Straddling the border between Israel and Jordan, deep within a region torn by decades of political strife, is the stark, desolate, intensely saline Dead Sea. Virtually barren of life, and imbued with a stillness that bespeaks extreme antiquity, the Dead Sea, in geologic terms, is actually quite young--a mere several million years of age. Both the sea itself and the entire Dead Sea valley in which it lies are the result of north-south motion at the boundary between two tectonic plates--two parcels of the Earth's rocky crust.

Continents might appear to be indestructible, but when tectonic forces pull blocks of crust in different directions, eventually even a continent will break. Until 20 million to 30 million years ago the African and Arabian plates were a single massive block of our planet's lithosphere. But then the floor of what soon became the Red Sea began to spread--launching the Arabian plate to the north-northeast, toward Eurasia; breaking the small Sinai subplate away from the African plate; and tearing up Earth's crust along the way.

Nowadays the Arabian plate is diverging from the Sinai subplate at a rate of about four millimeters a year. That's slow even by geologic standards (and a dozen times slower than human fingernails grow), but given enough time, even a slight but continual movement of a tectonic plate can cause inexorable, prodigious changes. In the past 20 million years or so, the Red Sea has opened, the Arabian Peninsula has taken shape, and the eastern flank of the Dead Sea fault has shifted about sixty miles northward with respect to the western side--enough to sculpt the Dead Sea valley, a long, prominent path through the landscape of the Levantine Corridor [see map on next page]. From that distant time until what was, geologically speaking, yesterday, the Dead Sea valley became the main land route out of Africa for both flora and fauna. Among the fauna, of course, were some of our earliest hominid ancestors. And as the geologic story of the Levantine Corridor has come into focus, an intriguing plotline has emerged: for perhaps the first time, investigators have shown that large-scale geologic processes have helped shape the course of human history.

Like so much else in nature, the topography of the Earth eschews straight lines. When a markedly linear feature does emerge, say from a subtle topographic trend discernible only in a satellite photograph, the trained eye of a geologist invariably sees an active fault--a feature along which earthquakes persistently recur. But you don't have to be a geologist to see the Dead Sea fault zone. Viewed from high above, its linear morphology, running up the middle of the Levant, is a dramatic--and decidedly unsubtle--indicator of its geologic character [see illustration at left on page 47].

The tectonic forces on the Arabian plate and the Sinai subplate are pulling in slightly different directions, and at different rates, creating what geologists call a transform fault [see diagram at right on page 47]. From the Dead Sea the fault extends almost due south to the Red Sea, and almost due north along the Jordan River and up into Lebanon, eventually wending its way into southern Turkey. Flanked by margins as high as 7,000 feet above its floor, the rift valley created by the Dead Sea fault is one of the deepest and most abrupt depressions on Earth.

Investigators can point to compelling evidence for ancient, damaging earthquakes in the anthropologically and archaeologically crucial area bordering the Jordan River. The first-century A.D. Jewish historian Flavius Josephus, writing of the destructive earthquake of 31 B.C., describes "an earthquake in Judea, such as had not occurred before, which killed many cattle.... And about thirty thousand persons also perished in the ruins of their houses." Characteristically toppled and fractured blocks of limestone that were once columns are evident among the ruins of early Jericho and elsewhere. And the sediments in the Dead Sea basin incorporate evidence of a good deal of seismic upheaval during the past 70,000 years.

For now, however, the Dead Sea rift valley offers geologists a nearly unparalleled opportunity to see and investigate continental breakup in action. Natural processes can be studied here without impediment, because the area is both sparsely populated and largely free of vegetation. The shores of the Dead Sea itself are the lowest dry land on Earth, and among the Dead Sea valley's unique characteristics is that, even though it has sunk to hundreds of feet below sea level, much of it is not submerged. Rock formations are thus well exposed.

Tens of millions of years ago, before the Arabian plate set off on its Eurasian journey, the Mediterranean Sea was far bigger than it is today, and covered much of the Levant. Later activity at the boundary between the Sinai subplate and the Arabian plate caused major upheavals of the seabed; between the two plates a block of crust sank, forming a valley known as a graben. As the huge, salty Mediterranean evaporated and receded, that graben retained some of the water. Eventually the large body of salty water that occupied most of the Dead Sea valley shrank because of evaporation. Several lakes subsequently appeared and disappeared in the rift valley; the present Dead Sea, comprising two sub-basins, was left behind about 10,000 years ago. Its northern subbasin was, and remains, by far the deeper: it now holds less than a thousand feet of water, and its bottom lies about 2,350 feet below sea level. The much shallower, southern sub-basin is now dry.