5. Types of Plate Margin

Question 1

What type of processes happen at destructive plate boundaries?

Answer 1

• oceanic meets continental plate
• oceanic plate meets oceanic plate
• continental plate meets continental plate

Question 2

What are the key processes/ features when oceanic plate meets continental plate

Answer 2

• Oceanic plate is denser than continental plate, when plates collide the oceanic plate slides beneath the continental plate into the mantle and melts.
• Deep ocean trenches mark the place where the oceanic plate sinks beneath the continental.
• Subduction also creates fold mountains, as the plates collide, the continental plate is folded and slowly pushed up, forming fold mountains. These continue to grow as plates move towards each other.
• Friction from subduction causes intermediate and deep earthquakes in an area called the Benioff zone.
• Volcanic eruptions are also created as magma created by melting oceanic plates pushes up through faults in the continental crust to form explosive eruptions.

Question 3

What are the key processes/ features when oceanic plate meets oceanic plate?

Answer 3

• When two oceanic plates collide, the faster or denser plate subjects beneath the other.
• Leads to formation of a deep ocean trench and melting.
• Resulting rising magma from the Benioff zone forms crescents of submarine volcanoes along the plate margins which may grow to form island arcs.

Question 4

What are the key processes/ features when continental plate meets continental plate?

Answer 4

• Continental plates are of lower density than asthenosphere beneath, meaning subduction doesn’t occur.
• Colliding plates and any sediments deposited between them become uplifted and buckle to form fold mountains.
• Volcanic activity doesn’t occur -> no subduction but shallow earthquakes triggered.

Question 5

Global example of oceanic meets continental plate

Answer 5

Where the Nazca Plate (oceanic) meets the South American (continental)

Question 6

Global example of oceanic meets oceanic plate

Answer 6

The Mariana Trench and the Marianas Islands in the Western Pacific

Question 7

Global example of continental plate meets continental plate

Answer 7

The Himalayas- continually compressing and growing higher

Question 8

What types of processes happen on constructive plate boundaries?

Answer 8

• mid ocean ridges
• rift valley

Question 9

What are the key processes/ features on mid-ocean ridges

Answer 9

• Oceanic divergence forms chains of submarine mountain ridges that extend for 1000s of km across the ocean floor.
• If we could drain water from oceans, ridges would look like giant bending spinal cords
• Transform faults cut across ridges
• Faults occur at right angles to the plate boundary, separating sections of the ridge. May widen at different rates, leading to frictional stresses building up, with shallow focus earthquakes releasing tension.
• Mid ocean ridges can rise up to 4000m above the ocean floor.
• Middle of ridges is marked by deep rift valleys in all but the most rapidly separating plate margins found in the East Pacific.
• Over centuries, the rift valleys are widened by magma rising from asthensophere, which Colls and solidifies to form new crust.

Question 10

What are the key processes/ features of a rift valley?

Answer 10

• Formed from continental divergence
• Valleys are formed when lithosphere stretches, causing it to fracture into sets of parallel faults.
• The land between the faults collapses into deep, wide valleys separated by upright blocks of land called horsts.

Question 11

Global example of mid-ocean ridges

Answer 11

Volcanic eruptions can build submarine volcanoes. Over time, these may grow to rise above sea level, creating volcanic islands such as Ascension Island, Tristan da Cunha and Surtsey in the mid Atlantic Ocean.

Question 12

Global example of a rift valley

Answer 12

The Great African Rift Valley

Question 13

What are the key processes/ features of a conservative plate boundary?

Answer 13

• When two plates slide past each other.
• Crust isn’t being destroyed by subduction- there’s no melting of rock and therefore no volcanic activity or formation of new crust.
• Friction between two moving plates leads to stresses building up whenever any ‘sticking occurs’
• Stresses may be released as powerful shallow focus earthquakes.

Question 14

Global example of conservative plate boundary

Answer 14

San Andreas fault
Los Angeles
San Francisco

Question 15

How do the magma plumes work?

Answer 15

• Radioactive decay within the Earth’s core generates very hot temps.
• If the decay is concentrated, hot spots will form around the core.
• These hot spots heat the lower mantle, creating localised thermal currents where magma plumes rise vertically.
• Although usually found close to plate margins, such as beneath Iceland, these plumes occasionally rise within the centre of plates and then ‘burn’ through the lithosphere to create volcanic activity across the surface.
• As the hot spot remains stationary, the movement of the overlying plate results in the formation of a chain of active and subsequently active volcanoes as the plate moves away from the hotspot.