2.2 Plate Tectonics

Question 1

What is the lithosphere broken up into?

Answer 1

Plates

Question 2

What are the different types of crust?

Answer 2

Oceanic and continental

Question 3

Where do volcanoes mostly occur?

Answer 3

Along plate boundaries

Question 4

What can the earth being a geological machine also be known as?

Answer 4

Plate tectonics

Question 5

What did the concept of plate tectonics originate with?

Answer 5

The idea of continental drift (the continents moving about the planet)

Question 6

Who first put forward the idea of continental drift?

Answer 6

Alfred Wegener in 1915

Question 7

Who was Alfred Wegener?

Answer 7

A meteorologist that came up with continental drift

Question 8

What evidence shows that continental drift is a correct theory?

Answer 8

• the shapes of continents are fitted
• ancient rocks and fossils
• glaciers
• mountain chains
• magnetic attractions

Question 9

How are the gaps between today’s continents when fitted together accounted for?

Answer 9

The gaps are due to some pieces of land being submerged due to difference in sea level, as well as erosion and deposition

Question 10

How can ancient crystalline rocks support continental drift?

Answer 10

Different rock types and ancient mountain belts match up

Question 11

How can fossils support continental drift?

Answer 11

Distribution of plant and animal fossils found in S. America, Africa, Antarctica, Australia - showing all southern continents were once joined

Question 12

How can glaciers support continental drift?

Answer 12

Distribution of late palaeozoic glaciations can show movement of glacial ice

Question 13

How can mountain chains support continental drift?

Answer 13

Mapping sea floors - underwater mountain ranges that line up with continental pattern → spreading ridges formed at constructive plate margins

Question 14

How can magnetic attractions support continental drift?

Answer 14

In metals, the tiny compasses point towards where the north pole was when the lava cooled → use this to figure out where continents were at the time

Question 15

Where is today’s present north magnetic pole?

Answer 15

In the south

Question 16

If the sea floor is spreading, what should be shown on the rock?

Answer 16

The rock should show symmetrical magnetic stripes moving away from the ridge

Question 17

What is a hot spot?

Answer 17

An area of very high heat flow in the mantle below the plates - with the heat burning through the plate to create volcanic activity at the surface

Question 18

What can hot spots also be known as?

Answer 18

Mantle plume

Question 19

Why can hot spots indicate direction of plate movement?

Answer 19

Although plates move - hot spot stays in same place e.g. direction that Hawaiian islands get older indicate direction of plate movement

Question 20

What led geologists to believe that the lithosphere and crust were moving?

Answer 20

Patterns of earthquakes and volcanoes

Question 21

How can plates move?

Answer 21

Convection currents in the lower mantle

Question 22

What is the process of convection currents in the lower mantle?

Answer 22

• hot material rises upwards through the mantle
• as this nears the lithosphere, it moves sideways and carries a plate with it (this happens as mid-ocean ridges)
• this material cools and descends at certain points - dragging lithosphere with it at subduction zones
• lower part of mantle heated and material rises again to complete convection cell

Question 23

How do mid-ocean ridges form, with relation to convection currents?

Answer 23

When hot material in the mantle rises and then move sideways and carries a plate with it

Question 24

How do subduction zones form, with relation to convection currents?

Answer 24

When material at top of mantle cools and descends, dragging lithosphere down with it

Question 25

Is the mantle a liquid?

Answer 25

No

Question 26

How do convection currents occur in the mantle, if the mantle is a solid?

Answer 26

Slow deformation of the solid mantle

Question 27

What is slab pull?

Answer 27

The weight of the crust pull the plate downwards

Question 28

How is it thought that constructive plate margins begin as?

Answer 28

As grabens or rift valleys on land

Question 29

What are rift valleys?

Answer 29

Where the crust is being split under great tension, with sets of normal faults forming opposite one another, and blocks of crust sinking in between

Question 30

How does an ocean develop as a result of plate tectonics?

Answer 30

* it must start as a rift valley * this widens - water floods in and forms a long narrow sea * sea widens by sea-floor spreading and opens up into an ocean

Question 31

Which way do the plates move in constructive boundaries?

Answer 31

Move apart - with magma brought up from the upper mantle

Question 32

What happens to magma that is brought up from the mantle at constructive margins?

Answer 32

It crystallises on the surface as basalt pillow lavas, and beneath as dolerite dykes or plutons of gabbro

Question 33

What is formed when crust is pushed away in opposite directions at constructive margins?

Answer 33

Mid-oceanic ridges - marked by lines of volcanoes

Question 34

What are the fractures in the line of the mid-ocean ridge?

Answer 34

Transform faults

Question 35

How are transform faults formed?

Answer 35

Formed at the same time as the ridge, and are curved to allow the plates to move at different speeds across the earths spherical surface

Question 36

What happens when one plate moves in the opposite direction to another on either side of a transform fault?

Answer 36

Great pressure build up and shallow earthquakes result

Question 37

What do conservative margins result in?

Answer 37

Build up of pressure as friction prevents rocks moving along these large transform fault zones

Question 38

How is pressure released at a conservative margin?

