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LEC.171 > Lecture 4: The Ocean Basins > Flashcards

Lecture 4: The Ocean Basins Flashcards

1
Q

How much of the Earth’s surface is covered by Ocean Basins

A

Three-fifths

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2
Q

When did we start exploring Ocean basins and what boosted our exploration?

A

Explored since 1870s, boosted by massive increase in funding from 1950s onwards (triggered by the cold war)

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3
Q

What did people previously believe ocean basins were?

A

‘Sunken Continents’

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4
Q

How do we use echo sounding to survey ocean basins?

A

Use a sound source to fire seismic waves down to the sub-bottom sedimentary layers
Measure time since deployed and picked up again
Use known travel time in water to calculate depth

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5
Q

How do we use depth sounding to survey ocean basins?

A

Lower a weight over the side of a ship, attached to piano wire.
They’d then measure how much wire had been unreeled by the time the weight hit the sea bed

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6
Q

What ways do we survey the ocean basins?

A 
Depth sounding
Echo sounding
Seismic reflection profiling
Dredging and dating of samples
Drilling
Submersibles
Magnetic field measurements 
Gravity field measurements
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7
Q

What are the max depths in the ocean?

A

Ocean trenches - up to 11km deep (Mariana Trench)

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8
Q

How deep are large flat ocean basins?

A

4km deep

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9
Q

How long and high are mid-ocean ridges?

A

65km long and approx. 2km high (ocean only 2km deep)

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10
Q

What is the seafloor covered with, and what is underneath?

A

Sediment, but underneath is volcanic rocks

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11
Q

Where does current/recent underwater volcanic activity occur?

A

MORs and at the end of aseismic ridges

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12
Q

What are aseismic ridges?

A

chains of volcanoes on the sea floor (can be big enough to be above sea level)

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13
Q

What is bathymetry?

A

Underwater topography

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14
Q

Are mid-ocean ridges continuous?

A

No

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15
Q

What are mid-oceans ridges broken up with?

A

Fracture zones

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16
Q

What do fracture zones do?

A

Break up mid-ocean ridges and offset them

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17
Q

What angle are offsets always to mid-ocean ridge segments?

A

Perpendicular

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18
Q

What is the part of fracture zones between offset ridges?

A

a transform fault (plate boundary)

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19
Q

How thick is layer 1 of the oceanic crusts?

A

0-10km thick (average approx 0.4km)

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20
Q

What are the three main sources for layer 1 of oceanic crusts?

A

Terrigenous sediment
Pelagic clays
Pelagic oozes

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21
Q

What is terrigenous sediment?

A

Sediment transported from land to ocean by rivers

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22
Q

Where is terrigenous sediment thickest?

A

Near continents

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23
Q

What is pelagic clays?

A

Fine particles, carried in suspension

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24
Q

Where do pelagic clays settle?

A

only settle out in deep calm water

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25
Q

What are pelagic oozes?

A

Remains of microscopic organisms (plankton)

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26
Q

How thick can layer 1 of the oceanic crust be?

A

On old oceanic crust, sediment can be up to 10 kilometres thick (but squashed down by the pressure of its own weight)

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27
Q

How thick is layer 2 of the oceanic crusts?

A

1-2.5km thick

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28
Q

What is the structure of layer 2 of the oceanic crust like?

A

Fairly homogenous

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29
Q

Pillow lavas definition

A

blobs of lava that cooled rapidly upon contact with cold seawater

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30
Q

Sheeted dykes definition

A

vertical volcanic layers

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31
Q

What is layer 2 of the oceanic crust mostly made of?

A

Basalt

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32
Q

What is layer 3 of the oceanic crust mostly made of

A

Gabbro

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33
Q

How thick is layer 3 of the oceanic crust?

A

Approx 5km

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34
Q

What is the different between gabbro and basalt?

A

Same chemical composition, but gabbro cools slower and therefore has larger crystals

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35
Q

What is below layer 3 of the oceanic crust and what is it made of?

A

The upper mantle, made of peridotite

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36
Q

What is the total thickness of the oceanic crust?

A

5-8km

37
Q

What depths can earthquakes occur at?

A

At a range of focal depths

38
Q

What depth range is a shallow earthquake?

A

0-50km

39
Q

What % of Earthquakes are considered shallow?

A

75%

40
Q

What depth range is a intermediate earthquake?

A

50-300km

41
Q

What % of Earthquakes are considered intermediate?

A

20%

42
Q

What depth range is a deep earthquake?

A

300-700km

43
Q

What % of Earthquakes are considered deep?

A

5%

44
Q

Where can intermediate and deep earthquakes be found?

A

Most are around the edge of the Pacific

45
Q

Where do shallow earthquakes occur?

