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OCR A LEVEL GEOLOGY > Plate tectonics > Flashcards

Plate tectonics Flashcards

1
Q

What are Wilson cycles?

A

Cycles of the crust breaking and re-joining. Happens every 500 million years.

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2
Q

Who proposed the theory of continental drift?

A

Alfred Wegener

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3
Q

What evidence was there for continental drift?

A

Rocks
Ice
Fold Mountains
Fossils
Jigsaw fit
Magnetic

RIFFJM

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4
Q

Name 9 continental plates

A

NA plate
SA plate
African Plate
Eurasian Plate
Australasian Plate
Antarctic plate
Pacific plate
Nazca plate
Indian plate

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5
Q

How was matching geology (rocks) used as evidence for continental drift?

A

Rocks of the same type and age occur in bands broken by oceans. Such as the Precambrian cratons which match up across the south Atlantic.

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6
Q

How were fold mountain chains used as evidence for continental drift?

A

Fold mountains with the same age and strike match up between SA and Africa as well as cratons.

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7
Q

How were fossils used as evidence for continental drift?

A

Fossils of land plants and animals of the same species are found in different continents such as the Mesosaurus was found in Africa and South America.

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8
Q

What is the difference between Pangea and Gondwanaland?

A

Pangea was all the continents connected
Gondwanaland was the Southern continents connected separated from the Northern continents.

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9
Q

How were glaciers (Ice) used to prove continental drift?

A

There is evidence of an ice sheet covering large parts of southern continents about 300 Ma.
Tillites (Lithified “boulder clay”)
Erratics (Rocks out of place moved by ice)
Striations (Scratches in bed from moving ice)
all these are found over the Southern continents explaining Gondwanaland.

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10
Q

How was magnetic evidence use to prove continental drift?

A

Apparent polar wandering curve
If a plate moved it will appear the poles have moved in the “rock record”

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11
Q

How do you work out the annual rate of sea floor spreading?

A

If rate on one side is 1 cm/yr it would be 2cm/yr for the ocean in total

Annual rate of sea floor spreading = Distance moved in cm / Number of years taken

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12
Q

What are plates?

A

Plates are lithospheres at the top 100km of the Earth is a plate. It is a rigid layer made of crust and rigid upper mantle above the low velocity zone.

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13
Q

What is crust?

A

Crust is the material above the moho discontinuity

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14
Q

How thick is oceanic crust compared to continental crust?

A

Oceanic - 7km thick
Continental - 35km thick

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15
Q

What is the main composition of oceanic crust compared to continental?

A

Oceanic - Basaltic
Continental - Granitic

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16
Q

What is the density of oceanic crust compared to continental crust?

A

Oceanic - 2.9-3.0 g/cm^3
Continental - 2.7 g/cm^3

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17
Q

How was the crust formed in oceanic and continental crust?

A

Oceanic - formed at MOR’S
Continental - Formed by accretion of lighter elements slowly rising through the mantle

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18
Q

How old is oceanic crust compared to continental crust?

A

Oceanic - All less than 200 Ma due to sea floor spreading
Continental - 3.8 Ga or less

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19
Q

What are the two sources of heat in the Earth?

A
  1. Original heat from the formation of the Earth
  2. Radioactive decay

Heat travels slow through rocks

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20
Q

How does sea floor spreading work?

A
  1. The two plates at an MOR move apart and Mafic magma moves upwards, filling the gap
  2. The younger rocks are closer to the MOR
  3. Magnetic reversals produce “striped” oceanic crusts parallel to the MOR
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21
Q

What allows MORS to fit on the Earths curved surface?

A

They are offset by transform faults perpendicular to MORS

22
Q

What is a fracture zone?

A

Where both sides of the plate are moving the same direction, and a “Crack” forms.

23
Q

What are 3 ways an earthquake can happen in MOR?

A
  1. Rocks sticking/moving along transform faults (Shallow, and low magnitude)
  2. Step faults along the MOR
  3. Magma moving up a dyke in the MOR
24
Q

Why can decompression melting occur at MORS?

A

Because of the MOR, there is less crust above the dyke, so there is less pressure which means there is a lower melting points so more magma forms (5% partially molten peridotite)

25
Q

What is a rift valley?

A

A valley that forms when two continents move away from eachother and oceanic crust fills the gap leaving low land

26
Q

What is the Benioff zone?

A

A zone of earthquakes which occur on the boundary of oceanic crust being subducted below continental crust. Is a seismology method to find where oceanic crust is.

27
Q

What are 7 oceanic crust features?

A
  1. Continental shelf
  2. Continental slope
  3. Seamount
  4. Guyot
  5. Abyssal plain
  6. Axial rift valley
28
Q

What are 7 plate boundries?

