Week 5: How to build a planet: Tectonic Plates

Question 1

When was Plate Tectonic Theory accepted?

Answer 1

mid to late 1960s
*1968

Question 2

What aspect of the Earth system does Plate Tectonic theory describe?

Answer 2

describes the movement of plates and the forces acting on them
(heat is the driving force)

Question 3

What was Wegener’s continental drift hypothesis?

Answer 3

Continental drift= large-scale movement of the continents through time
Alfred Wegener noted jig-saw puzzle fit

Question 4

How does the following evidence support the theory of continental drift?: fit of coastlines around the Atlantic?

Answer 4

Palaeozoic mountain belts on both coasts (N. America and Africa)

Question 5

How does the following evidence support the theory of continental drift?: geologic fit?

Answer 5

Wegener noted jig-saw puzzle fit
similarity of rock assemblages and ages across oceans
Palaeozoic mountain belts on both coasts

Question 6

How does the following evidence support the theory of continental drift?: fossil distribution?

Answer 6

Fossil remains across multiple continents:
-Glossopteris (fern)= found in all Southern continents shows that they were once joined as seed is large and bulky therefore could not have drifted across the oceans to a separate continent

-Cynognathus (reptile)= fossils found in S.Africa and S.America as a land dominating species would not have been capable of migrating across Atlantic

-Mesosaurus (reptile)= found in S. Africa and Eastern S.America. If continents were in present positions mesosaurus would nit be capable to swim such distance as a coastal animal

-Lystrosaurus (reptile)= found in Antartica, India and S.Africa would not have the swimming capability to traverse any ocean

Question 7

How does the following evidence support the theory of continental drift?: distribution of late Palaeozoic glaciation?

Answer 7

Late Palaeozoic glacial till found on continents currently not at high latitudes
Wegener’s reconstruction: the glaciated areas connect to outline a region of late Palaeozoic southern polar ice caps
Currently the Palaeozoic striations point inland, away from sea

Question 8

How does the following evidence support the theory of continental drift?: distribution of climate belts?

Answer 8

1980s discovery of “polar” dinosaurs at Dinosaur Cove:
-warm blooded
-keen night vision
-adapted to freezing or sub-freezing temperatures and long winter nights

around 100million years ago Dinosaur Cove area was well within the Antarctic Circle, more than 40 degrees closer to the South Pole than it is today.
australia has drifted North during the past 100myrs

Question 9

Why was continental drift initially rejected?

Answer 9

he had compelling observations but proposed that continents dragged over the solid oceanic rust by tidal forces of sun and moon

Question 10

What scientific developments led to the formulation of plate tectonic theory? How?

Answer 10

4 major scientific developments:

1.ruggedness and youth of the ocean floor
2. earthquake and volcanic activity along ocean trenches and submarine mountain trenches
3. repeated reversals of the Earth magnetic field
4. sea-floor spreading hypothesis and associated recycling of oceanic crust

Question 11

Describe the basic bathymetric characteristics of mid-ocean ridges, deep-ocean trenches, and seamount chains?

Answer 11

Mid-ocean ridges:
-sea floor made of young basalt
-thin layer of clay
-tiny shells of dead plankton

Deep-ocean trenches/seamount chains
-earthquake/volcanic activity concentrated
-earthquake zones parallel to trenches
-inclined 40-60 degrees
-extend several hundred km into Earth

Question 12

Describe the hypothesis of seafloor spreading. What is the evidence for it?

Answer 12

New rock created at the ridges, old rocks recycled by the trenches
EVIDENCE:
-rocks at/near the crest=very young, become progressively older away from ridge crest
-youngest rocks at ridge crest=present-day polarity
-stripes of rock parallel to the ridge crest alternated in magnetic polarity= record that Earth’s magnetic field flip-flopped

Question 13

What is palaeomagnetism and how does it form?

Answer 13

Palaeomagnetism is the study of the Earth’s ancient magnetic field as preserved in rocks.
Formed via thermoremanent magnetization:
-when iron-rich lavas cool, the Earth’s magnetic polarity (normal or reverse) at time of cooling is locked in by the alignment of magnetic minerals.
(circulation in outer core generates the magnetic fields)

Question 14

What is a magnetic reversal?

Answer 14

A magnetic reversal=polarity reversal:
-change in the Earth’s magnetic field in which the magnetic north and south poles switch places.
(normal to reverse or vice versa)

Question 15

What is a marine magnetic anomaly?

Answer 15

the magnetic field of rocks on the sea floor alternate between high and low intesnity values in long, narrow parallel bands= magnetic anomalies
-almost perfectly symmetrical with respect to the crest of the mid-ocean ridges

Question 16

What is the basis for the distinction between the lithosphere and asthenosphere? What are the key differences?

Answer 16

-can differentiate them based on material strength
KEY DIFFERENCES:
-lithosphere is the cool, strong outer shell while lithosphere is the hot weak interior
-lithosphere= crust +top of mantle while asthenosphere= mantle below lithosphere
-lithosphere behaves like a relatively rigid brittle shell while asthenosphere is a ductile solid

Question 17

How does the lithosphere respond to stress?

Answer 17

shatters

Question 18

What is the difference between the lithosphere and asthenosphere and the crust and mantle?

