 October 2003 |
Promised Land
Several million years ago tectonic forces began to create an edenic
corridor that led early humans out of Africa and into the Near East.
By Zvi Ben-Avraham and Susan Hough
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.
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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.” |
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 first map]. 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.
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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 second map]. 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.
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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 sub-basin 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.
Geologists have devised a number of clever ways to reconstruct the past movements of tectonic plates. One important clue comes from estimating the rate at which sediments were deposited. Investigators have found that the upper, hence later, layers of sediment in the Dead Sea valley built up more quickly than the lower, earlier layers. Hence the water that carried the sediments from higher land to the flat valley floor must have flowed more quickly as the millennia passed. That implies the surrounding landscape was becoming increasingly mountainous. Accordingly, geologists have inferred that before about 5 million years ago the entire Dead Sea fault zone was relatively flat, but that in the past 2 million to 3 million years, accelerating tectonic processes have strongly uplifted its flanks.
There is further evidence of rapid change in the Wadi ‘Araba, or Arava Valley, the southern part of the Dead Sea valley, which reaches from the southern end of the Dead Sea to the Gulf of ‘Aqaba. In the past 2 million years the Wadi ‘Araba subsided, as the neighboring Negev region to the west—as well as the Trans-Jordanian plateau to the east--was uplifted and tilted. Here, as elsewhere, seemingly gradual geologic motion has led to dramatic cumulative changes. In the blink of a geologic eye the Dead Sea transform fault carved up enough of the landscape to reconfigure the patterns of river drainage in the Negev desert. As a result, within just the past 2 to 3 million years sizable freshwater lakes formed in newly created depressions in the Wadi ‘Araba.
| In the southern Levant the new lakes would have beckoned to flora and fauna alike, just as in more recent times in North America, trailer parks have sprung up near new bodies of water such as southeastern California’s Salton Sea (suddenly created as a result of an engineering gaffe combined with major flooding on the Colorado River in 1905). |
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The appearance of large bodies of freshwater in an otherwise arid zone invariably gives rise to a wetter, more temperate local climate. The air becomes more humid because of evaporation from the lakes, and large bodies of water also tend to buffer the extremes of hot and cold, making the region altogether more inviting and hospitable to terrestrial life.
In the southern Levant the new lakes would have beckoned to flora and fauna alike, just as in more recent times in North America, trailer parks have sprung up near new bodies of water such as southeastern California’s Salton Sea (suddenly created as a result of an engineering gaffe combined with major flooding on the Colorado River in 1905). Of course Airstreams and RVs were hard to come by during the Pliocene and Pleistocene Epochs, but the migrating creatures of Africa, including early modern humans, gradually made their own kinds of living arrangements near the lakes that appeared in the rift valley. Thus did the Dead Sea fault carve the land route out of Africa: a navigable, habitable corridor flanked by raw, rugged rock.
We Homo sapiens are descended from bipedal primates--usually called hominids--that first appeared in Africa some 5 million years ago. The earliest hominid remains have been unearthed in East Africa--that is, in present-day Ethiopia, Kenya, and Tanzania. The earliest hominid tools that appear to be part of a standardized toolmaking tradition were discovered in the same region: hand axes related to the Acheulean culture. Such axes were a Paleolithic invention--rounded at one end to fit in the palm, pointed at the other end, chipped and fractured to a cutting edge along the perimeter.
Early Acheulean axes, about 1.4 million years old, were found at Olduvai Gorge in Tanzania, the site made famous by the findings of the archaeologists Louis and Mary Leakey. Outside Africa the earliest hand axes whose dating is uncontested are also Acheulean and also about 1.4 million years old. They were unearthed in the Dead Sea valley about sixty miles north of the Dead Sea--at the extensive site of ‘Ubeidiya, a vanished lake just south of what was once the biblical Sea of Galilee, now variously called Lake Tiberias or Lake Kinneret.