A

Transform boundary - Plates moving past eachother
Hot spots
Convergent (Oceanic - Oceanic)
Convergent (Oceanic - Continental)
Convergent (Continental - Continental)
Divergent (Continental)
Divergent (Oceanic)

29
Q

What are isostatic adjustments?

A

Is a vertical movement of the crust when pressure is applied. When the pressure is released, the crust rebounds. This is seen, with glaciers, fold mountains, lower density rocks and volcanos.

30
Q

What is the CCD (Carbonate Compensation Depth)?

A

Is the depth in the oceans (3.5km) below which carbonate material dissolves. This is because of CO2 dissolving the oceans producing carbonic acid.

31
Q

What is meant by moving chain islands?

A

Volcano is built when a vent is created through the lithosphere. The oceanic crust then moves the moves the volcano off of the mantle plume, and becomes extinct (Island). A new volcano is created and process repeats making an island chain.

32
Q

What features for conservative plate margins have?

A

Strike-Slip faults (Sideways movement)

33
Q

How does the fault move in a conservative plate margin?

A

A section breaks and moves. This causes stress transfer to the next section which is where the next earthquake will be.

34
Q

What conservative plate margin is running through Istanbul?

A

North Anatolian Fault

35
Q

What theory has not been disproved for the movement of plates?

A

Slab pull - Old, dense crust sinks due to gravity and the rest of the crus gets pulled
Convection currents - Move plates through movement in the magma below

36
Q

What direct evidence has helped us learn more about MORs?

A
  1. Satellites - Can show the location of MORs and GPS can be used to measure movements
  2. Side-Scan sonar - Shows the detailed shape of the sea floor along the MOR
  3. Multi-beam echo sounders - Show the overall length of the MOR
  4. Magnetic surveys - using magnetometers show weakened filed strength at MORs suggesting hot rock (needs to be below the curie point to have a strong magnetic field)
  5. Seismic tomography - where enough seismometers exist we can see seismic waves are retarded by >10% as they travel through hot rock below the MORs.
  6. Gravity surveys (using gravimeters) show light, buoyant (hot) rock below MORs
  7. Heat flow is typically 0.3 W/m^2 higher at MORs
  8. Ophiolites are where crust has been obducted, which can show the composition of the oceanic crust that would have sunk into the MOR
37
Q

What do slow spreading rates (2-4 cm / yr) create?

A

-Lots of faulting
- Median rift valleys (20km wide and 3km deep) with rugged mountains at crest
- Discontinues magma chamber under MOR
-Occasional lava flows and pillow eruptions
-Lots of faulting
-Example is the Mid Atlantic Ridge

38
Q

What do fast spreading rates (16 cm / yr) create?

A

-Continuous magma chamber
-Lots of layers of lava
-Not so much rifting
-Smooth dome
-Example is the East Pacific Rise

39
Q

What effects do MORs have on sea level?

A

When there are lots of fast, active MORs, they displace sea water so global sea level rise (like in the cretaceous when the Atlantic was opening fast

40
Q

What does adiabatic mean?

A

A temperature change due to a pressure change

41
Q

What does decompression melting mean?

A

Making a rock start to melt by dropping the pressure (the temperature will fall adiabatically but so will the melting point)

42
Q

What are mush zones?

A

Zones where there is a mixture of crystals and melt

43
Q

What is the geotherm?

A

Shows the actual temperature at different depths

44
Q

What is the solidus?

A

Line separating all solid material from mush (“partial melt”)

45
Q

What is the liquidus?

A

Line separating all mush from liquid

46
Q

What is the adiabatic curve?

A

Theoretical pathway showing the history of a rock being forced to rise (or sink), and is used to say at which depths partial melting starts

47
Q

What are black smokers?

A

A hydrothermal vent associated with volcanic activity on an MOR. Emits dark clouds of fine sulphide particles where water becomes superheated and becomes less dense so gushes out of the sea floor at high speeds.

48
Q

What is serpentine, how is it formed, and where is it found?

A

Is a mineral formed by the hydration of olivine
Olivine reacts with hot water to form a hydrated by incorporating the water into the atomic structure of olivine, so there is an increase in volume and a reduction of density.
Found in Cornwall

49
Q

What is the name of the process that occurs in the formation of serpentine?

A

Metasomatism

50
Q

What is metasomatism?

A

Is the chemical alteration of a rock by hydrothermal and other percolating fluids. The chemicals of the original minerals react with hot water and its dissolved chemicals to form new minerals which replace the original crystals.

51
Q

What are white smokers?

A

Cooler hydrothermal vents that emit barium, calcium and silicon. Precipitation of anhydrite and silica gives it the white colour

OCR A LEVEL GEOLOGY (12 decks)

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