The Son In Italy

Given his family history, it is not surprising that young Walter became a scientist himself. No one could have followed in his father's footsteps, and wisely in retrospect, Walter chose not to try but to follow his own love, geology. He earned his doctorate at Princeton under Professor Harry Hess and, until Qaddafi expelled the Americans, worked as a petroleum geologist in Libya. In 1971 he joined the faculty at the Lamont-Doherty Geological Observatory at Columbia University, where much of the research that led to the plate tectonic revolution had been done. After a few years there, Walter accepted a position at Berkeley, the university where his father was in residence and where Walter soon received tenure and the title of full professor. Had Walter remained in Libya or at Lamont-Doherty, we might still be scratching our heads over the mystery of dinosaur extinction.

During the 1970s, Walter summered in the pleasant northern Italian town of Gubbio, where he studied an unusually complete section of sedimentary rock that spanned the time from the middle of the Mesozoic era well up into the Cenozoic era. Geologists divide the Mesozoic into three periods: The oldest, the Triassic, is overlain by the Jurassic, which in turn is overlain by the Cretaceous (Figure 2). The earliest, lowest period in the Cenozoic is called the Tertiary. Its name hearkens back to an earlier time when there were thought to be four ages of rocks: primary, secondary, tertiary, and quaternary; only the last two are used today. The Mesozoic-Cenozoic boundary marks the point in geologic time at which the dinosaurs perished. It has become customary, however, to nickname the boundary for the two adjacent periods, the Cretaceous and Tertiary, rather than their eras. Geologists call it the "K-T" boundary. ("K" is used instead of "C" to avoid confusion with the older, Cambrian period; Cretaceous, which comes from the Latin creta, for chalk, also happens to be Kreide in German.)

FIGURE 2 The sweep of geologic time and evolution. Geologic time is divided into eras, periods, and epochs. The earth formed 4.5 billion years ago; life began sometime in the Archean, possibly as early as 3.8 billion years ago, and exploded in the Cambrian. Dinosaurs arose in the Permian and disappeared about 160 million years (abbreviated m.y.) later at the boundary between the Cretaceous period of the Mesozoic era and the Tertiary period of the Cenozoic era. [After Geologic Time, U.S. Geological Survey Publication.]

FIGURE 2 The sweep of geologic time and evolution. Geologic time is divided into eras, periods, and epochs. The earth formed 4.5 billion years ago; life began sometime in the Archean, possibly as early as 3.8 billion years ago, and exploded in the Cambrian. Dinosaurs arose in the Permian and disappeared about 160 million years (abbreviated m.y.) later at the boundary between the Cretaceous period of the Mesozoic era and the Tertiary period of the Cenozoic era. [After Geologic Time, U.S. Geological Survey Publication.]

The Cretaceous period began 145 million years ago and lasted until 65 million years ago, when the Tertiary began. Not surprisingly, geologists break the periods down more finely, first into epochs and then into stages. We are particularly concerned with the Maastrichtian, the last stage of the Cretaceous, which began about 75 million years ago and ended at 65 million years ago; and with the Danian, the first stage of the Tertiary, which started then and lasted until about 60 million years ago.

Walter had not gone to Gubbio to study dinosaur extinction. He and a group of American and Italian geologists were there to measure the magnetism frozen in the Cretaceous and Tertiary sedimentary rocks handsomely exposed in a deep gorge nearby. They hoped to be able to locate sections where the rocks had recorded reversals of the earth's magnetic field—times at which the north pole of the earth had acted as a south pole, and vice versa. (A magnetized rod or needle develops two poles that act oppositely. Because one end of the rod points toward the current north magnetic pole of the earth, we say that it is the north-seeking end. This property is the basis for the common compass.)

While his father, back at Berkeley, had begun to worry that physics had started to leave him behind and that his career had stalled, Walter and his co-workers were in Italy, engaged in research that no one could have expected would aid in jump-starting Luis's career. The geologists were attempting to determine the precise patterns of magnetic reversals in rocks of known age, which would then allow those same unique patterns to be used to date rocks of unknown age. Thus Walter Alvarez and his colleagues were aiming to fill in a gap in geologic knowledge, a vastly more common endeavor than launching a paradigm shift.

