Geology Reference
In-Depth Information
Seismologists found the solution. In the cold war climate of the 1960s, nuclear weapons
became the central focus (and primary source of funding) of seismology. Following the
1962 Cuban missile crisis, the United States and the Soviet Union agreed to a Limited Test
BanTreaty,whichrestricted nuclear weaponstesting toundergrounddetonations. Verifica-
tionofthetreatyreliedoncontinuousseismicmonitoringusinganextensive(read:expens-
ive) array of vibration-sensitive instruments deployed to the far reaches of the globe. The
resulting World-Wide Standardized Seismograph Network (WWSSN) linked 120 stations
to a central computer-processing center in Golden, Colorado, home of one branch of the
United States Geological Survey. For the first time ever, it was possible to pinpoint the ex-
act locations, depths, magnitudes, and motions of small earthquakes (and big explosions)
anywhere on Earth.
ThefringebenefitsforEarthscienceweretremendous.Armedwiththeirnewtools,geo-
physicists could sense thousands of previously undetectable Earth movements and thus
document previously unrecognized planetwide patterns of earthquakes. They found that
almost all of Earth's sudden crustal motions occur along narrow lines of intense seismic
activity—places like the midocean ridges. Many other quakes occur close to chains of vol-
canoes near the continental margins—for example, around the Pacific Ocean's notorious
“ring of fire.” These violence-prone regions of the Pacific rim, including the Philippines,
Japan, Alaska, Chile, and other danger zones, formed a common pattern.
It had long been known that relatively shallow earthquakes (those from depths of a few
miles or less) originate just offshore in the vicinity of deep trenches on the ocean floor,
while deeper and deeper earthquakes, some originating more than a hundred miles down,
occur farther and farther inland from the coast. The deepest known earthquakes commonly
take place beneath chains of dangerous explosive volcanoes like Mount St. Helens and
Mount Rainier in Washington State—volcanoes typically located at considerable distance
inland.
Bythelate1960s,newdatafromWWSSNclarifiedthedetailsoftherelationshipamong
deep ocean trenches, earthquakes, and volcanoes. The distinctive pattern of quake depths
increasing inland from the trenches painted a picture of huge slabs of ocean crust plunging
downintothemantle, underneaththecontinents, alongwhatwerecalled subductionzones.
Old basaltic crust, which is much colder and thus denser than the hot mantle, is literally
swallowed up by Earth. As subducting basalt snags and buckles the adjacent crust down-
ward, the deep ocean trenches form. For every square mile of new crust generated at ocean
ridges, a square mile of old crust disappears at a subduction zone. The new exactly bal-
ances the old.
As if a veil had been lifted, the new science of plate tectonics came into sharp focus.
Ocean ridges and subduction zones define the boundaries of about a dozen shifting plates,
each of which is cold (compared with the deeper mantle), brittle (hence subject to earth-
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