(Divergent Plate Boundaries)

Answer 12

Volcanic activity is absent along transform faults, but as they slip, energy is released in the form of earthquakes.

Question 13

Mid-ocean ridges vary in what? What determines this?

(Mid-ocean ridges and transform faults)
(Divergent Plate Boundaries)

Answer 13

Vary in topography depending on the rate of spreading.

This is determined by the amount and rate of magma brought to the surface by convection currents.

Question 14

How are underwater rift valleys formed?

(Underwater rift valleys)
(Divergent Plate Boundaries)

Answer 14

As magma rises towards the surface, the pressure reduces and it liquefies.

This force overlying rocks upwards, as the lithosphere is placed under stress, fracturing.

This produces the underwater rift valleys, along mid-ocean ridges.

Question 15

What are pillow lavas? How do they form?

(Pillow lavas)
(Divergent Plate Boundaries)

Answer 15

Rounded mounds of erupted material.

Magma eruption along divergent boundaries occurs mostly underwater; on the sea bed this cools rapidly, forming pillow lavas.

Question 16

What are black smokers? How do they form?

(Black smokers)
(Divergent Plate Boundaries)

Answer 16

Hydrothermal vents located on the seafloor, characterised by high-temperature, mineral-rich fluids that are released from the Earth’s interior. They support unique and highly specialised organisms and ecosystems.

At mid-ocean ridges, sea water seeps into rifts and is superheated.

As it rises towards the surface it causes chemical changes in the basaltic rocks.

Superheated jets of water may re-emerge on the ocean floor containing metal sulphides.

Question 17

How can rift zones give reason to continent break up? What must crust be like?

(Rifting away from mid-oceanic ridges)
(Divergent Plate Boundaries)

Answer 17

Rift zones happen on the ocean floor and on land, in part explaining how continents can break up.

Continental crust must thin considerably for rifting to occur.

Question 18

Outline the Red Sea Rift.

(Red Sea Rift)
(Rifting away from mid-oceanic ridges)
(Divergent Plate Boundaries)

Answer 18

Stretches from the Red Sea northwards to Turkey.

It has been uplifted and stretched, causing faulting and forming a graben (sunken valley).

As the rift widened, magma erupted at the surface.

Eventually the rift valley sank below sea level, forming the present day Red Sea.

If the crust continues to thin, magma will well up to form a new boundary between Africa and the Arabian Peninsula.

North in Israel, the rift dips below sea level, forming the Dead Sea.

Question 19

What are the 3 types of convergent plate boundary?

(Convergent Plate Boundaries)

Answer 19

• Oceanic-continental.
• Continental-continental.
• Oceanic-oceanic.

Question 20

What is slab pull?

(Convergent Plate Boundaries)

Answer 20

Where the oceanic crust descends down into the asthenosphere, it pulls the rest of the oceanic plate with it.

Question 21

What is subduction?

(Convergent Plate Boundaries)

Answer 21

When tectonic plates of different densities converge, leading to the denser oceanic plate being forced under the continental plate.

Question 22

What happens at oceanic-continental convergent plate boundaries?

(Oceanic-Continental)
(Convergent Plate Boundaries)

Answer 22

The tectonic plates are of different densities which leads to the denser oceanic plate being forced under the continental plate when they converge; (subduction).

Subduction causes a deepening of the ocean at the plate boundary and forms and ocean trench.

Question 23

What are ocean trenches? How are they formed?

(Ocean trenches)
(Oceanic-Continental)
(Convergent Plate Boundaries)

Answer 23

Ocean trenches are long, narrow depressions with depths of 6000m - 11,000m.

They mark the zone of subduction where the oceanic crust descends into the asthenosphere, pulling the rest of the oceanic plate with it, (slab pull).

As it does so, the leading edge of the overriding continental plate is buckled to form a trench.

Question 24

What are mountain chains? How are they formed?

(Mountain chains)
(Oceanic-Continental)
(Convergent Plate Boundaries)

Answer 24

Layers of sediment and sedimentary rock develops on oceanic plates adjacent to continents.

The sources of these sediments are from cliff erosion and fluvial (river) sources.

As the plates converge, these sediments and rocks crumple, fold and are uplifted.

