Word: rocke
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Dates: during 1970-1979
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...California's quake-prone San Andreas fault, where the North American plate and the adjacent Pacific plate are grinding horizontally against each other as they move in opposite directions. When friction causes these plates to stick, stresses build up that are eventually released in a quake when the rock suddenly fractures and the plates lurch ahead. Yet the New Madrid area lies in the very heart of the North American plate, far from its boundaries. Why should it have shaken so violently in the early 1800s and, in fact, continued to quiver occasionally ever since...
Seismologists now believe they have part of the answer. Using oil-exploration equipment, they bounced sound waves off the subterranean rock where the tremors were centered. The echoes yielded a surprising profile of this hidden structure...
Along a large crack, corresponding layers of rock were offset vertically-in some places by as much as 1,000 meters (3,300 ft.). In other words, a break had occurred and some of the rock sank. Apparently, at some time in the remote past, the rock had been uplifted-perhaps by volcanism...
Then, as the volcanic flows cooled, part of the rock collapsed, creating sharp breaks. Subsequently the breaks were obscured by the soft sediments of the Mississippi River Valley...
...fault zone apparently also lies within an even larger geological structure discovered previously during magnetic and gravitational surveys: a great underground rift in the earth's crust marked by the subsidence of rock over an area at least 190 km (120 miles) long and 50 km (30 miles) wide. Scientists believe this rift was created several hundred million years ago, when the North American plate began to split and molten rock from the interior welled toward the surface. Though the breakup halted-for reasons as inexplicable as the original movement-a weak area remains in the form...