Geologist Walter Alvarez was on an expedition in Italy during the early 1970s when he noticed something fascinating in the limestone mountains outside Gubbio: two dark layers of rock sandwiching a lighter, half-inch-thick seam. The darker sections were filled with fossils of microorganisms known as forams, while the center swath was virtually devoid of fossil life. Alvarez and colleagues from Columbia University’s Lamont-Doherty Earth Observatory later determined that the middle layer was laid down at the exact time of the extinction of the dinosaurs. More exciting, he and his father, Nobel Prize–winning physicist Luis W. Alvarez, found that the fossil-free layer was rich in iridium, an element that is rare on the earth but relatively abundant in rocks from space. Piecing those clues together, the two Alvarezes proposed a radical idea: The mass extinction that wiped out the dinosaurs 65 million years ago was caused by the impact of a giant asteroid, which unleashed a globe-spanning cloud of debris and plunged the planet into darkness for months. In 1990, geologists found the crater from this disaster off the north coast of the Yucatan peninsula, validating the impact theory for most of the scientific world.
After that triumph, Walter Alvarez, now at the University of California at Berkeley, began seeking other ways of using science to illuminate history—not just remote geological epochs, but also the events of the human era. “Naturally, a geologist thinks historically, and human history entangled itself with the work I was doing,” he says. That thinking began to take concrete form one day when he received an e-mail from Fred Spier, a sociologist at the University of Amsterdam. Spier was working in the field of “big history,” a unified, multidisciplinary narrative of everything from the Big Bang to the present. He asked Alvarez if he would be interested in teaching a course on this topic, and Alvarez agreed. “It was a seed that fell on fertile ground,” he says.
Before long Alvarez was writing The Mountains of Saint Francis, a book that traces Italy’s physical and cultural past going back 250 million years. He developed a wildly popular course at Berkeley that integrates cosmology and geology with world wars, sports, and Barack Obama. And he expanded the scope of big history by adding to it his concept of the contingency—the rare, unexpected event (like an asteroid collision) that changes the world in a blink.
DISCOVER senior editor Pamela Weintraub interviewed Alvarez in his Berkeley office, a comfortable space decorated with his own homages to the past: a black-and-white photo of his wife, Milly, taken in her youth, and a piece of the Apennines that dates to the demise of the dinosaurs. Soft-spoken and reflexively sunny, Alvarez turns intense when it comes to the big message behind big history. “Geology is the most important science of the 21st century because there is only one earth, and it’s becoming clear that we could damage it beyond the point where it could support us,” he says. “Geologists are studying the historical record to understand the controls before we tip into an unstable state.”
You seem to elevate geology above other sciences. Why?
Geology is a lot more complicated than astronomy and physics. If you were an early astronomer, you looked at points of light in the sky. The stars stayed in a fixed arrangement and the planets moved against that pattern. It didn’t make any difference what happened a billion years before; all that mattered was what you saw now. It took the genius of Tycho Brahe and Johannes Kepler to figure out the movement of heavenly bodies and the genius of Isaac Newton to give us the laws of motion, but these were tractable problems. If you look at a mountain range like the Alps or the Apennines, on the other hand, you see an extremely complicated pattern of rocks that have the configuration they do because of hundreds of millions of years of history.
How did your predecessors put geology on the map?
The pioneering geologist Nicolaus Steno had a great insight that seems trivial today but wasn’t in the 17th century: If you have a pile of stuff, the older stuff is on the bottom. And it’s not only the papers on your desk; it’s also the rocks in the field. The older ones were laid down first and the younger ones were placed on top. Steno was a great genius because nobody ever previously had the notion that history is written in the rocks.
How did you decide to study that geological history?
I like being out in the mountains, up in the fresh air and the sunshine with the breeze blowing, and you can see all the way across from one side of Italy to the other, so geology appealed to me.
The time you spent in Italy helped you explain the extinction of the dinosaurs 65 million years ago. How did that come about?
In 1970 I received a fellowship and spent a year and a half in Rome as the geologist on an archeological project. Another geologist there invited me on a field research project up in the Apennines, about halfway to Florence. I fell in love with those mountains. When I was invited to go to Columbia to be a researcher at Lamont, I kept wanting to get back to Italy and to those mountains that I found so fascinating.
And fortunately you were able to return to the Apennines with a very helpful colleague.
Yes, Bill Lowrie, a paleomagnetist who could record the direction of the magnetic field at the time a given rock or fossil was formed. This was 1972, ’73, and plate tectonics [the theory, now verified, that sections of the earth’s crust shift around over millions of years] was just coming along. We became friends, and we thought, hey, it would be really fun to go to Italy to see whether the land had rotated because of plate movement. The local limestone was red, indicating the presence of iron and thus fossil compasses. If we found the compasses tilted around, it would show that Italy and the rest of the continent had rotated. It turned out the idea didn’t work very well because there were thousands of layers of bedrock and the layers could slip. But we discovered something else by accident—that these limestones recorded reversals in the earth’s magnetic field.
What did these magnetic reversals signify?
Every now and then the earth’s magnetic field, which here points north and down, would reverse and point south and up, all due to the convection of the liquid outer core of the earth. Scientists first found evidence that this magnetic field had reversed many times over the history of the earth by measuring the magnetization of rocks that filled in between two diverging tectonic plates under the ocean at the Mid-Atlantic Ridge. If you tow a magnetometer behind a ship or an airplane above the ocean floor, you will find that there are these magnetic stripes and each stripe represents a reversal in the magnetic field. It’s literally a magnetic tape recorder.
In this case, you were looking at the magnetic reversals recorded not under the sea but in the layers of rock in a mountain.
The concept was similar. The mountains moved and pushed the earth up, just the way the ocean floor spread out. We also saw black and white stripes, but here’s the cool thing. Look at these distances: It requires almost a thousand kilometers of seafloor spreading to record the same amount of time that you find in 150 meters of mountain sediment, so the earth is running two magnetic tape recorders. The one in the seafloor runs 6,000 times faster, and so it captures more detail, but it is less useful than the one in the mountains because we can date the mountain rocks to specific periods of time.
