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LEC.171 > Lecture 6: How Plates Move > Flashcards

Lecture 6: How Plates Move Flashcards

1
Q

How do divergent plate boundaries create new lithosphere?

A

Plates moving away from each other, creating new lithosphere

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2
Q

What does a divergent plate boundary look like on a map?

A

A double line with two arrows pointing away from the boundary (the directions the plates are moving)

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3
Q

What is the most common type of divergent boundary?

A

Mid-ocean ridge

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4
Q

What is a continental rift zone?

A

A divergent plate boundary on land

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5
Q

What features do continental rift zones typically have?

A

Thinning lithosphere
Normal faults causing shallow earthquakes
Chains of Volcanoes

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6
Q

What can continental rift zones evolve into?

A

Mid-Ocean Ridges

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7
Q

What can ocean-ocean convergence plate boundaries form?

A

Island arcs

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8
Q

Example of an ocean-ocean convergence plate boundary formed Island Arc?

A

The Aleutian arc in the Northern Pacific

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9
Q

What is an island arc?

A

A chain of islands that is parallel to a subduction zone

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10
Q

Which plate normally subducts at an ocean-ocean convergence plate boundary?

A

The older plate subducts (colder and denser)

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11
Q

What does a converging plate boundary look like on a map

A

A single line with arrows converging on either side. The line has triangles (teeth) on the overwriting plate, pointing towards the side being subducted

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12
Q

What is formed at an ocean-continent convergence?

A

An active continental margin

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13
Q

Give an example of an active continental margin

A

The Andrean continental margin

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14
Q

Which plate subducts at an ocean-continent convergence?

A

the oceanic plate always subducts (continental plate is thicker, with lower density rock)

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15
Q

What is formed when an continental plate subducts an oceanic?

A

A chain of volcanoes along the coast

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16
Q

What is special about a chain of volcanoes formed by an ocean-continent convergence?

A

They can all be active (all of the volcanoes are underlain by subduction)

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17
Q

Which plate subducts at a continent-continent convergence boundary?

A

The continental lithosphere is too thick and buoyant to subduct - so neither

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18
Q

What happens at a continent-continent convergence boundary?

A

The two plates ‘fold up’ - creating a mountain range

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19
Q

Give an example of a landform formed by a convergent continent-continent boundary?

A

The Himalayas

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20
Q

What general height and depth do fold mountains reach?

A

A lot higher than sea level, and delve deeply into the mantle

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21
Q

What do we get beneath a converging continent-continent boundary?

A

Plutonism and possibly volcanism

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22
Q

Why do we get plutonism (and maybe volcanism) at a converging continent-continent boundary?

A

Because the crust and lithosphere are so thick here, may be deep enough for pressure and temperature to be sufficiently high to melt some of the crustal rock into granite type magma.
Possible that they might reach the surface as eruptions as well

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23
Q

What plates are conservative boundaries nearly always between?

A

both oceanic plates, or both continental plates

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24
Q

Can there be a transformative plate between a continental plate and an oceanic?

A

yes, but only very rarely

25
Q

Where is the most common type of transformative plate and what does it do?

A

On the seafloor - offsets two segments of a mid-ocean ridge

26
Q

What does a transformative plate in a MOR cause?

A

A step profile with fracture zones on either side, causing a scarp on the seafloor

27
Q

What does a transform fault between a MOR look like on a map?

A

Two arrows separating two sets of two lines. The arrows represent a transform fault, the two sets of two lines represent mid-ocean ridges

28
Q

What do the arrows mean on a transformative plate boundary?

A

The Mid-ocean ridge is creating a new part of the plate (new crust) and pushing it away in both directions - the arrows are determined by the direction that the mid-ocean ridge is pushing in

29
Q

What does a transformative plate between a MOR and a trench look like?

A

Same as a normal one with 2 MORs, but with one replaced by a line with teeth pointing away

30
Q

What does a transformative plate between two trenches look like?

A

Same as a normal one with 2 MORs, but with the MORs replaced with lines with teeth pointing inwards

31
Q

What is created at a continental transform fault system?

A

A complex band of more-or-less parallel faults (because the continental crust is much thicker and more complex than the oceanic)

32
Q

Does the plate break cleanly at a continent-continent transformative boundary?

A

No

33
Q

What is an example of a continent-continent transformative boundary?

A

The Californian transformative plate boundary between the Pacific and North American plate

34
Q

What is the dominant fault on the Californian boundary?

A

The San Andreas fault

35
Q

What is a triple junction?

A

Points on the Earth’s surface where 3 plates meet

36
Q

How many different possible combinations of triple junctions can we have?

A

16

37
Q

What three different parts to a triple junction can we have?

A

Ridge (R), Trench (T) and Fault (F)

38
Q

What is the most common triple junction?

A

Ridge, Ridge, Ridge (RRR)

39
Q

Can we have a triple junction of three transform faults?

A

No

40
Q

What are triple junctions essential for?

A

Discovering how transformations of plates are changing overtime

41
Q

Are all continental margins plate boundaries?

A

No

42
Q

What happens at a passive continental margin?

A

continental lithosphere is welded on to oceanic lithosphere, with no relative motion between them

43
Q

What features does a passive continental margin have?

A

Have no trench (gradual slope), no volcanoes and low seismicity

44
Q

What is the safest and most stable plate boundary

A

A passive continental margin

45
Q

Give an example of a passive continental margin

A

The edges of the Atlantic Ocean or offshore Northern Britain

46
Q

When does a passive continental margin form?

A

Form when you have new crust being created at mid-ocean ridges, but instead of becoming two continents, become 2 passive continental margins

47
Q

What are all plate movements equivalent to?

A

rotations about a point on the surface called the Euler pole/pole of rotation

48
Q

Do plates have the same Euler point?

A

No, they are all different

49
Q

What can we think of plate movement as instead?

A

Rotations around the Euler point

50
Q

What reveals the direction of relative plate motion and why?

A

The transform faults of the plate and another

Because they are just sliding past each other (no convergence or divergence) here, this must be their dominant direction

51
Q

How can we find the Euler pole of a plate?

A

The Euler pole is:
At the centre of all the circles traced by transform faults
Where all lines perpendicular to the Transform Fault intersect

52
Q

How can we find relative speed of a plate?

A

Using magnetic stripes on the seafloor

53
Q

Process to calculate relative speed of a plate?

A

Measure the distance (km) between equivalent magnetic reversals of known ages (Ma) on each side of the MOR
We then use Speed=distance/time
Gives us the full spreading rate of the MOR (how fast a point on one plate is moving away from a point on the other)
We can then divide by 2 to give us the half spreading rate (plate speed relative to the MOR)

54
Q

Is relative spreading speed constant?

A

No, varies with distance

55
Q

At what angle from the Euler point is relative spreading speed greatest?

A

90 Degrees

56
Q

What can we use instead of cm/y for spreading speed?

A

Angular speeds in degrees per million years

57
Q

How can we use hot spots to find absolute rates of plate motion?

A

If we then plot the ages of rocks in a particular hot spot chain, vs how far away it is from the hot spot, we get a rough straight line graph.
If we average our data, we can calculate the absolute speed (giving the pacific plate an absolute speed of approx. 10cm/y)

58
Q

How can we find absolute rates of plate motion now?

A

GPS is one method we use

Daily measurements can be plotted on a graph overtime to show us a plates movement and direction

59
Q

Do GPS velocity vectors agree with vectors predicted by geological methods?

A

GPS velocity vectors agree closely (but not exactly) with vectors predicted by geological methods

LEC.171 (8 decks)

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