Destructive, constructive and conservative plate margins

Question 1

Where do volcanoes/earthquakes mainly occur?

Answer 1

Along plate boundaries.
Where magma can escape the earth’s mantle.
Where stresses build up from two plates rubbing against each other.

Question 2

What features form at constructive (divergent) margins?

Answer 2

Oceanic ridges.

Rift valleys.

Question 3

How are oceanic ridges formed?

Answer 3

Oceanic/oceanic.
Two plates move apart (DIVERGE) - convection currents moving in opposite directions.
Fissures (cracks) are created.
Magma fills the gap.
Eventually erupts onto the surface to create new land.
Andesitic lava - can travel long distances.
Can be 60,000km long.
Can rise 3,000m above the ocean floor.
Transform faults occur at right angles to the plate boundary.

Question 4

What is an example of an oceanic ridge?

Answer 4

Mid-Atlantic Ridge.
Eurasian plate moves away from the North American plate.
4cm per year.
E.g. Iceland.
Surtsey 1963-67 eruption - Jolnir island created in 1965.
Proves new material is being created.

Question 5

What does a slow of rate of diverging plates (10-15mm per year) produce?

Answer 5

As seen in parts of the Mid-Atlantic Ridge.
Wide axis (30-50km).
Deep (3,000km) central rift valley with inward-facing fault scarps.

Question 6

What does an intermediate of rate of diverging plates (50-90mm per year) produce?

Answer 6

Galapagos ridge (Pacific).
Less well-marked rift (50-200m deep).
Smoother outline.

Question 7

What does a rapid of rate of diverging plates (90mm+ per year) produce?

Answer 7

East Pacific rise.
Smooth crest.
No rift.

Question 8

What occurs along the oceanic ridge? 4

Answer 8

1. Volcanic activity - forms submarine volcanoes.
   Can sometimes rise above sea level e.g. Surtsey.
   Gentle sides, low viscosity, basaltic lava, effusive.
2. Abyssal plain - underwater plain of deep ocean floor (3,000-6,000km deep).
3. Axial rift - valley occurring at slow rate spreading.
4. Transform faults - leads to earthquakes.
   Due to two parts moving at different rates - friction.

Question 9

How are rift valleys formed?

Answer 9

Continental/continental.
DIVERGING - plates are moving away.
Brittle crust fractures as sections move apart.
Areas of crust drop down between parallel faults.

Question 10

What is an example of rift valleys?

Answer 10

Great African Rift valley in Eastern Africa.
The Eastern section is moving north east whilst the Western section is moving West and North West.
It extends 4,000km from Mozambique to the Red Sea.

Question 11

What occurs in rift valleys?

Answer 11

Volcanic activity - Mount Kilimanjaro.
Thinner crust - lithosphere tension - thinner plate as it starts to split.
African rift line - emergent plate boundary - a new ocean will form as eastern Africa splits away from the rest of the continent.

Question 12

What features form at destructive (convergent) margins? 3

Answer 12

Oceanic trenches.
Young fold mountains.
Island arcs.

Question 13

How are oceanic trenches formed?

Answer 13

Oceanic/continental, oceanic/oceanic.
Denser oceanic plate subducts under the lighter continental plate.
Oceanic plate subduction forms a very deep part of the ocean - trench.
As it descends friction, increasing pressure and heat from the asthenosphere (and mantle) melts the plate.

Question 14

What is an example of an oceanic/continental oceanic trench?

Answer 14

Peru-Chile trench.
Nazca plate subducts under the South American plate.
160km deep.

Question 15

What is an example of an oceanic/oceanic oceanic trench?

Answer 15

Marianas trench.
Pacific plate subducts under the Philippine plate.
Beneath Peru - subducts 10-15 degrees.
Below Japan - subducts 40-45 degrees.

Question 16

How are young fold mountains formed?

Answer 16

Oceanic/continental.
Accumulated sediment on the continental shelf are deformed by folding and faulting.
Continental crust is folded up (anticlines) and down (synclines).
Continental plate edge - uplifted to form YFM.

Question 17

What is an example of YFM?

Answer 17

The Andes Mountains.
South American plate is crumpled up above the subducting Nazca plate.
7,000 km long.
200-700 km wide.
Average height - 4,000 m.

Question 18

How are island arcs and volcanoes formed?

Answer 18

More subduction - more heat.
Subduction friction heat - melts the oceanic plate into magma - the Benioff zone.
Less dense than the surrounding asthenosphere - begins to rise as magma plutons.
Eventually reaches surface - forms volcanoes.
Andesitic lava/explosive.
Offshore eruptions - a line of volcanic islands - island arcs appear.

Question 19

What is an example of an island arc?

Answer 19

The Philippines.
Oceanic trench on the convex side.
Series of volcanoes.
E.g. Mount Pinatubo 1991 eruption.

Question 20

What features are at conservative margins?

+ EXAMPLE.

Answer 20

NO subduction - NO volcanic activity.
Friction is created - shallow earthquakes.
E.g. San Franciso 1906.
San Andres Fault - the Pacific and North American plates move parallel to each other.
Transform faults can develop due to different plate speeds (despite same direction) - at right angles.