Plate Tectonics: A Unifying Theory - The Changing Earth: Exploring Geology and Evolution - page 50

Aleutian
Islands

Alaskan
coast

Kamchatka

Sea level Sea level

Emperor
Seamounts

Kauai
3.8 - 5.6

Direction
of plate movement

Oahu
2.3 - 3.3

Sea level

Molokai
1.3 - 1.8

Maui
0.8 - 1.3

Hawaiian

hot spot

Upper mantle

Hawaiian Islands

Oceanic crust

Hawaii 0.7 to
present

Mantle
plume

Asthenosphere

◗

Figure 2.22 Hot Spots A hot spot is the location where a stationary mantle plume has risen to the surface and formed a volcano. The
Emperor Seamount—Hawaiian Island chain formed as a result of the Pacifi c plate moving over a mantle plume, and the line of volcanic
islands in this chain traces the direction of plate movement. The island of Hawaii and the Loihi Seamount are the only current hot spots of
this island chain. The numbers indicate the ages of the Hawaiian Islands in millions of years.

◗ geologists to determine absolute motion because they pro-

vide an apparently fi xed reference point from which the rate
and direction of plate movement can be measured.
The previously mentioned Emperor Seamount-
Hawaiian Island chain formed as a result of movement over
a hot spot. Thus, the line of the volcanic islands traces the di-
rection of plate movement, and dating the volcanoes enables
geologists to determine the rate of movement.

Figure 2.23 Reconstructing Plate Positions Using Magnetic
Anomalies

Anomaly
31

Present
Mid-Atlantic
Ridge

Anomaly
31

Eurasian
Plate

North American
Plate

THE DRIVING MECHANISM
OF PLATE TECTONICS
A major obstacle to the acceptance of the continental drift hy-
pothesis was the lack of a driving mechanism to explain con-
tinental movement. When it was shown that continents and
ocean fl oors moved together, not separately, and that new crust
is formed at spreading ridges by rising magma, most geologists
accepted some type of convective heat system (convection cells)
as the basic process responsible for plate motion. The question
still remains, however: What exactly drives the plates?
Most of the heat from Earth's interior results from the de-
cay of radioactive elements, such as uranium (see Chapter 17),
in the core and lower mantle. The most efficient way for
this heat to escape Earth's interior is through some type of
slow convection of mantle rock in which hot rock from the
interior rises toward the surface, loses heat to the overlying
lithosphere, becomes denser as it cools, and then sinks back
into the interior where it is heated, and the process repeats
itself. This type of convective heat system is analogous to a
pot of stew cooking on a stove (

African
Plate

a The present North Atlantic, showing the Mid-Atlantic Ridge
and magnetic anomaly 31, which formed 67 million years ago.

Eurasian
Plate

North American
Plate

African
Plate

Figure 2.24).
Two models involving thermal convection cells have
been proposed to explain plate movement (

◗

b The Atlantic 67 million years ago. Anomaly 31 marks the plate
boundary 67 million years ago. By moving the anomalies back
together, along with the plates they are on, we can reconstruct the
former positions of the continents.

Figure 2.25).
In one model, thermal convection cells are restricted to the

◗