(N-S) of the planet at the time magma cools into volcanic rock. Earth scientists
had previously realized that the polarity of Earth’s magnetic field suddenly
reverses from N-S and S-N. Reversals have been noted as long as 780,000 years
apart and as short as 25,000 years. As new rock is created it records these polarity
reversals. The stunning geography discovered by Vine and Matthews was that
there are numerous stripes of matching polarity running in parallel out from the
Indian Ridge. Indeed, the ages of these seafloor rocks increases with increasing
distance from oceanic ridges. This was direct evidence of seafloor spreading and
opened a cornucopia of research that is called plate tectonics.
There is now no doubt that Earth’s crust is arranged in a series of a dozen major
plates and many minor ones. The Pacific Plate is the largest at 103,000,000 sq km.
Others are as small as 18,000 sq km and our knowledge of sizes and boundaries are
often being refined. The size of a plate may not bespeak its significance in land-
forming processes. For instance, Juan De Fuca is a modest size plate caught
between the Pacific Plate and the North American Plate and is responsible for
large earthquakes and volcanoes in the northwestern United States and
southwestern Canada.
The vertical structure of Earth’s crust has been described elsewhere in this book
(seeEarth). The plates themselves range up to 100 km in depth. In the horizontal
realm, the plates are defined by their very definite edges. They have irregular,
jagged shapes that have been likened to the pieces of a jigsaw puzzle. Some plates
are almost entirely covered by ocean (e.g., the Pacific Plate) and some are covered
entirely by land (e.g., the Turkish-Aegean plate). Most plates are covered by both
land and ocean (e.g., the North American Plate).
The energy for plate motion is provided by slow convection within the mantle.
The relatively cool and rigid crustal plates ride slowly on the hot, weak rocks of
the asthenosphere, a part of the mantle usually 50 to 150 km deep. In the astheno-
sphere the hot temperatures of near 1600°C bring rocks to near-melting condi-
tions. These rocks are plastic and capable of convectional flow.
Isostasy is a vital concept in understanding the behavior of plates. Because of
materials and density differences, the crust floats on the asthenosphere. As materi-
als are floated toward and away from a particular area, the altitude of the crust will
adjust according to the amount and density of the materials. The compensation
may be rapid over a few hundreds or thousands of years or slow over millions of
years. Isostasy gives us the wherewithal to understand the grand movements at
theboundariesof crustal plates.
Crustal plates move away from areas of seafloor spread in various directions.
Motion ranges from 1 to 10 cm per year. Although these are slow in terms of familiar
environmental events, they can cause massive crustal impacts. The Atlantic Ocean,
Plate Tectonics 257