Significant amounts of molten material are injected into it.

This results in mountain chains, e.g. the Andes.

Question 25

What is the Benioff zone?

(Oceanic-Continental)
(Convergent Plate Boundaries)

Answer 25

The boundary between a subducting oceanic plate and the over-riding continental plate at a destructive boundary.

Question 26

How are earthquakes formed on oceanic-continental convergent plate boundaries?

(Earthquakes)
(Oceanic-Continental)
(Convergent Plate Boundaries)

Answer 26

Oceanic plates are subducted at an angle between 30° and 70°.

As it descends, the oceanic plate comes under intense pressure and friction.

Faulting and fracturing occur in the Benioff zone, where the descending plate is at an angle close to 45°.

This energy is released as earthquakes.

Shallow earthquakes are normally the most devastating.

Question 26

How are volcanoes formed on oceanic-continental convergent plate boundaries?

(Volcanoes)
(Oceanic-Continental)
(Convergent Plate Boundaries)

Answer 26

Oceanic plates contain substantial amounts of water.

As subduction takes place and pressure and temperature increase, dehydration occurs.

This triggers processes that partially melt the overlying plate and this molten material rises towards the surface as plutons of magma.

Huge intrusions of magma create further uplift of fold mountains and where rising magma reaches the surface it forms volcanoes.

Question 27

What happens when two oceanic plates converge?

(Oceanic-Oceanic)
(Convergent Plate Boundaries)

Answer 27

When two oceanic plates meet, the slightly older and therefore denser one will subduct, creating a trench.

Dehydration of the subducting plate takes place and leads to partial melting.

This magma rises to the surface and forms chains of volcanic islands known as island arcs.

E.g. in the Atlantic, the North American plate is subducted below the Caribbean plate, forming island arcs such as the Antilles.

Question 28

What is extremely common on oceanic-oceanic convergent plate boundaries?

(Oceanic-Oceanic)
(Convergent Plate Boundaries)

Answer 28

Large earthquakes.

Question 29

Where are oceanic-oceanic convergent plate boundaries mainly found?

(Oceanic-Oceanic)
(Convergent Plate Boundaries)

Answer 29

Found around the Pacific.

80% of earthquakes, and many volcanoes, are located in these regions.

Question 30

Give an example of of an island arc and an ocean trench.

(Oceanic-Oceanic)
(Convergent Plate Boundaries)

Answer 30

The Northwest zone of the Pacific ‘Ring of Fire’ contains island arcs such as the Aleutian Islands, which extend westwards from Alaska.

Nearby is the Challenger Deep, the deepest part of the ocean and part of the Mariana Trench (11,000m deep). Here the Pacific plate is subducted beneath the Philippine plate.

Question 31

What happens when two continental plates converge?

(Continental-Continental)
(Convergent Plate Boundaries)

Answer 31

When two continental plates converge, little if any subduction takes place.

This is because the two plates have similar densities.

Question 32

Give an example of where two continental plates converge.

(Continental-Continental)
(Convergent Plate Boundaries)

Answer 32

E.g. in Europe, the collision of the African and Eurasian plates over the past 40 million years has created the Alps.

E.g. the Indo-Australian and Eurasian plates converging has formed the Himalayas and Mount Everest.

Question 33

What happens at transform plate boundaries?

(Transform Plate Boundaries)

Answer 33

Plates slide past each other.

Question 34

What is absent on transform plate boundaries? Why?

(Transform Plate Boundaries)

Answer 34

Volcanic activity is absent at these plate margins because no subduction takes place.​

Question 35

How are earthquakes formed on transform plate boundaries?

(Transform Plate Boundaries)

Answer 35

Frictional resistance to movement along the plate boundaries often causes the build-up of pressure.

These pressures cause rocks to fracture, releasing enormous amounts of energy, causing earthquakes.

Question 36

Transform plate margins do not have spectacular landforms. What is an exception? Give an example.

(Transform Plate Boundaries)

Answer 36

In some places, a giant tear can mark the location of the active boundary.

Rivers can also be diverted by movements along these faults.

An example of a transform plate boundary is in California, where the North America and Pacific plates are moving in the same direction but at different speeds.