Chapter 2

Question 1

Accretionary wedge

Answer 1

Sediment and pieces of oceanic crust scrape off downgoing plate and accumulate as an accretionary wedge along leading edge of overriding plate.

Question 2

Asthenosphere

Answer 2

Rises and melts in the ocean.

Question 3

Collage tectonics

Answer 3

A collage is an assemblage of micro-continents, arcs, and other crustal fragments accreted along the edge of a continent adjacent to a subduction zone.

Question 4

Continental drift

Answer 4

A hypothesis, credited largely to Alfred Wegener, that suggested all present continents once existed as a single supercontinent. Further, beginning about 200 million years ago, the supercontinent began breaking into smaller continents, which then “drifted” to their present positions.

Question 5

Convergent boundary

Answer 5

Convergent (destructive):

– Two plates collide with one possibly subducting beneath the other. Creates new mountain systems.

Question 6

Deep-ocean trench

Answer 6

Trench:

– Deep depression on seafloor where oceanic plate bends downward. A narrow, elongated depression of the seafloor.

Question 7

Divergent boundary

Answer 7

Divergent (constructive):
– Two plates move apart from
one another. Creates new seafloor

Question 8

Forearc basin

Answer 8

Volcanic material and sediment accumulate within forearc basin.

Question 9

Fracture zone

Answer 9

Linear zone of irregular topography on the deep-ocean floor that follows transform faults and their inactive extensions.

Question 10

Geomagnetic reversal

Answer 10

A change in Earth’s magnetic field from normal to reverse or vice versa.

Question 11

Hot spot

Answer 11

A concentration of heat in the mantle, capable of producing magma that, in turn, extrudes onto Earth’s surface. The intraplate volcanism that produced the Hawaiian Islands is one example.

Question 12

Island arc

Answer 12

A chain of volcanic islands generally located a few hundred km from a trench where there is active subduction of one oceanic plate beneath another.

Question 13

Oceanic Lithosphere

Answer 13

New oceanic lithosphere moves laterally away from spreading ridges at rates ranging from 1 - 18cm/yr:
– Youngest oceanic lithosphere occurs along the ridge axis;
– Oceanic lithosphere becomes progressively older away from the ridge axis;
– Oldest oceanic lithosphere occurs along edge of the ocean basin adjacent to a subduction zone and/or continental margin.

Question 14

Magnetic time scale

Answer 14

Time scale of the Earth’s magnetic field in the recent past. Developed by establishing the magnetic polarity for lava flows of known age. Major divisions are called “chrons.”

Question 15

Mantle plume

Answer 15

A mass of hotter-than-normal mantle material that ascends toward the surface, where it may lead to igneous activity. These plumes of solid yet mobile material may originate as deep as the core-mantle boundary.

Question 16

Normal polarity

Answer 16

A magnetic field the same as that which presently exists.

Question 17

Oceanic ridge system

Answer 17

A continuous mountainous ridge on the floor of all the major ocean basins and varying in width from 500 to 5000 km (300-3000 miles). The rifts at the crests of these ridges represent divergent plate boundaries.

Question 18

Paleomagnetism

Answer 18

Magnetic properties of ancient rocks provide further evidence that continents drifted over time. The natural remnant magnetism in rock bodies. The permanent magnetization acquired by rock that can be used to determine the location of the magnetic poles and the latitude of the rock at the time it became magnetized.

Question 19

Plate

Answer 19

A coherent unit of Earth’s rigid outer layer that includes the crust and upper unit.

Question 20

Plate tectonics

Answer 20

The theory that proposes that Earth’s outer shell consists of individual plates that interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself.

Question 21

Reverse polarity

Answer 21

A magnetic field opposite to that which presently exists.

Question 22

Ridge push

Answer 22

A gravity driven force that results from the elevated position of the ridge. A mechanism that may contribute to plate motion. It involves the oceanic lithosphere sliding down the oceanic ridge under the pull of gravity.

Question 23

Rift or rift valley

Answer 23

A long, narrow trough bounded by normal faults. It represents a region where divergence is taking place.

Question 24

Seafloor spreading

Answer 24

The growth and Bathymetry of ocean basins The hypothesis, first proposed in the 1960s by Harry Hess, which suggested that new oceanic crust is produced at the crests of mid-ocean ridges, which are the sites of divergence.

Question 25

Slab-pull

Answer 25

Results from the sinking of a cold dense slab of an oceanic lithosphere and is the major driving force of plate motion. A mechanism that contributes to plate motion in which cool, dense oceanic crust sinks into the mantle and “pulls” the trailing lithosphere along.

Question 26

Slab suction

Answer 26

One of the driving forces of plate motion, it arises from the drag of the subducting plate on the adjacent mantle. It is an induced mantle circulation that pulls both the subducting and overriding plates toward the trench.

Question 27

Spreading center

Answer 27

A boundary in which two plates move apart, resulting in upwelling of material from the mantle to create new seafloor.

Question 28

Subduction zone

Answer 28

A long, narrow zone where one lithospheric plate descends beneath another.
Consists of:
Trench
Accretionary wedge 
Forearm Basin
Volcanic Arc

Question 29

Transform boundary

Answer 29

Transform (lithosphere conserved):

– Two plates slide laterally past one another.

Question 30

Volcanic arc

Answer 30

Magma reaches surface and erupts to form volcanic arc.