HAZARDS : Types of plate margin

Question 1

What is a constructive plate margin?

Answer 1

When two plates are moving apart.

Question 2

What are constructive margins also known as?

Answer 2

Diverging plate boundaries.

Question 3

What hazards can occur at constructive plate margins?

How is each formed?

Answer 3

Volcanes - mantle is under pressure from plates above, and when they move apart, the pressure is released at the margin. Mantle melts, forming magma which is less dense than the plate above - it rises and can erupt, forming a volcano.

Earthquakes - plates don’t move apart in a uniform way - some move faster than others, so pressure builds up. This can cause the plate to crack, making a fault line and causing an earthquake.

Question 4

What two landforms do constructive plate margins create?

Answer 4

Ocean ridges and rift valleys.

Question 5

How are ocean ridges formed?

Answer 5

• As tectonic plates diverge, basaltic lava rises up to fill gap created, then cools to form new crust.
• New crust is dragged apart and even more new crust forms between it.
• This is known as sea-floor spreading and can form a mid-ocean ridge.

Question 6

What hazard can form on ocean ridges?

Give an example.

Answer 6

Underwater volcanoes erupt along mid-ocean ridges and they can build up to be above sea level.

Build up of underwater volcanoes along the Mid-Atlantic Ridge led to the formation of Iceland.

Question 7

What is a fault line?

Answer 7

Where a plate has cracked under pressure.

Question 8

Describe the formation of a rift valley:

Answer 8

• At diverging plates, rising magma causes the continental crust to bulge and fracture, forming fault lines.
• Plates keep moving apart, and crust between parallel faults drops down to form a rift valley.

Question 9

At what type of plate margin are ocean ridges found?

Answer 9

Constructive margins.

Question 10

At what type of plate margins are rift valleys formed?

Answer 10

Constructive margins.

Question 11

Give an example of a rift valley:

Answer 11

East African Rift System - a series of rift vallyes that stretch from Mozambique to the Red Sea.

Formed due to Nubian plate and Somalian plates diverging.

Question 12

What type of hazard can be found in rift valleys?

Answer 12

Volcanoes.

Kilimanjaro and Mount Kenya found in East African Rift.

Question 13

The area between two parallel rift valleys that forms an upstanding block.

Answer 13

A horst.

Question 14

nm A horst.

Answer 14

The area between two parallel rift valleys that forms an upstanding block.

Question 15

The line of the African Rift is thought to be what?

Answer 15

An emergent plate boundary - the beginning of the new ocean, as eastern Africa splits away from the remainder of the continent.

Question 16

What is a destructive plate boundary?

Answer 16

Where two plates are moving towards each other

Question 17

What is a destructive plate boundary also known as?

Answer 17

A converging margin.

Question 18

What determines what happens at a destructive plate margin?

Answer 18

The types of plate involved in the boundary.

Question 19

What hazards are associated with destructive margins?

Answer 19

Volcanoes and earthquakes.

Question 20

What happens when an oceanic and continental plate converge?

Answer 20

• More dense oceanic crust is forced under the less dense continental crust - it is subducted.

Question 21

What landforms are created at destructive boundaries?

Answer 21

Deep sea trench or fold mountains (oceanic-continental).

Deep sea trench or island arcs (oceanic-oceanic).

Fold mountains (continental-continental).

Question 22

What type of margin are deep sea trenches found at?

Answer 22

Destructive

Question 23

How is a deep sea trench formed?

Answer 23

When the denser oceanic crust subducts under the less dense continental crust.

Question 24

How are fold mountains formed?

Answer 24

• By two converging plates.
• Edges and sediment bewteen the two plates are forced up into fold mountains.
• There is little subduction and no volcanic activity.

Question 25

On what type of margin are fold mountains formed?

Answer 25

Destructive.

Question 26

How do volcanoes form on destructive margins?

Answer 26

• Oceanic crust subducts under continental crust.
• Oceanic crust is heated by friction and contact with the upper mantle, which melts it into magma.
• Magma is less dense than the continental crust above and will rise back to the surface to form volcanoes.

Question 27

How do earthquakes form on destructive margins?

Answer 27

• As one plate moves under another it can get stuck.
• This causes pressure to build up.
• When pressure becomes too much, the plates jerk past each other, causing an earthquake.

