Chapter 10

Question 1

List some of the evidence used by Wegener to support his idea of moving continents.

Answer 1

The evidence used by Wegener to support his idea of moving continents included matching continental shapes and geological features on either side of the Atlantic; common terrestrial fossils in South America, Africa, Australia, and India; and data on the rate of separation between Greenland and Europe.

Question 2

What was the primary technical weakness with Wegener’s continental drift theory?

Answer 2

The primary technical weakness of Wegener’s theory was that he had no realistic mechanism for making continents move.

Question 3

How were mountains thought to be formed (a) by contractionists and (b) by permanentists?

Answer 3

Contractionists assumed that mountains formed because as the Earth contracted the crust wrinkled into mountains.
Permanentists assumed that mountains formed by the geosynclinal process.

Question 4

How were the trans-Atlantic paleontological matchups explained in the late 19th century?

Answer 4

In the late 19th century the trans-Atlantic paleontological matchups were explained by assuming that there must have been land bridges between the continents at some time in the past, or that terrestrial organisms had floated across the ocean on logs.

Question 5

In the context of isostasy, what would prevent an area of continental crust from becoming part of an ocean?

Answer 5

Continental crust is lighter than oceanic crust and cannot sink low enough into the mantle to become an ocean (although this can happen over limited areas, and commonly does happen along coastal areas of continental plates).

Question 6

How did we learn about the topography of the sea floor in the early part of the 20th century?

Answer 6

Prior to 1920, ocean depths were measured by dropping a weighted line over the side of ship. Echo sounding techniques were developed at around that time and that greatly facilitated the measurement of ocean depths.

Question 7

How does the temperature profile of the crust and the mantle indicate that part of the mantle must be convecting?

Answer 7

Temperature increases quite rapidly with depth in the crust, but much less so in the mantle, and this implies mantle convection.

Question 8

What evidence from paleomagnetic studies provided support for continental drift?

Answer 8

Paleomagnetic studies showed that old rocks on the continents had different pole positions than they do today, and also that they were progressively more different with time past. This implied either that the poles had moved or that the continents had moved. It was also found that the apparent polar wandering paths for different continents were different, and this supported the concept that the continents had moved.

Question 9

Which parts of the oceans are the deepest?

Answer 9

The trenches associated with subductions zones are the deepest parts of the oceans.

Question 10

Why is there less sediment in the ocean ridge areas than in other parts of the sea floor?

Answer 10

The ocean ridge areas are the youngest parts of the sea floor and thus there hasn’t been time for much sediment to accumulate.

Question 11

How were the oceanic heat-flow data related to mantle convection?

Answer 11

It was (and still is) assumed that high heat flow exists where mantle convection cells are moving hot rock from the lower mantle toward the surface, and that low heat flow exists where there is downward movement of mantle rock.

Question 12

Describe the spatial and depth distribution of earthquakes at ocean ridges and ocean trenches.

Answer 12

Earthquakes are consistently shallow and relatively small at ocean ridges. At ocean trenches earthquakes get increasingly deep in the direction that the subducting plate is moving. The earthquakes near to surface can be very large, while those at depth tend to be small.

Question 13

In the model for ocean basins developed by Harold Hess, what took place at oceanic ridges and what took place at oceanic trenches?

Answer 13

In the Hess model new crust was formed at ocean ridges and then was consumed back into the mantle at the trenches.

Question 14

What aspect of plate tectonics was not included in the Hess theory?

Answer 14

Hess’s theory did not include the concept of tectonic plates.

Question 15

What is a mantle plume and what is its expected lifespan?

Answer 15

A mantle plume is a column of hot rock (not magma) that ascends toward the surface from the lower mantle. It is hypothesized that mantle plumes ascend as much as 10 times faster than the rate of mantle convection.

Question 16

Describe the nature of movement at an ocean ridge transform fault (a) between the ridge segments, and (b) outside the ridge segments.

Answer 16

etween the ridge segments there is movement in opposite directions along a transform fault.
Outside of the ridge segments the two plates are moving in the same direction and likely at about the same rate. In this case there is no faulting, and it is known as a fracture zone.

Question 17

How is it possible for a plate to include both oceanic and continental crust?

Answer 17

Tectonic plates are made up of crust and the lithospheric (rigid) part of the underlying mantle. The mantle part ensures that the very different oceanic and continental crust sections of a plate can act as one unit.

Question 18

What is the likely relationship between mantle plumes and the development of a continental rift?

Answer 18

mantle plume beneath a continent can cause the crust to form a dome which might eventually split open. Several mantle plumes along a line within a continent could lead to rifting.

Question 19

Why does subduction not take place at a continent-continent convergent zone?

Answer 19

Subduction does not take place at a continent-continent convergent zone because neither plate is dense enough to sink into the mantle.

Question 20

Where are Earth’s most recent sites of continental rifting and creation of new ocean floor?

Answer 20

Continental rifting is taking place along the East Africa Rift, and sea floor has recently been created in the Red Sea and also in the Gulf of California.

Question 21

What is likely to happen to western California over the next 50 million years?

Answer 21

Over the next 50 million years California is likely to split away from the rest of North America along the San Andreas Fault and then move north toward Alaska.

Question 22

What geological situation might eventually lead to the generation of a subduction zone at a passive ocean-continent boundary such as the eastern coast of North America?

Answer 22

The accumulation of sediment at a passive ocean-continent boundary will lead to the depression of the lithosphere and could eventually result in the separation of the oceanic and continental parts of the plate and the beginning of subduction.