L5: Plate Tectonics

Question 1

What is seafloor spreading?

Answer 1

A theory proposed during the 60s in which suggesting lateral movement of the oceanic crust away from mid-ocean ridges

Question 2

What occurs during seafloor spreading?

Answer 2

• The release of pressure at mid-ocean ridges as the plates move apart, which allows hot magma from the asthenosphere to reach the ocean floor.
• There it cools and crystallizes to form new lithosphere
• The system of mid-ocean ridges along divergent plate boundaries forms linear submarine mountain chains
• Youngest rocks are located closest to the ocean ridges; progressively older as you travel further

Question 3

Divergent plate margins.

Answer 3

• Fractures in the lithosphere where 2 plates move apart
• AKA spreading centers
• Can occur in plates capped by either continental or oceanic crust

Question 4

Convergent plate margins

Answer 4

• Occur where 2 plates are moving towards each other
• Can occur in oceanic, continental, or both
• Because of their very different properties, the results of the convergence will be fundamentally different

Question 5

Transform fault plate margins.

Answer 5

Fractures in the lithosphere where 2 plates slide past each other, grinding and abrading their edges as they do so. Can occur in both oceanic or in continental crust

Question 6

Ocean crust is _____ than continental crust, hence the reason for very old ________.

Answer 6

• Stable

- Continents

Question 7

Define continental drift.

Answer 7

Slow movement of continents across the face of earth

Question 8

Define continental-rift zone.

Answer 8

Rifting and spreading zones on continents are characterized by rift valley, volcanism, and earthquakes

Question 9

What is continental rifting?

Answer 9

Mantle material also rises from near the core-mantle boundary in the form of thermal plumes

Question 10

What are thermal plumes?

Answer 10

• Occurs in continental rifting
• When a plume nears the lithosphere with overlying continent, it dreads laterally, doming the overlying plate and moving the rifted segments outward from the central area
• Uplifts results in fractures that eventually can open up to form narrow oceanic tracts

Question 11

Describe the process rifting/rifting Africa (?)

Answer 11

The dissection of a continent and potential birth of a plate
Huge continental masses act as thermal blankets; they slowly heat up and expand
Notes bc that shit LONG

Question 12

What is the Wilson Cycle?

Answer 12

• The opening of new ocean basins along divergent zones, the expansion of the basin as seafloor spreading continues, and the ultimate closure of the basin as plates converge
• Occurs bc spreading is not continuous; eventually needs to close

Question 13

List the forms of plate margins.

Answer 13

• Divergent boundaries
• Convergent boundaries
• Transform boundaries
• Hot spots

Question 14

Define ocean-continent boundaries.

Answer 14

• Continental lithosphere is relatively low in density and buoyant
• Fe rich mantle

Question 15

Describe the process of ocean-continent boundaries.

Answer 15

Oceanic lithosphere is more similar in density to the underlying asthenosphere, so it’s easily forced under the continental material when the plates converge. (subduction)

Question 16

When _____ lithosphere meets _______ lithosphere, the _______ lithosphere is subducted, and a _______ ________ belt is formed at the __________ __________.

Answer 16

• Oceanic
• Continental
• Oceanic
• Volcanic mount
• Continental margin

Question 17

What is subduction?

Answer 17

When 2 plates capped by oceanic crust converge, one of the plates will undergo subduction and descend into the mantel underneath the other plate; the sinking of old, cold oceanic lithosphere into the asthenosphere

Question 18

What happens to mid-ocean ridges created in new lithosphere?

Answer 18

• Subduction zones of the world balance seafloor creation, so these ridges would be consumed in subduction
• Buoyant continents cannot be subducted, so they’re preserved hence the old pieces of crust on each continent.

Question 19

How does subduction work?

Answer 19

• The subducted plate is heated, melted, and re-incorporated into the asthenosphere
• Volcanoes form where the melted material rises up through the overlying plate to the surface
• Meanwhile the continental crust is compressed, folded, and faulted leading to mountains forming

Question 20

Describe ocean-ocean convergence.

Answer 20

• Where oceanic lithosphere meets oceanic lithosphere
• On plate gets subducted under the other
• A deep sea trench and a volcanic island arc are formed
• The older, more dense plate sinks below the younger, less dense plate, which leads to the formation of “island arcs”

Question 21

Continental Shield

Answer 21

Assemblage of ancient cratons and orogens

Question 22

What is a supercontinent?

Answer 22

The assemblage of cratons into large continental complexes

Question 23

Define icostasy

Answer 23

• The property whereby the lithosphere maintains floatation balance
• This means that his mass is stable w/ respect to the underlying asthenosphere and is neither rising nor sinking
• Icostasy is what prevents “ from sinking

Question 24

Define mid-ocean ridge transform fault.

Answer 24

Mid-ocean ridges are typically offset by transform faults

Question 25

Define continental transform fault.

Answer 25

At the transform faults, plates slip horizontally past each other

Question 26

What occurs at a spreading ridge?

Answer 26

• The rift is not a long continuous crack
• Actually forms numerous short segments where 2 opposite plates scrape past each other
• These transform faults have a lot of stress leading to earthquakes

Question 27

What are hot spots?

Answer 27

• Arise when isolated plumes of magma (smalled versions of those in rifting) rise from deep within the asthenosphere to the surface
• These plumes are thought to be fixed in the mantle
• As plates move over it there becomes a progressive trail of volcanoes at the Earths surface

Question 28

Fill out the table

Answer 28

Notes

Question 29

Orogens

Answer 29

• Bands of mountains
• Younger along current active margins
• Ancient that are remnant mountains (often just the roots), some welding cratons together

Question 30

Craton

Answer 30

The oldest part of a continent, formed, evolved, and altered over billions of years (stable)

Question 31

How are mountain belts formed?

Answer 31

• Collision results in large zones of deformation where continental crust is involved in subduction
• Typically form high mountain ranges (orogens) causing crust to thicken = rocks within orogens are crumpled (deformed)
• Plates are not perfectly rigid!

Question 32

Continent-continent convergence.

Answer 32

• Where 2 continents converge, the crust crumples and thickens, creating high mountains and a wide plateau
• 2 landmasses come together to form a mountain range bc both plates are too buoyant to sink into the mantle
• @ buoyant plates lead to the thickening of the crust and the resultant formation of a mountain chain

Question 33

List convergent boundaries.

Answer 33

• Ocean-ocean convergence

- Ocean-continent