Plate Tectonics

Question 1

What is plate tectonics?

Answer 1

a theory that explains the global distribution of geological phenomena in terms of the formation, destruction, movement, and interaction of Earth’s lithospheric plates

Question 2

What evidence did Wegener provide that continents were once fitted together like a jigsaw puzzle as the supercontinent Pangaea?

Answer 2

1. Occurrence of Late Paleozoic fossils make more sense when continents are fitted together
2. Inferred ice flow directions (based on grooves on bedrock found under ancient glacial sediments)
3. mountain belts of same age connect to form single mountain belt

Question 3

What did geophysical surveys indicate about the magnetic polarity within the ocean floor rock?

Answer 3

the magnetic polarity of magnetite crystals within the ocean floor rock (basalt) alternates from normal to reversed in a symmetrical pattern

Question 4

What are the ages of the ocean floor

Answer 4

dating of ocean floor reveal symmetric pattern in relation to mid-ocean ridges, youngest age along crests of mid-ocean ridges + oldest age near the edges of continents
- youngest regions shown in midlines of oceans + oldest near edges of continents

Question 5

How thick is the seafloor sediment actually?

Answer 5

• surveys of the ocean floor indicated that thickness of oceanic sediment overlying the basalt foundation increases away from the axes of mid-ocean ridges

Question 6

What was the explanation for mid-ocean ridge system?

Answer 6

• harry hess proposed:
• new seafloor (therefore new oceanic crust) is formed along the axis of mid-ocean ridge
• new lithosphere moved laterally away from ridge, eventually plunging downward into a submarine trench (commonly along a continental margin)

Question 7

What was the hypothesis of “seafloor spreading”?

Answer 7

formed the foundation for concept of plate tectonics
- explained why seafloor basalt is youngest at the crests of mid-ocean ridges
- magma, sourced from the mantle is injected into central rift of a mid-ocean ridge, forming new seafloor

Question 8

What was the explanation of Magnetic Stripes?

Answer 8

• new ocean crust formed along axis of mid-ocean ridge, magnetite crystals that crystallize from basaltic magma (extruded as lava) are aligned in the direction of magnetic field
• as it cools, magnetite crystals are “frozen into place”, preserving their orientation during the time of crystallization
• due to symmetrical spreading of seafloor, a symmetrical pattern of polarity “stripes” is preserved

Question 9

What was the explanation of sediment thickness?

Answer 9

-w/o seafloor spreading, entire ocean expected to be covered w/ thick blanket of oceanic sediment
- w/ seafloor spreading, sediment pile would be expected to thicken away from the ridge axis (older crust is furthest away from ridge)

Question 10

What was the explanation for Seamount pattern?

Answer 10

-volcanoes deduced to have formed above stationary mantle heat plumes (“hotspots”)
- chains of hotspots are produced as plate moves over the hotspot
- explains why these hotspot volcanoes (+ other submerged seamounts) are older + more deeply submerged w/ increasing distance from hotspot
- they sink as the crust cools

Question 11

What is the explanation of the Trenches?

Answer 11

• if seafloor is made at mid-ocean ridges, it must be destroyed somewhere else (otherwise the earth would be expanding)
• To Hess, submarine trenches seemed to be the obvious places where ocean floor would be destroyed (reasoned, the crust underlying seafloor plunged downward (was subducted) under an adjacent plate to ultimately be assimilated in the mantle

Question 12

What was the evidence of subduction

Answer 12

• areas w/ most severe earthquakes (indicating severe compression + subsequent release of energy)
• focal points of earthquakes are deeper inboard of the trench
• the oblique array of earthquake occurrences that indicate the position of the descending slab

Question 13

What is mantle convection

Answer 13

• upward-flowing currents would deliver hot magma to mid-ocean ridges, whereas downward-flowing currents would drag cold crust into mantle at trenches

Question 14

What are the 3 types of plate boundaries?

