Ch.2: Plate Tectonics

Question 1

Wegner’s hypothesis

Answer 1

continents are mobile and once formed a supercontinent, Pangaea
didn’t know the mechanism that moves
the continents—that distinguishes Continental Drift
from Plate Tectonics

Question 2

Wedger’s Evidence of Pangea (4)

Answer 2

“Jigsaw Fit” of continents
• Distribution of climate belts preserved in rocks (ex.
glacial deposits, coal deposits)
•Distribution of fossils
•Matching rock type and structures

Question 3

Newer evidence to support plate tectonics (7)

Answer 3

Bathymetric profiles and ocean floor maps
Patterns of earthquakes 
Seafloor spreading
High heat flow at oceanic ridges
Paleomagnetism
Magnetic reversals
Marine magnetic anomalies

Question 4

Bathymetric Profiles

Answer 4

illustrates that mid-ocean ridges are elevated above deeper abyssal plains

Question 5

Seafloor spreading

Answer 5

Harry Hess, 1962
• Measured high heat flow at ocean ridges
New ocean crust is created at mid-ocean ridges
(regions of high heat flow), moves away from the
ridges, and then sinks back into the mantle.
Robert Dietz termed this process subduction.

Question 6

Magnetic Inclination

Answer 6

Magnetic inclination indicates the paleo-latitude
of where the rock originally formed.
For example, a rock with magnetic inclination of 0°
formed at the magnetic equator.

Question 7

Paleomagnetism: Apparent Polar Wandering

Answer 7

Magnetic changes in a layered succession of lavas can

preserve the history of tectonic plates moving a continent.

Question 8

Magnetic reversals

Answer 8

During reverse magnetic polarity, the magnetic dipole
points in the opposite direction.
During these conditions, a compass needle would
point to the south magnetic pole. Points up

Question 9

Paleomagnetism: “Frozen” into Basalt

Answer 9

Iron-rich minerals in rocks record the magnetic
field at time the rock formed.
In hot magma, magnetic dipoles are randomly
oriented and no magnetization is possible.
As magma cools, magnetic dipoles in iron-bearing
minerals become aligned with Earth’s magnetic field.
The magnetic signature is then frozen in the rock.

Question 10

Marine magnetic anomalies

Answer 10

Magnetometers towed by ships moving across the midocean ridges recorded alternating strong and weak
magnetic fields.
Close to MOR

Question 11

Divergent

Answer 11

plates move away from each other
Two plates move away from one another; seafloor
spreading produces new oceanic crust at a mid-ocean
ridge. Also called a constructive plate boundary because new crust is formed.

Question 12

Convergent

Answer 12

plates move towards each other

The denser plate subducts beneath the overriding plate in this destructive plate margin.

Question 13

Transform

Answer 13

plates move sideways past each other

Question 14

Rifting

Answer 14

When continental lithosphere stretches and thins, the upper crust breaks. The upwelling asthenosphere
initiates volcanism. May eventually split a continent apart

Question 15

Ocean-continent convergence

Answer 15

forms a magmatic arc on the edge of the overriding

continental plate.

Question 16

Ocean-ocean convergence

Answer 16

forms a volcanic island arc on the edge of the

overriding oceanic plate

Question 17

Subduction

Answer 17

The Wadati-Benioff zone is an inclined plane of earthquakes that defines the position of the
downgoing tectonic plate.
When denser plate subducts under overriding plate, water (oceanic crust under continental) melts rock from mantle

Question 18

Continent-continent convergence

Answer 18

starts after subduction has consumed the oceanic plate that was once between two continents. Continental crust is too buoyant to subduct; when two continents converge, rock undergoes compression and shearing,
and a mountain range like the Himalayas develops.

Question 19

Volcanoes

Answer 19

As a plate drifts over a mantle plume, a volcano
forms over the hot spot. A chain of nowextinct volcanoes forms behind the active volcano (like in Hawaii). First proposed by J. Tuzo Wilson,
1960s.

Question 20

Youngest vs. oldest seas floor

Answer 20

youngest close to MOR

oldest away from MOR

Question 21

Ridge push

Answer 21

“Ridge push” develops by the gravitational
energy associated with the topographic
elevation of the midocean ridge.
Divergence at MORproduces high ridge that pushes down

Question 22

Convection

Answer 22

Warm areas in mantle rises, cool areas sink

Question 23

Slab pull

Answer 23

“Slab pull” develops because the old oceanic
lithosphere is more dense than the underlying
asthenosphere, so it sinks
Weight of downgoing slab pulls and out and away from ridge subducting plate down

Question 24

Active

Answer 24

Active plate boundaries

Question 25

Passive

Answer 25

Not active plate boundaries, result