Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 27

Plate Tectonics
The basis of plate tectonics is the idea that the Earth's surface is broken into six large
and many smaller rigid plates, like a huge cracked sphere (Fig. 2.6). The plates consist
of portions of both continents and ocean basins, each about 100 km (∼60 mi) thick, that
are moving in various directions at rates varying from ∼1 to 10 cm (∼¼-4 in.) per year,
or about as fast as fingernails grow. Where plates pull apart, new volcanic material fills
the void, but where they come together, one oceanic plate dives beneath the other and
is absorbed back into the Earth. If the second plate is a continent, its rocks are com-
monly squeezed and buckled up into mountains.
The concept of plate tectonics was born in the 1960s by combining two preexisting
ideas—continental drift and sea-floor spreading (Fig. 2.7). Initial evidence for plate tec-
tonics came from the sea floor. Systematic surveys in the late 1950s and 1960s revealed
that the globe is virtually encircled by spectacular undersea mountain ranges or oceanic
ridges. The development of sophisticated seismic, sonar, and computerized equipment
allowed detailed sampling and analysis of the ocean floor. It became evident that, simil-
ar to those on land, the undersea mountain ranges are the loci of frequent volcanic and
earthquake activity, and that they are areas of abnormally high heat flow. The largest
and best-known undersea range is the Mid-Atlantic Ridge, which extends north-south
for several thousand kilometers, roughly parallel to the coastlines of Europe, Africa, and
the Americas. The active volcanic island of Iceland is a place where the ridge is large
enough to protrude above sea level.
A corollary discovery based on dating of rocks from the sea floor revealed that the
youngest volcanic rocks are near the centers of the undersea ridges; rock ages increase
with distance outward from the ridge in both directions. This led to the discovery of
sea-floor spreading, the idea that new volcanic crust is created at mid-ocean ridges and
then spreads outward by dividing between the two plates moving in opposite directions.
In addition, other studies revealed distinct magnetic strips of volcanic rock paralleling
the axis of the mid-ocean ridges. These strips occur in symmetrically distributed pairs,
with the pattern on one side of the ridge forming a mirror image of the other. The mag-
netic orientation of iron traces contained in the formerly molten rock in these strips
can reveal opposite headings. It is well known that the Earth's magnetic field has re-
versed itself repeatedly throughout geologic time—north has become south and south
has become north (although the magnetic poles have stayed in about the same loca-
tions). When lava was erupted along the mid-ocean crest, it became magnetized in the
direction of the prevailing magnetic field. As the sea floor spread, the rock was carried
away from the center in both directions like two gigantic conveyor belts. Later erup-
tions deposited new material, which was in turn magnetized in the direction of the new
magnetic field and then transported away from the ridge axis. By dating the rocks in
these strips, it is possible to calculate the rates of sea-floor spreading. Rates of plate
movement are also measured over short time periods by using astronomical and satel-
lite laser ranging techniques, as well as repeat measurements of continental plate dis-
placement by the global positioning navigation satellites (Gripp and Gordon 1990; Her-
ring 1996; Turcotte and Schubert 2002).