Question 28

What happens when oceanic and oceanic plates converge?

Answer 28

The denser one of the two is subducted.

Deep sea trenches form, and earthquakes and volcanic eruptions can be triggered.

Question 29

How are island arcs formed?

Answer 29

• During subduction, the descending plate encounters hotter surroundings, and this coupled with the heat generated from friction, begins to melt the plate.
• As the material is less dense than the surrounding asthenosphere, it begins to rise towards the surface as plutons of magma.
• These reach the surface and form volcanoes.
• If eruptions take place off shore, a line of volcanic islands forms - known as island arcs.

Question 30

What happens when continental and continental plates meet at a destructive margin?

Answer 30

Neither is subducted.

Question 31

What landforms are created when continental and continental plates converge?

Answer 31

Fold mountains.

Eg. the Himalayas.

Question 32

What hazards are associated with continental-continental destructive plates?

Answer 32

Earthquakes, but not volcanoes as neither plate is subducted.

Question 33

What happens at a conservative plate margin?

Answer 33

Two plates are moving past each other.

Question 34

What hazards can occur at conservative plate margins?

Answer 34

Only earthquakes.

• Plates lock together in places and pressure builds up.
• This causes plares to jerk past each other (or crack, forming fault lines), releasing energy as an earthquake.

Question 35

Why did the San Andreas Fault form?

What two plates does it involve?

Answer 35

Because of the conservative plate boundary.

• Pacific plate is moving past the North American plate - both moving NW, but Pacific moves faster than North American.*
• Pacific plate moves due to sea floor spreading from East Pacific Rise (divergent) in Gulf of California.*
• North American plate pushed by sea floor spreading from Mid Atlantic Ridge (divergent).*

Question 36

What are hotspots?

Answer 36

The ground above a magma plume - areas of intense volcanic activity that aren’t near any plate boundaries.

Question 37

Who first proposed the hotspot theory?

What did they specifically hypothesise?

Answer 37

Wilson, in the 60s proposed that volcanic chains form when a tectonic plate drifts over a ‘hot spot’ mantle.

Specifically hypothesized the distinctive linear shape of Hawaiian Island-Emperor Seamounts chain resulted from pacific plate moving over deep stationary hotspot in mantle, located beneath present-day Hawaii.

Question 38

What did geophysicist Morgan suggest?

Answer 38

That hotspots were fueled by narrow plumes of hot mantle rock rising from the core-mantle boundary.

Question 39

Is the plume hypothesis ever been widely accepted?

Why/why not?

Answer 39

No; however is the most widely held explanation for anomalous volcanism.

Because no plume hat yet been found to satisfy all the criteria attributed to plumes.

Question 40

What criteria has been attributed to magma plumes in the hotspot theory?

Answer 40

• Fixed relative to one another.
• Produce time-progressive volcanic chains.
• Break up continents.
• Drive plate tectonics.

Question 41

What is the alternative to hotspot theory as an explanation for intraplate volcanicity?

Answer 41

A plate hypothesis - lithospheric streching allows already melted rock to escape from the mantle to the surface.

Proposed by Anderson et al (2000s).

Question 42

What is a magma plume?

Answer 42

A vertical column of extra-hot magma that rises up from the mantle.

Question 43

How do magma plumes form?

Answer 43

Generated in the lower mantle and rise slowly through the mantle by convection.

Question 44

What is said to be the shape of a magma plume?

How does the shape change when it reaches the lithosphere?

Answer 44

A bulbous plume head fed by a long, narrow plume tail.

At the lithosphere the shape spreads out to a mushroom shape.

Question 45

How do hotpots form intraplate islands?

Answer 45

• Hotspots may represent top of mantle plume.
• Mantle plumes are largely unaffected by plate motions and tend to remain stationary over time.
• As lithospheric plates move across the stationary hotspots, volcanism generates volcanic islands.
• These are active over the mantle plume.
• Become inactive when they move away from the mantle plume, due to plate movement.
• New oceanic lithosphere positioned over the hotspot and a new island will beign to form.
• Cycle repeats and island chain formed.

Question 46

Name and describe an example of an island chain:

Answer 46

Hawaiian Archipelago.

• Island of Hawaii is only one volcanically active.
• 7 islands that become progressively older to the NW.