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Geology > Plates and crust > Flashcards

Plates and crust Flashcards

1
Q

LOOK AT OCEAN FLOOR SLIDES FOR BATHYMETRY LECTURE

A
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2
Q

describe the oceanic crust

A

covered by sediment. thickest near continents. thinnest or absent at MOR. primarily basalt. heat flow greater at MOR. earthquake occur in distant belts in oceanic regions (oceanic fracture zones, MOR axes, deep ocean trenches)

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3
Q

describe sea-floor spreading

A
  • sediment thickens away from MOR
  • earthquakes cause cracks at MOR and cracked crust splits apart
  • molten rock rises into cracked crust from high heat flow
  • new ocean floor at MOR
  • old ocean floor consumed into mantle at subduction
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4
Q
A
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5
Q

SEE OCEAN FLOOR AND OCEANIC CRUST SLIDES IN BATHYMETRY LECTURE

A
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6
Q

describe sea-floor spreading

A

-sediment thickens away from ridges
-at MOR, earthquakes cause cracked crust to split apart
- high heat flor from molten rock rises into cracked crust
-new ocean floor at MOR
-old ocean floor consumed into mantle at subduction

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7
Q

what is the evidence of sea-floor spreading?

A

magnetism in sea-floor rocks varies farther from MOR. oceanic crustal rocks form on the ridges. in sea-floor basalts, we see stripes of normal (s to n) and reverse (n to s) magnetic fields. pattern is symmetric on either side of the MOR.

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8
Q

why does striping occur at sea-floor?

A

magnetic reversals. layered lava reveals reversals in magnetic polarity.

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9
Q

why do magnetic reversals occur?

A

magnetic field flips sometimes. WE DONT KNOW WHY. they are geologically rapid and expressed worldwide.

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10
Q

what are plate tectonics?

A

earth’s outer shell is broken into rigid plates that move. provides explanations for distribution of earthquakes, volcanoes, changes in past positions of continents and ocean basins, origins of mountain belts, etc.

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11
Q

what are the two types of lithosphere and their characteristics?

A

continental:
- felsic
- 150-400 km thick
- light and more buoyant
- crust is 25-70 km thick
oceanic:
- mafic (basalt and gabbro)
- crust is 7-10 km thick
- heavier and less buoyant

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12
Q

how many plates do we have and how fast do they move?

A

about 12
move at 1-15 cm/year which is vey fast geologically

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13
Q

where do earthquakes occur?

A

at plate boundaries, where tectonic plates meet

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14
Q

what are continental margins?

A

where land meets ocean. near plate boundaries =”active” EARTHQUAKES. margins far from boundaries = “passive”.

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15
Q

describe passive margin

A

passive margin continental crust thins seaward. traps eroded sediment and develops into continental shelf

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16
Q

what are the three types of plate boundaries?

A

divergent, convergent, transform

17
Q

describe divergent boundaries

A
  • tectonic plates move apart
  • lithosphere thickens away from ridge axis
  • new lithosphere created here
  • also called MOR or just ridge
  • ocean depth increases w distance from ridge
18
Q

describe convergent boundary

A
  • tectonic plates move together
  • plate consumption is called subduction
  • aka convergent margin, subduction zone, trench
  • trenches are deepest part of oceans
19
Q

describe transform boundary

A
  • tectonic plates slide sideways
  • plate material not created or destroyed
  • aka fault/transform
20
Q

how are mid-ocean ridges formed?

A
  • sea floor spreading opens axial rift valley
  • rising asthenosphere melts, forming mafic magma
  • pooled magma solidifies into oceanic crustal rock
21
Q

what is pillow basalt?

A

magma quenched at the seafloor
- gabbro = deeper, basalt = shallower

22
Q

what are black smokers?

A

-found at MORs
- water entering fractured rock is heated by magma
- hot water dissolves mineral, iron therefore black

23
Q

where are new and old oceanic crusts found?

A

new crust closer to ridge. older crust farther away. oldest found at far edge of basin. aging ocean crust move away from heat source of asthenosphere at base of MOR. older lithosphere sinks deeper into mantle allows for ocean deepening

24
Q

what lithosphere is the subducting plate?

A

subducting plate is always oceanic. continetal crust cannot be subducted, too buoyant. subduction recycles oceanic lithosphere. balanced by sea-floor spreading

25
Q

what is angle of descending subducting plate?

A

descends at angle of about 45 degrees. Different angle of subduction depending on age of the plate
We can kind of predict where an earthquake will occur depending on angle and location of subduction

26
Q

what depth to earthquakes cease?

A

below 660 km. plate descent can continue past earthquake limit. low mantle=plate graveyard

27
Q

name 4 geological features occurring due to subduction

A

deep-ocean trenches
accretionary prisms
volcanic arcs
back-arc basins

28
Q

what is a volcanic arc?

A

chain of volcanoes on overriding plate. continental crust = continental arc. oceanic crust = island arc

29
Q

what is the difference between volcanic island arc and continental arc?

A

Volcanic island arc = oceanic arc subducting under oceanic arc
Continental is more explosive = oceanic under continental arcs

30
Q

what are back-arc basins?

A

marginal sea behind an arc. forms between island arc and a continent.

31
Q

how are back-arc basins formed?

A

one tectonic plate subducts beneath another, creating a trench and causing the overlying plate to stretch and thin

32
Q

how are transform boundaries identified?

A

earthquakes
absence of volcanism
continental transforms cut across continental crust

33
Q

what are hot spots?

A

plumes of deep mantle material not linked to plate boundaries

34
Q

how are hot spots formed?

A

upward flow of hot material from the mantle melts lithosphere. magma rises to surface. PERFORATION of plates.

35
Q

how is a chain of extinct volcanoes formed due to hot spots?

A

volcanoes build above hot spot. plate motion pulls volcano off the plume. volcano goes extinct and erodes. hot spot seamounts age away from originating hot spot. indicates direction of plate motion

36
Q

what is continental rifting?

A

continental lithosphere breaks apart.
lithosphere stretches and thins.
brittle upper crust faults and ductile lower curst flows.
asthenosphere rises and melts

37
Q

what are the driving mechanisms of plate motions?

A

ridge-push: elevated MOR pushes lithosphere away
slab-pull: denser subducting plate is pulled downward. convection in asthenosphere

38
Q

how are plate velocities mapped?

A
  • plotting plate motion relative to fixed spot in mantle
  • measuring volcano ages/distances along hot spot track

Geology (19 decks)

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