A

basically happen all over the world (mostly on plate boundaries)

46
Q

Where do all earthquakes occur (within the earth)

A

in the rigid outer layer of the Earth – called the lithosphere

47
Q

Why do all earthquakes occur in the lithosphere?

A

it is the only layer rigid enough to undergo brittle fracturing

48
Q

Why do earthquakes not occur in the asthenosphere

A

It is close to melting point and so behaves plastically

49
Q

What types of earthquakes occur in mid-ocean ridges?

A

Earthquakes here are normally shallow

50
Q

What are mid-ocean ridge earthquakes normally formed by?

A

Extensional stress (normal faulting)

51
Q

What is normal faulting

A

Where plates are being pulled apart

52
Q

What types of earthquakes occur in transform faults?

A

Mostly shallow

53
Q

What are transform fault earthquakes normally formed by?

A

Strike-slip faulting

54
Q

What is strike-slip faulting

A

Blocks of lithosphere are sliding past each other horizontally, creating friction and earthquakes

55
Q

What % of earthquakes occur in oceanic trenches?

A

95%

56
Q

How do focal depths range in oceanic trenches?

A

From shallow to deep

57
Q

What stresses can be in oceanic trenches?

A

can be extensional (normal faulting) or compressional (thrust faulting)

58
Q

What is the Benioff zone?

A

the area where an oceanic lithosphere subducts into the mantle

59
Q

What does a plot of earthquake foci around a subduction zone reveal?

A

reveals a dipping plane called a Benioff zone

60
Q

How can the dip of a Benioff zone vary (and average)

A

Dip can vary between 15 and 75 degrees (average is 45)

61
Q

What does the Benioff zone show?

A

the lithosphere remains rigid as it subducts, but only down to a depth of 700km

62
Q

What happens to the lithosphere beneath 700km?

A

it is probably to hot and plastic to fracture

63
Q

Why can’t we date dredged basalts easily?

A

They’ve been altered by water

64
Q

Why can’t we use deep sea drilling for large scale detailed maps?

A

It is expensive and difficult

65
Q

What magnetic material does basalts and gabbro’s contain?

A

Magnetite

66
Q

What happens to basalt and gabbro when molten?

A

It is ferrimagnetic (above the Curie Point, 500 degrees), therefore groups of atoms can align themselves with any external magnetic field
When the rock is molten, the atoms align themselves with the Earth’s magnetic field
When it cools down the atoms are locked in place

67
Q

What is the alignment of basalt and gabbro’s atoms with the earth’s magnetic field called?

A

Thermoremanent magnetization (TRM)

68
Q

What did sampling of Basalts and Gabbro’s show?

A

different directions of atoms

This is due to reversals in the earths field

69
Q

What is the reversals in the direction of atoms in Basalts and Gabbro’s called?

A

Magnetic straigraphy

70
Q

What can we use magnetic stratigraphy

A

Divide the earth’s history into ‘normal’ and reverse periods

71
Q

What is one period of time between magnetic reversals called?

A

A chron

72
Q

How many chrons are there?

A

4

73
Q

What are Oceanic Magnetic Anomalies parallel to and symmetrical with?

A

MORs

74
Q

What do the pattern of reversals match?

A

Patterns already established on the continents

75
Q

The fact that the pattern of reversals matches the pattern on land allows us to do what?

A

Calculate the time period of oceanic rocks

76
Q

How do we calculate the date of underground rocks using magnetic alignment on land?

A

The symmetrical pattern of stripes is time-calibrated so ocean floor anywhere can be dated simply by counting up the stripes

77
Q

Ocean basin definition

A

places where there is oceanic crust beneath the sea

78
Q

How did we originally measure sea depth?

A

We lowered a weight over the side of a ship, attached to piano wire. They’d then measure how much wire had been unreeled by the time the weight hit the sea bed

79
Q

What was the majority of the rock dredged up off the sea bed?

A

Volcanic rock - basalt

80
Q

What did drilling of the seabed reveal?

A

Volcanic rock is everywhere on the ocean bed, but is just overlaid by lots of sediment in some places

81
Q

What is the section where the seabed starts rising on a passive continental margin called?

A

the continental rise

82
Q

What is the steepest part of a passive continental shelf called?

A

the continental slope

83
Q

What is the shallow section of a passive continental boundary closest to land called?

A

The continental shelf

84
Q

Does a passive continental margin have a trench?

A

No

85
Q

What is a typical gradient for a continental slope?

A

1m up for every 14m across

86
Q

Why do Mid-Ocean ridges break up?

A

because the plates are moving across the surface of a sphere

87
Q

What is the absolute limit of earthquake depth?

A

700km

88
Q

What is the chemical composition of magnetite?

A

Fe3O4

LEC.171 (8 decks)

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