Geologists had discovered that, for reasons unknown, magnetic reversals were frequent (on their time scale), occurring on the average about every 500,000 years. Because all rocks of a certain age, wherever found, show the same magnetism—either normal (defined as the situation today) or reversed—we know that the reversals affected the entire earth at once. In the '960s, analysis of the magnetic reversal patterns in rocks from the seafloor showed that sections of the floor on one side of, and parallel to, a mid-oceanic, deep-sea volcanic ridge, could be matched exactly with the pattern on the other side. Some clever scientists deduced that lavas were being extruded at these ridges and, as they cooled, took on normal or reversed magnetism, whichever was prevalent at the time. Later, the frozen lavas were dragged out to either side as the seafloor spread away from the ridge, to be replaced by a new batch of lava that, if the earth's magnetic field had meanwhile flipped, would be magnetized in the opposite direction. This proved that the seafloors diverged from ridges, and it was only a small leap to conjecture that continents, made of light, buoyant rock, would ride on top of the spreading seafloors. Thus emerged the theory of plate tectonics, the modern version of the theory of continental drift.

In order for the new magnetic time scale to be used to date rocks, it had to be tied into the standard geologic time scale that had been built up through the decades based on the diagnostic fossils contained in sedimentary rocks. To do so scientists needed to find a cross section of fossil-bearing rock of known age that had been deposited steadily and slowly, allowing the magnetic minerals in the parent sediment to capture the fine details as the earth's magnetic field repeatedly reversed itself. Gubbio was ideal. In a 400-m gorge outside the town, rocks of middle Cretaceous age, 100 million years old, are exposed at the bottom and are successively overlain by younger beds that reach well up into the Tertiary, to an age of about 50 million years. Especially prominent are thick beds of a beautiful rosy limestone—scaglia rossa—a favorite Italian building stone. These were exactly the kinds of rocks required by Walter Alvarez and his colleagues, for such limestones build up slowly on the deep ocean floor and their magnetism would have captured each change in the earth's magnetic field.

Not only did the team find the reversals in the rocks of the gorge, they were expressed so intricately that the geologists proposed the Gubbio section as the "type"—the world standard—for the Cretaceous-Tertiary part of the magnetic reversal time scale.7 Walter Alvarez and his co-workers had succeeded in their effort to fill an important hole in geological knowledge. Were it not for the unique coincidence of scientific and paternal circumstances described in the Prologue, that likely would have been that.

The K-T boundary in the rocks of the Gubbio gorge can be spotted just with the naked eye (Figure 3). The white limestone below the boundary is rich in sand-sized fossils of a one-celled organism, a kind of plankton called foraminifera, many belonging to the genus Globotruncana. In the red limestone above the boundary, however, Globotruncana completely disappears, replaced by a much more scarce and much smaller foraminifer with the awkward name of Parvularugoglobigerina eugubina. Clearly, at this boundary something happened that killed off almost all of the "forams," as the micropaleontologists call them. Exactly at the boundary, between the two units, lies a 1 -cm-thick layer of reddish clay, without fossils.

Walter brought home to Berkeley a polished specimen from Gubbio that included each of the three layers at the K-T bound-ary—the K, the T, and the clay in between—showed it to his father, and explained that it captured the time of the great mass extinction and marked the disappearance not only of most forams but of the dinosaurs as well. Although most nongeologists viewing this chunk of rock would have registered at most a polite curiosity (did

Scaglia Rossa
FIGURE 3 Luis and Walter Alvarez studying the K-T section at Gubbio, Italy. Walter Alvarez has his finger on the boundary. [Photo courtesy of University of California Lawrence Berkeley National Laboratory.]

it have potential as a paperweight?), Luis Alvarez commented that his son's description was one of the most fascinating revelations he had ever heard. Walter explained that the limestones above and below the layer contained about 5 percent clay, and suggested that perhaps the limy portion had simply not been deposited for a time, allowing the layer of pure clay to build up. Then, perhaps, lime deposition had started again, leaving behind a thin layer of clay sandwiched between two layers of limestone. Walter estimated that this might have taken 5,000 years, which would mean that the great K-T mass extinction had taken place in a mere instant of geologic time. Luis immediately proposed that he and Walter measure the length of time the clay layer had taken to form. Walter had succeeded in gaining his father's formidable attention and the game was afoot.

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