Plate Tectonics Revisited

Question 1

How is heat generated from inside the earth transmitted?

Answer 1

Conduction and convection

Question 2

Define conduction

Answer 2

Heat energy transmitted through solid by molecular forces

Question 3

Define convection

Answer 3

Heat energy transmitted by density

Question 4

What mode of heat transfer does the mantle use?

Answer 4

convection

Question 5

Define post glacial rebound, and what it proves

Answer 5

Crust pushed by glaciers rebounds upwards when glacier melts. This proves that the mantle is viscous

Question 6

How do we know that the mantle convects?

Answer 6

It has a very high Rayleigh number

Question 7

Rayleigh number. Define it

Answer 7

A parameter that indicates whether or not convection occurs in a fluid being heated from below. Convection occurs when Ra > 2000

Question 8

How is Ra calculated?

Answer 8

(Variables that promote convection)/(variables that hinder convection)

Question 9

Some problems with thermal-convection-driven model of plate tectonics (4)

Answer 9

• Plate vary significantly in size, so regular-sized convection cells not possible.
• Convection cells in mantle should be stable long-term features, but spreading ridges
move rapidly over Earth’s surface (in places at rates approaching the spreading rates
themselves), indicating they are superficial features.
• Some plates (e.g., African plate) surrounded by ridges; no subduction zones that could
correspond to downwelling limb of convection cell.
• Spreading ridges can be subducted (nail in coffin)

Question 10

What is responsible for convection of the mantle?

Answer 10

Plate tectonics, when plates get secreted and subducted, will cycle is the heat in convection

Question 11

What is the Beinoff zone?

Answer 11

Planar zone os seismicity associated with down-going slab at subduction zone

Question 12

Is there evidence of mantle convection independent of plate tectonics?

Answer 12

Hotspots

Question 13

What three types of plate tectonics

Answer 13

Divergent, Transform, and Convergent

Question 14

What characterizes divergent plate boundaries?

Answer 14

Normal faults); crustal thinning; rifting of continental crust;
eventually mafic volcanism

Question 15

What characterizes transform plate boundaries

Answer 15

Plates slide horizontally past eachother.
no thinning or
extension of crust; lithosphere not created or detroyed; limited to no volcanism

Question 16

How many plate boundary types?

Answer 16

3

Question 17

Oceanic-oceanic plate boundaries characteristics (4)

Answer 17

1. Older plate subducts under younger one;
2. water brought with it acts like flux (flux melting) that lowers the melting point of
   the slab,
3. mafic to intermediate magmas are
   produced,
4. generates a volcanic island arc.

Question 18

Define Volcanic island arc

Answer 18

A chain of volcanic islands in the middle of the ocean

Question 19

What characterizes Oceanic-continental plate boundaries (4)

Answer 19

1. Oceanic lithosphere subducts under continental lithosphere; 2. water is brought in with it, acting as a flux (flux melting)
2. mafic to felsic magmas
   produced
3. generates a
   continental arc

Question 20

Define continental arcs

Answer 20

A chain of volcanic mountains along the edge of the continent

Question 21

What defines Continental-continental plate boundaries (4)

Answer 21

1. Neither plate subducts completely (too buoyant) 2. no volatiles being delivered to
   depth ,therefore no volcanism.
2. Significant crustal thickening occurs (intense folding and
   thrust faulting).
3. Shallow earthquake activity only (no subduction, no Benioff zone).

Question 22

Define Seamounts

Answer 22

older, extinct volcanoes generated at spreading ridges or hotspots

Question 23

What are guyuts?

Answer 23

flat-topped seamounts

Question 24

How are many of the smaller morphological elements on the seafloor are generated ?volcanism

Answer 24

volcanism

Question 25

What do hotspots generally look like?

Answer 25

Big heads, narrow tails

Question 26

Which of the following observations suggest the mantle is viscoelastic?

a) The mantle transmits P-waves and S-waves.
b) The mantle transmits P-waves, as does the outer core.
c) The mantle transmits S-waves. Post-glacial rebound occurs.
d) The mantle transmits S-waves but the outer core does not.

Answer 26

c

Question 27

The mantle convects because

a) of its low viscosity.
b) of its high thermal conductivity.
c) of the minimal temperature gradient between its top and bottom.
d) None of the above.

Answer 27

d

Question 28

Which of the following challenges the idea that mantle convection drives plate
tectonics?
a) Spreading ridges can migrate rapidly across the Earth’s surface
b) Spreading ridges can be subducted
c) Some plates (e.g., African plate) are bordered almost entirely by spreading
ridges
d) All of the above.

Answer 28

d

Question 29

Many researchers hypothesize that plate tectonics is driven by the plates
themselves. Explain the forces that likely facilitate this.

Answer 29

Slab pull (dominant) and ridge push (subordinate).

Question 30

Earthquakes and faults are confined to the lithosphere because
a) the mantle is incapable of brittle failure.
b) the lithosphere can deform by brittle failure whereas the mantle that underlies
it cannot.
c) the mantle deforms through viscous dissipation.
d) All of the above.

Answer 30

c

Question 31

Define the term Benioff zone and explain why it occurs.

Answer 31

Benioff zones are dipping, planar zones of increased earthquake activity caused
by subduction of one tectonic plate beneath another. See slide 18.

Question 32

Volcanoes are only common at some types of convergent plate boundaries.
Why?

Answer 32

Because subduction does not occur at continental-continental convergent plate
boundaries. See slide 36.

Question 33

In contrast to the Pacific margin, the Atlantic margin of North America is
aseismic (i.e., characterized by few earthquakes) and lacks volcanoes, even
though both margins occur at boundaries between continental and oceanic
lithosphere. Why?

Answer 33

The Atlantic margin is a passive margin, whereas the Pacific margin is an active
one. See slides 47 to 49.

Question 34

Define aseismic

Answer 34

characterized by few earthquakes