Answer 14

1. divergent
2. convergent
3. transform

Question 15

what is divergent plate boundary?

Answer 15

2 plates moving away from each other (+ where new oceanic lithosphere forms)

Question 16

what is convergent plate boundary?

Answer 16

2 plates moving toward one another (where old oceanic lithosphere is consumed)

Question 17

what is transform plate boundary?

Answer 17

2 plates are moving alongside one another (lithosphere neither created, nor destroyed)

Question 18

Oceanic lithosphere is _____ recycled, continental lithosphere is not

Answer 18

constantly

Question 19

what does the lithosphere consist of

Answer 19

crust + lithospheric mantle

Question 20

Continental lithosphere is ____ and ____ than Oceanic lithosphere

Answer 20

lighter + thicker

Question 21

What is Isostasy

Answer 21

balance of all large portions of Earth’s lithosphere
- Thick continental lithosphere stands higher on the asthenosphere than thin oceanic lithosphere

Question 22

Why are continents floating rafts?

Answer 22

-oceanic lithosphere is continuously created + destroyed (quite young)
- slabs of continental lithosphere float like rafts (continental lithosphere can never be completely assimilated back into the mantle (too buoyant)

Question 23

How are ocean basins formed in the first place?

Answer 23

-heat builds up under a continent (possibly over a mantle heat plume)
- continental lithosphere warp upward + thins (puts lithosphere in state of tension)
- Asthenosphere rock wells up + melts (decompression melting), producing mafic magma
- mafic magma rises to produce oceanic crust in rift valley (develops into wider linear sea, process continues, widening into an ocean basin as continental fragments drift apart)

Question 24

What are triple junctions + rifting?

Answer 24

• rifts form as 3 “arms” (meeting as triple junction)
• 2/3 “arms” continue to spread eventually form ocean basin
• remaining arm stops spreading + becomes “failed rift” (that just fills w/ sediment from then onward)

Question 25

Igneous rocks at Divergent plate boundaries

Answer 25

“runny” mafic magma sourced from the mantle intruded into fractures as plates move apart at a spreading centre (mid-ocean ridge)

Upper crust of oceanic plate is made of basalt (aphanitic mafic rock)

Lower part of crust is made of gabbro (phaneritic mafic rock)

Underlying these mafic crustal rocks in mantle peridotite (ultramafic mantle rock)

Ocean basins originate when continental landmasses split apart

Question 26

Igneous Rocks at convergent plate boundaries

Answer 26

A slab of oceanic lithosphere can be subducted under continental lithosphere (or another slab of oceanic lithosphere)

Magma is produced via hydration melting

At surface, igneous rocks include pyroclastic deposits + andesite or rhyolite

At depth, magma cools slowly to produce diorite or granite

Question 27

how do plate tectonics explain how largest mountain ranges are produced?

Answer 27

Continents are too “light” to be subducted, they collide when an ocean “closes”

Due to compressive stress, continental lithosphere is shortened + produces mountains at the surface

Scraps of oceanic crust (including sediments) can be found high + dry in the highest mountain ranges

Question 28

how do plate tectonics explain regional metamorphism?

Answer 28

Compression created by converging plates, together w/ heating of rock as the crust is thickened and lowered downward produces regional metamorphism (metamorphic grade increases w/ depth)

Question 29

Where do clastic sediments derive from

Answer 29

derived from wearing-down of mountains that owe their existence to convergence of plates

Question 30

Epicentres of earthquakes and volcanoes follow _______

Answer 30

plate boundary pattern

Question 31

how do we explain the movement of lithospheric plates?

Answer 31

Convection: hot mantle rock rises, cools and sinks, dragging the plates along

Ridge Push (ridge sliding): Plates slide down the flanks of mid-ocean ridges under the downward “push” of gravity (like toboggans down either side of a hill)

Slab Pull: The dense, leading edge of a plate “pulls” the rest of the plate down into the mantle (via subduction)