Excavations at ‘Ubeidiya have uncovered the best available hard evidence for the migration of early hominids [see “Down in the Valley,” below]. Since 1960 some thirty archaeological layers, showing multiple distinct periods of occupation, have been exposed. On the basis of several kinds of converging evidence--magnetic characteristics of the rock layers, changes in sedimentation, ecological changes reflected in pollen grains, and deposits of bones, boulders, fossils, and tools--investigators have been able to date the occupations with confidence. Later sites elsewhere in the Dead Sea fault zone have yielded a wealth of additional evidence of hominid and human occupation, such as uncracked, hard-shelled pistachios, acorns, and water chestnuts, accompanied by pitted stone hammers and anvils. The earliest known raisins and olives have been found in this region. And, tellingly, within natural oases are trees of Sudanese origin: emigrants from the southwest.
A million years ago, in Pleistocene times, when freshwater was far more abundant than it is today, the Dead Sea rift valley would have been lush, verdant, and full of things for the indigenous fauna to eat. And the paleontological record clearly shows that the splendid new corridor attracted a remarkable influx of species--birds, mammals, invertebrates, plants--between 2 million and 3 million years ago. Arriving in step with East African flora were the creatures of the East African savannas: gazelle, giant deer, hippopotamus, rhinoceros, wart hog. Life arrived and blossomed once tectonic movements had pried the region’s blocks of crust apart and tilted them to form mountains, providing the water from which all else flowed.
Today, unfortunately, that land of milk and honey is no more. The freshwater lakes that once dotted the Wadi ‘Araba are permanently dry. The Dead Sea itself is nearly lifeless. Almost 35 percent (by weight) of its “water” is made up of dissolved solids--not only sodium chloride but also potassium, bromine, and magnesium salts—giving it the highest salinity of any lake on Earth. With each passing year, evaporation further drops its level and raises the salinity of the remaining fluid. Since 1929, when hydrologists began keeping records, the Dead Sea has dropped by more than seventy feet. Only highly specialized communities of salt-tolerant microorganisms make their home in it today.
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The paleontological record clearly shows that the splendid new corridor attracted a remarkable influx of species--birds, mammals, invertebrates, plants—between 2 million and 3 million years ago. Arriving in step with East African flora were the creatures of the East African savannas: gazelle, giant deer, hippopotamus, rhinoceros, wart hog. |
Scholars tend to seek meaning along sharply different timescales. A historian typically searches across decades or centuries for the written word. An evolutionary biologist may study a species across hundreds of thousands of years. A physical anthropologist considers the few million years that hominids have walked the Earth. The frame of reference for a terrestrial geologist may be longer still--as long as Earth’s 4.6-billion-year history. The longest timescale of all is the cosmologist’s 13.7-billion-year age of the universe.
Yet rarely, it seems, have the disciplines met. For the historian, the questions addressed by evolutionary biology, anthropology, geology, and cosmology have generally (except for the occasional natural catastrophe) fused with the unchanging background against which the real action takes place. Increasingly, though, physical scientists and historians are seeing connections. The physiologist and evolutionary biologist Jared Diamond of the University of California, Los Angeles, for instance, has advanced the thesis in his notable book Guns, Germs, and Steel that the exigencies of geography, if not geology, have played a critical role in shaping the development of cultures.In the study of the Dead Sea fault zone, one can extend the connections further still. Creakingly slow geologic forces opened up the corridor for humanity’s earliest ancestors to take their first steps out of Africa and into the world beyond. That exodus was probably inevitable, but the timing and direction of the migration were determined by plate tectonics. Perhaps it behooves our species, now poised to shape the planet in dramatic and potentially disastrous ways, to realize how fundamentally the planet has shaped us.
For the past twenty-five years geophysicist Zvi Ben-Avraham has been investigating the geologic and cultural history of the fault zone around the world’s lowest, saltiest lake, the Dead Sea. He is director of the Minerva Dead Sea Research Center at Tel Aviv University in Israel and a professor of marine geoscience at the University of Cape Town in South Africa. Ben-Avraham is the author or editor of more than 150 scientific papers, as well as eight monographs and books. Susan Hough, a seismologist with the U.S. Geological Survey in Pasadena, California, focuses her research on faults and earthquakes elsewhere in the world. Her article, written with Roger Bilham, on the earthquake zone in western India (“Shaken to the Core”) appeared in the February 2003 issue of Natural History.
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Copyright © Natural History Magazine, Inc., 2003