chapter

(Figure 7.24). Visible evidence of the age of Kauai can
be seen by examining its extinct volcanoes, which have
been eroded into jagged peaks and vast canyons. By contrast,
the south slopes of the relatively youthful island of
Hawaii consist of fresh lava flows, and two of Hawaii’s
volcanoes, Mauna Loa and Kilauea, remain active.
This evidence supports the fact that the plates do
indeed move relative to Earth’s interior. The hot spot
tracks also trace the direction of plate motion. Notice, for
example, in Figure 7.24 (lower left) that the Hawaiian
Island–Emperor Seamount chain bends. This particular
bend in the trace occurred about 40 million years
ago when the motion of the Pacific plate changed from
nearly due north to its present northwesterly path.
Suiko
65 my
Midway
Islands
27 my
Kauai
3.8—5.6
Direction of
plate motion
Emperor
Seamount chain
Hawaiian chain
Oahu
2.2–3.3
Hawaii
Molokai
1.3–1.8
Oceanic
lithosphere
Ages given
in millions of
years
Measuring Plate Motion
Chapter 7 Plate Tectonics 217
Figure 7.23 The JOIDES Resolution,
the drilling ship of the Ocean Drilling
Program. This modern drilling ship
replaced the Glomar Challenger in the
important work of sampling the floors
of the world’s oceans. JOIDES is an
acronym for Joint Oceanographic
Institutions for Deep Earth Sampling.
(Photo courtesy of Ocean Drilling
Program)
A number of methods have been employed to establish
the direction and rate of plate motion. As noted earlier,
hot spot “tracks” like those of the Hawaiian Island–
Emperor Seamount chain trace the movement of the
Pacific Plate relative to the mantle below. Further, by
measuring the length of this volcanic chain and the time
interval between the formation of the oldest structure
(Suiko Seamount) and youngest structure (Hawaii), an
average rate of plate motion can be calculated. In this
case the volcanic chain is roughly 3000 kilometers long
and has formed over the past 65 million years—making
the average rate of movement about 9 centimeters
Maui
less than 1.0
Hot spot
Hawaii
0.7 to present
Mantle
plume
Figure 7.24 The chain of islands and
seamounts that extends from Hawaii
to the Aleutian trench results from the
movement of the Pacific plate over
an apparently stationary hot spot.
Radiometric dating of the Hawaiian
Islands shows that the volcanic activity
decreases in age toward the island of
Hawaii.