Answer 18

Lithosphere
-Cool, strong, outer shell
-Crust + ~top 100km of mantle kms - 200km thick
-Behaves like a relatively rigid, brittle shell

Asthenosphere
-Hot, weaker interior
-Mantle below lithosphere ~300km thick
-Flows like a ductile soli

Question 19

What are the tectonic plates made of? How many plates are there and are they all the same shape and size?

Answer 19

7
no they are all different shape and sizes
make Earth’s surface a mosaic of rigid plates
bound by mid-ocean ridges and deep-ocean trenches

Question 20

On which tectonic plate is Britain located?

Answer 20

Eurasian

Question 21

What are the three types of plate boundaries? How do the plates move with respect to each other at each boundary type?

Answer 21

-divergent= separate (apart from each other)
-convergent= collide (towards each other)
-transform= slide (grind past each other)

Question 22

What is the relationship between earthquake activity/volcanism and plate margins?

Answer 22

activity is higher/mostly at plate margins

Question 23

What are the key differences in the behaviour of continental and ocean lithosphere at plate boundaries?

Answer 23

Continental= derived by a sequence of melting
Oceanic= result of mantle melting deeper levels

-continental underlies continents while oceanic underlies sea floor
-continental is thicker (40km vs 7km) and less dense
-seismic wave velocities slower in continental (7 vs 6)

Composition:
continental= more felsic
oceanic= mafic

Question 24

How do continental-oceanic versus continental-continental convergent margins differ?

Answer 24

Convergent=move towards

Continental oceanic:
-oceanic plate subducts below continental
-forms= volcanic mountains chains, folded and thrusted mountains, deep earthquakes

Continental-continental:
-neither plate subducts
-forms: crustal thickening, folded and thrusted mountains and earthquakes

Question 25

At what rates do the plates move? How do the rates of spreading compare at the three different types of boundaries? If new material is constantly being added to the Earth, why is the Earth not expanding?

Answer 25

rates vary, generally cms a year (range 6mm-150mm)

divergent may have fastest rates but generally similar to convergent on average transform tend to have slower rates (all plate types have a range tho)

Earth does not expand as material is also being consumed at convergent boundaries through processes like subduction.

Question 26

Where are the current highest and lowest rates of spreading and convergence?

Answer 26

highest rate of spreading (divergence)= 150mm/yr (Nazca-Pacific plate)
lowest rate of spreading (divergence= 14mm/yr (African-Antartic)
highest rate of convergence= 89mm/yr (Caribbean-Cocos)
lowest rate of converegence=6mm/yr (Eurasian-African)

Question 27

What role does mantle convection play in plate tectonics? What two mechanisms drive plate movement?

Answer 27

plate motion is driven by it
two forces:
-ridge push
-slab pull (including basal drag)

Question 28

What are the following and where do they occur with respect to plate boundaries: mid ocean ridges, volcanic (island) arcs, (deep) ocean trench, Wadati-Benioff zone?

Answer 28

Wadati-Benioff zone:
-earthquake zones parallel to trenches

Question 29

What is the composition of the oceanic crust? Where is the majority of new oceanic crust formed?

Answer 29

mafic composition: basalt and gabbro
majority= mid ocean ridges

Question 30

What is the evidence for sea floor spreading?

Answer 30

-samples collected from the sea floor
-age of oceanic crust increases with distance from the spreading centre

Question 31

How does the age of the oceanic crust vary with distance from the spreading centres?

Answer 31

further from spreading centre=older

Question 32

How does the rock cycle relate igneous, metamorphic and sedimentary rocks?

Answer 32

Igneous to Sedimentary: Exposed igneous rocks can be weathered and eroded into sediments=undergo transportation and deposition=sedimentary rocks.

Sedimentary to Metamorphic: Sedimentary rocks buried deep within the Earth’s crust can be subjected to heat and pressure= metamorphism

Metamorphic to Igneous: Metamorphic exposed to high temperatures= melt and crystallize to form new igneous rocks.

Question 33

How does plate tectonics control the rock cycle?

Answer 33

movement of Earth’s lithospheric plates is responsible for various geological processes that drive the formation, transformation, and recycling of rocks

Question 34

What is a supercontinent?

Answer 34

large continents that include most of all of the existing continents

Question 35

What is the supercontinent cycle? What events (and evidence) are associated with continental break up and assembly of the supercontinents?

Answer 35

Geologic time scale of events in the history of Earth’s continents
supercontinent assembly and breakup is episodic

Break up=requires rifting
assembly= requires collision to orogeny

Question 36

What is an orogeny? In which plate tectonic settings do the most intense orogenesis occur? Present day orogens are identified by finding regions of high, rugged peaks. How can geologists identify orogens that have been eroded away?

Answer 36

Orogeny: episodes of mountain building processes of folding, faulting, terrain accretion, magmatic addition and metamorphism

Occurs most:
Oceanic-Continental
Continental-Continental (most intense)

Question 37

What is the nature of the relationship between supercontinent formation, global sea level and global climate?

Answer 37

SEA LEVEL
supercontinent:
short length of mid-ocean ridges/subduction zones= 1.more heat trapped= thermal continental uplift= low sea level
2.low magmatic activity=low CO2 supply to atmosphere= cooling

dispersed continents:
long length of mid-ocean ridges/subduction zones=
1.less heat trapped= continents sit low= high sea level
2.high magmatic activity=high CO2 supply to atmosphere= global warming