Answer 38

In the form of an earthquake

Question 39

Which direction are plates at destructive margins moving?

Answer 39

Towards each other

Question 40

What does the plate destroyed at destructive plate boundaries compensate for?

Answer 40

The new plates forming along spreading ridges

Question 41

What are subduction zones?

Answer 41

When one plate is going underneath another

Question 42

What are Benioff zones?

Answer 42

Inclined zones of seismicity - when earthquake foci occur in lines at distinct angles leading down from plate margins

Question 43

When do trench systems occur?

Answer 43

When an oceanic plate is subducted underneath another oceanic plate

Question 44

What is marked when an oceanic plate subducts another oceanic plate?

Answer 44

A deep ocean trench

Question 45

How can earthquakes and volcanoes form from the subduction of oceanic-oceanic plates?

Answer 45

* earthquakes - friction with the other plates (and so a Benioff zone formed) * volcanoes - when depth reached where temperature high enough plate will melt, forming magma, which will rise through cracks

Question 46

What type of volcanoes are formed from the subduction of oceanic-oceanic plates?

Answer 46

Andesitic

Question 47

What are volcanic arcs?

Answer 47

Chain of volcanic islands created in an arc shape, due to the curvature of the earth

Question 48

Where do oceanic-continental mountain belts occur?

Answer 48

When an oceanic plate is subducted under a continental plate

Question 49

Why is the oceanic plate subducted under the continental plate?

Answer 49

It is denser and thicker than the continental plate

Question 50

What happens to most of the magma when the subducted plate melts in a oceanic-continental subduction zone?

Answer 50

Although some reaches the surface to form andesitic volcanoes, most builds up in the continental crust as large granite plutons

Question 51

What will the granite plutons, formed when an oceanic plate is subducted underneath a continental plate, do to the surrounding rock?

Answer 51

Produce contact metamorphism of the rocks around them

Question 52

How are fold mountains formed?

Answer 52

When sediments that build up along oceanic-continental subduction margins are buckled into great folds and faults

Question 53

How is regional metamorphism related to destructive plate boundaries?

Answer 53

The immense pressures and temperatures created by movement along destructive plate margins also produce regional metamorphism on a large scale

Question 54

When do continental mountain belt systems occur?

Answer 54

When the subduction of an oceanic plate results in one continent moving towards another - until it eventually collides with it

Question 55

Why are two continental plates squeezed against one another at destructive plate margins?

Answer 55

Neither can be subducted

Question 56

What forms when two continental plates are squeezed against one another?

Answer 56

The sediments buckle and form fold mountains - also earthquake activity and intrusion of granitic plutons

Question 57

Why is there little volcanic activity at continental-continental destructive plate boundaries?

Answer 57

There is so must crust to be penetrated

Question 58

Why is thrust faulting common at continental-continental destructive plate boundaries?

Answer 58

Due to the great pressure involved

Question 59

What is the Joides Resolution 360 research cruise?

Answer 59

Boat that does ocean drilling of igneous rocks to understand the processes that create mid-ocean ridge basalt

Question 60

What does the mechanical behaviour of the outer earth involve?

Answer 60

* the lithosphere - cold, rigid outer shell | * underlain by the asthenosphere - weaker layer

Question 61

What is the lithosphere composed of?

Answer 61

Crust and upper mantle

Question 62

What is the asthenosphere composed of?

Answer 62

Upper mantle

Question 63

Are the mechanisms of how tectonic plates move completely understood?

Answer 63

No

Question 64

Who discovered evidence for sea floor spreading, and what year?

Answer 64

* Hess - 1960 * Vine and Matthews - 1963 * J. Tuzo Wilson - 1965

Question 65

Features at divergent plate boundaries?

Answer 65

* basalt extrusion * sea floor spreading * ocean ridges * melting of upper mantle to form basaltic magma * high heat flow * rift valleys * abyssal plane

Question 66

Example of a divergent plate boundary?

Answer 66

Mid-Atlantic Ridge

Question 67

Features at conservative plate boundaries?

Answer 67

* earthquake activity | * transform faults

Question 68

Example of a conservative plate boundary?

Answer 68

San Andreas fault zone

Question 69

Features at oceanic-oceanic convergent plate boundaries?

Answer 69

Island arc/trench systems

Question 70

Example of a oceanic-oceanic convergent plate boundary?

Answer 70

Java-Sumatra/Caribbean

Question 71

Features at oceanic-continental convergent plate boundaries?

Answer 71

* subduction zones * Benioff zone * partial melting producing andesitic and granitic magmas

Question 72

Example of a oceanic-continental convergent plate boundary?

Answer 72

The Andes

Question 73

Features at continental-continental convergent plate boundaries?

Answer 73

* mountain building * folding * thrust faulting * partial melting of crust producing granites * associated regional metamorphism

Question 74

Example of a continental-continental convergent plate boundary?

Answer 74

The Himalayas

Question 75

What is the only type of material that can be subducted? Why is this?

Answer 75

Oceanic plate material - because the granitic crust of the continents is not dense enough to descend into the asthenosphere