2.2 Plate Tectonics

Question 1

What is lithosphere?

Answer 1

The solid upper portion of the mantle and crust
Most rigid of earth’s layers
Coolest of earth’s layers
Divided into the world’s tectonic plates

Question 2

What are plate tectonics?

Answer 2

1. The earth is broken up into a number of lithospheric plates
2. The plates move via a process called continental drift
3. There are 3 types of plate boundaries:
   A) Conservative - sliding past each other
   B ) Convergent - moving towards each other
   C) Divergent- moving away from each other

Question 3

What is the earth’s structure?

Answer 3

Concentric structure based on chemical properties and mechanical behaviour

Question 4

What are the earth’s chemical properties?

Answer 4

Crust
Mantle
Core

Question 5

What is the earth’s mechanical behaviour?

Answer 5

The lithosphere is underlaid by asthenosphere.
The lithosphere is divided into tectonic plates which move relative to one another by mechanisms not yet completely understood

Question 6

What is the asthenosphere?

Answer 6

Composed of upper mantle
Weaker layer
Sits below the lithosphere
A ductile layer of the mantle which can flow allowing the plates to move

Question 7

What are the key developments in the history of plate tectonics?

Answer 7

1912- Wegener releases ‘continental drift’ articles - saw a jigsaw
1915 - Releases Origin of Continents and Oceans - suggests drift from fossil similarity
1959 - Hess releases ‘history of ocean basins’ - sea floor spreads to create flat top mountains called guyots- thought all crust moved rather than individual sections
1963 - Tested sea floor spreading causing switch of magnetic fields
1963 - Tuzo - Wilson suggested mantle hot spots causing volcanic island chains like Hawaii
1965 - Suggests conservative plate boundaries eg San Andreas fault
1966 - Mackenzie released ‘the viscosity of the lower mantle’ controlling plate tectonics
1993 - Wilson suggests slab pull force as primary force of plate motion
2002 - Conrad and lithgow bertelloni support slab pull dominance
2006 - New sea spread idea of symmetrical and asymmetrical spreading to different ridges

Question 8

Where are the continental divergent plate boundaries?

Answer 8

Mid Atlantic ridge
East African Rift Valley leading across Indian Ocean
Boundary between Pacific plate and Nazca plate and Antarctic plate

Question 9

How do divergent plate boundaries work?

Answer 9

Rising hot material and extensional forces cause crust and lithosphere to become thinner
Volcanoes are produced due to rising magma from partial melting of mantle and mafic magma and basalt lava rises
These are footwall upthrown normal faults - FUN faults

Question 10

Name the features that appear in oceanic divergent plate boundaries?

Answer 10

A layer of sediment 
Basalt Pillow lavas
Dolerite dykes
Homogeneous gabbro 
Layered gabbro
Mantle peridotite

Question 11

Explain the features of sediment in divergent boundaries?

Answer 11

Sediments are thinner near ridge where crust is younger

Thicker where crust is older as there is more time to accumulate

Question 12

Explain the features of pillow lavas in divergent boundaries?

Answer 12

Lava erupting underwater
Inside remains molten and sinks to form teardrop
Outside cools rapidly

Question 13

Explain the features of dolerite dykes in divergent plate boundaries?

Answer 13

Magma flows to the surface through fractures in the crust creating dykes and sills

Question 14

Explain the features of homogeneous gabbro in divergent plate boundaries?

Answer 14

Formed in magma chambers below the ridge and pushed to either side

Question 15

What are the magnetic anomalies in divergent boundaries?

Answer 15

Lava erupts and magnetic fields of minerals go in different directions
The magnetic fields of minerals are all aligned in the same direction as the earth’s magnetic field
The lava cools below the current temperature and magnetic fields are locked in place
Newer material nearest to the ridge, older material has moved away making strips of reverse polarity
Not perfectly symmetrical as it depends how lava flows

Question 16

How do convergent plate boundaries work -> oceanic - continental plate boundary?

Answer 16

Example of continental plate boundary is the Andes
Oceanic lithosphere subducts beneath continental plate because it is more dense
Regional metamorphism due to high temperature and pressure causes rocks to recrystallise
Deep focus earthquakes as subducting lithosphere fractures as it forces its way into asthenosphere
Shallow focus earthquakes occur due to build up and release of 2 plates against each other

Question 17

How does the convergent boundary work -> when oceanic plates meet?

Answer 17

Older and denser plate subducts
Ocean trench forms on the side of subducting plate
Partial melting of subducting mafic ocean crust leads to intermediate magmas - this causes andesite lava and explosive eruptions

Question 18

Explain the features of convergent plate boundaries -> continental - continental plates?

Answer 18

Eg Eurasia and India - Himalayas
Historic Tethys sea - where marine fossils are found
No subduction because both plates have same composition and density
Marginal ocean sediments formed upwards creating mountains
Shallow focus earthquakes close to rising surface
Folding occurs deeper where rocks are hotter
Silicic Magma produced but deep and rarely erupts at surface
Widespread regional metamorphism

Question 19

Explain symmetric spreading at the mid ocean ridge?

Answer 19

Material is created due to partial melting of mantle

New material moves away due to plate movement

Question 20

Explain asymmetric spreading in the ocean core complex?

Answer 20

Slow plate movement causes magma production to stop
Spreading continues; large scale FUN faults known as detachment faults form uplifting large amounts of mantle rocks onto the sea floor

Question 21

What is lithospheric drag and is it effective?

Answer 21

Occurs at convergent plate boundaries
Converting asthenosphere drags lithosphere down
Not the most effective movement

Question 22

What is slab pull and is it effective?

Answer 22

Denser plate subducts and pulls rest of the plate with it

It is effective

Question 23

What is ridge push and is it effective?

Answer 23

The ridge pushes the plates away
New material pushes plate away from the ridge
Not the most effective movement