3.1. Plate Tectonics

Question 1

Why is the Earth’s crust is broken up into moving plates?

Answer 1

• Convection currents in the mantle
• Heat from the core heats the mantle which makes the magma rise
• This current cools down as it comes closer to the surface of the Earth
• As a result it moves in a horizontal direction along the bottom of the crust
• When the current cools down more, the convection current descends and go towards the core

Question 2

Oceanic Crust

Answer 2

• Denser, newer and thicker than continental

- Made of dark basaltic rocks

Question 3

Continental Crust

Answer 3

• Colder, thicker and less dense than continental

- Made of light-colored granite rocks

Question 4

Mantle

Answer 4

Zone of molten Silicates and other minerals. Molten so it moves, the source of this is the Earth’s intense heat which sets up convection currents in the mantle

Question 5

Outer Core

Answer 5

• Made of liquid Iron and nickel
• Source of decay due to radioactive elements
• under slightly less pressure

Question 6

Inner Core

Answer 6

• Made of solid iron and nickel

- Solid, despite temperatures of 3700 degrees celsius because of the intense pressure there.

Question 7

Divergent Boundaries

Answer 7

• Occurs where 2 plates move away from each other

- Rifting occurs due to the plates moving apart.

Question 8

Examples of Divergent Boundaries

Answer 8

• oceanic + oceanic - Mid-Atlantic Ridge - American plates are moving away from the Eurasian and African plates (all oceanic crust sea-floor spreading)
• continental + continental - East African Rift Valley

Question 9

Convergent Boundaries

Answer 9

• Occurs where 2 plates converge (oceanic + continental or oceanic + oceanic)

Question 10

Examples of Convergent Boundaries

Answer 10

• oceanic + continental - Nazca plate (oceanic) and South American plate (continental) creating the Andes mountains
• oceanic + oceanic - Japan

Question 11

Collision Boundaries

Answer 11

• Occurs where 2 plates of continental crust converge

Question 12

Examples of Collision Boundaries

Answer 12

Indian plate collides with Eurasian plate (both continental crust) forming the Himalayas

Question 13

Conservative Boundaries

Answer 13

• Occurs where 2 plates move parallel to each other, land is neither created or destroyed

Question 14

Examples of Conservative Boundaries

Answer 14

North American plate and Pacific plate - The San Andreas fault

Question 15

Earth Structure

Answer 15

Crust 
Lithosphere 
Asthenosphere
Mantle 
Outer core 
Inner core

Question 16

Lithosphere

Answer 16

the rigid outer part of the earth, consisting of the crust and upper mantle.

Question 17

Asthenosphere

Answer 17

Semi-molten and what tectonic plates float on

Question 18

Thickness of continental crust

Answer 18

35-70km average

Question 19

Thickness of oceanic crust

Answer 19

6-10km average

Question 20

Nature of continental crust

Answer 20

light - silica and aluminium

Question 21

Nature of oceanic crust

Answer 21

dark - mainly basalt

Question 22

Mechanisms of plate movement

Answer 22

Ridge push, Convection drag, Slab pull

Question 23

Ridge Push

Answer 23

Intrusion of magma into spreading ocean ridges pushes plates apart

Question 24

Convection Drag

Answer 24

Convection currents in the mantle drags the crust

Question 25

Slab Pull

Answer 25

Cold, denser oceanic lithosphere sinks due to gravity, and drags the rest of the plate with it

Question 26

Sea-floor Spreading

Answer 26

• There is upwelling of magma as it pushes up through the Earth’s mantle and breaks through the crust at the mid-ocean ridge, resulting in doming of the surface
• The magma hardens to form new ocean crust
• The new ocean crust splits apart the old crust and pushes both halves away from the ridge
• The old crust gradually moves further away from the mid-ocean ridge, and the ocean floor gets wider
• The rock furthest away from the ridge is the most heavy, and so pulls the rest of the plate with it - slab pull

Question 27

Subduction

Answer 27

• Sinking of a plate beneath another
• Oceanic plates are heavier than continental plates. Zones usually dip at angles 30-70 but older, heavier plates dip more steeply.

Question 28

Fold mountains

Answer 28

• When plates move towards each other they are forced up as they collide
• The lighter plate buckles up and folds

Question 29

Ocean ridges

Answer 29

• The longest linear, uplifted features of the Earth’s surface
• Magma rises as the plates move away from each other and cool when it reaches the surface
• Convection currents cause the uplifting of ocean floors

Question 30

Ocean Trench

Answer 30

• Long, narrow depressions in the ocean floor with depths of over 6000m and up to 11,000m
• When plates converge, denser plates are pushed beneath the lighter plate and deep into the mantle

Question 31

Volcanic island arcs

Answer 31

• A series of volcanic islands
• When lithosphere is subducted the slab melts when the edges reach a depth which is sufficiently hot. Hot, remelted material from the subducting slab rises and leaks into the crust, forming a series of volcanoes.

Question 32

Processes at Convergent Boundaries

Answer 32

• two plates converge: an oceanic plate and continental plate and there is subduction of the oceanic plate under the continental plate
• at the point of subduction, an ocean trench is created on the ocean floor
• the descending ocean plate is heated by friction created at the subduction zone and melts due to heat and increasing pressure in the mantle
• the newly formed magma is less dense than the surrounding material and therefore rises through weaknesses towards the surface - where it rises through cracks in the crust

Question 33

Processes at Collision boundaries

Answer 33

• 2 plates of continental crust move together - both plates are rigid, low density and no subduction occurs
• Forces of collision causes the crust to deform and any ocean sediments between the two plates are forced up
• This results in Orogenesis as the folding and faulting creates fold mountains
• No volcanoes due to the absence of a magma source due to lack of subduction