exist in a kind of cone of uncertainty. It would certainly be the most interesting few months in the history of the world. And imagine the party if it passed safely.
“So how often does something like the Manson impact happen?” I asked Anderson and Witzke before leaving.
“Oh, about once every million years on average,” said Witzke.
“And remember,” added Anderson, “this was a relatively minor event. Do you know how many extinctions were associated with the Manson impact?”
“No idea,” I replied.
“None,” he said, with a strange air of satisfaction. “Not one.”
Of course, Witzke and Anderson added hastily and more or less in unison, there would have been terrible devastation across much of the Earth, as just described, and complete annihilation for hundreds of miles around ground zero. But life is hardy, and when the smoke cleared there were enough lucky survivors from every species that none permanently perished.
The good news, it appears, is that it takes an awful lot to extinguish a species. The bad news is that the good news can never be counted on. Worse still, it isn’t actually necessary to look to space for petrifying danger. As we are about to see, Earth can provide plenty of danger of its own.
14 THE FIRE BELOW
IN THE SUMMER of 1971, a young geologist named Mike Voorhies was scouting around on some grassy farmland in eastern Nebraska, not far from the little town of Orchard, where he had grown up. Passing through a steep-sided gully, he spotted a curious glint in the brush above and clambered up to have a look. What he had seen was the perfectly preserved skull of a young rhinoceros, which had been washed out by recent heavy rains.
A few yards beyond, it turned out, was one of the most extraordinary fossil beds ever discovered in North America, a dried-up water hole that had served as a mass grave for scores of animals-rhinoceroses, zebra-like horses, saber-toothed deer, camels, turtles. All had died from some mysterious cataclysm just under twelve million years ago in the time known to geology as the Miocene. In those days Nebraska stood on a vast, hot plain very like the Serengeti of Africa today. The animals had been found buried under volcanic ash up to ten feet deep. The puzzle of it was that there were not, and never had been, any volcanoes in Nebraska.
Today, the site of Voorhies’s discovery is called Ashfall Fossil Beds State Park, and it has a stylish new visitors’ center and museum, with thoughtful displays on the geology of Nebraska and the history of the fossil beds. The center incorporates a lab with a glass wall through which visitors can watch paleontologists cleaning bones. Working alone in the lab on the morning I passed through was a cheerfully grizzled-looking fellow in a blue work shirt whom I recognized as Mike Voorhies from a BBC television documentary in which he featured. They don’t get a huge number of visitors to Ashfall Fossil Beds State Park-it’s slightly in the middle of nowhere-and Voorhies seemed pleased to show me around. He took me to the spot atop a twenty-foot ravine where he had made his find.
“It was a dumb place to look for bones,” he said happily. “But I wasn’t looking for bones. I was thinking of making a geological map of eastern Nebraska at the time, and really just kind of poking around. If I hadn’t gone up this ravine or the rains hadn’t just washed out that skull, I’d have walked on by and this would never have been found.” He indicated a roofed enclosure nearby, which had become the main excavation site. Some two hundred animals had been found lying together in a jumble.
I asked him in what way it was a dumb place to hunt for bones. “Well, if you’re looking for bones, you really need exposed rock. That’s why most paleontology is done in hot, dry places. It’s not that there are more bones there. It’s just that you have some chance of spotting them. In a setting like this”-he made a sweeping gesture across the vast and unvarying prairie-“you wouldn’t know where to begin. There could be really magnificent stuff out there, but there’s no surface clues to show you where to start looking.”
At first they thought the animals were buried alive, and Voorhies stated as much in a
The BBC documentary had suggested that the existence of so much ash in Nebraska was a surprise. In fact, Nebraska’s huge ash deposits had been known about for a long time. For almost a century they had been mined to make household cleaning powders like Comet and Ajax. But curiously no one had ever thought to wonder where all the ash came from.
“I’m a little embarrassed to tell you,” Voorhies said, smiling briefly, “that the first I thought about it was when an editor at the
Voorhies sent samples to colleagues all over the western United States asking if there was anything about it that they recognized. Several months later a geologist named Bill Bonnichsen from the Idaho Geological Survey got in touch and told him that the ash matched a volcanic deposit from a place called Bruneau-Jarbidge in southwest Idaho. The event that killed the plains animals of Nebraska was a volcanic explosion on a scale previously unimagined-but big enough to leave an ash layer ten feet deep almost a thousand miles away in eastern Nebraska. It turned out that under the western United States there was a huge cauldron of magma, a colossal volcanic hot spot, which erupted cataclysmically every 600,000 years or so. The last such eruption was just over 600,000 years ago. The hot spot is still there. These days we call it Yellowstone National Park.
We know amazingly little about what happens beneath our feet. It is fairly remarkable to think that Ford has been building cars and baseball has been playing World Series for longer than we have known that the Earth has a core. And of course the idea that the continents move about on the surface like lily pads has been common wisdom for much less than a generation. “Strange as it may seem,” wrote Richard Feynman, “we understand the distribution of matter in the interior of the Sun far better than we understand the interior of the Earth.”
The distance from the surface of Earth to the center is 3,959 miles, which isn’t so very far. It has been calculated that if you sunk a well to the center and dropped a brick into it, it would take only forty-five minutes for it to hit the bottom (though at that point it would be weightless since all the Earth’s gravity would be above and around it rather than beneath it). Our own attempts to penetrate toward the middle have been modest indeed. One or two South African gold mines reach to a depth of two miles, but most mines on Earth go no more than about a quarter of a mile beneath the surface. If the planet were an apple, we wouldn’t yet have broken through the skin. Indeed, we haven’t even come close.
Until slightly under a century ago, what the best-informed scientific minds knew about Earth’s interior was not much more than what a coal miner knew-namely, that you could dig down through soil for a distance and then you’d hit rock and that was about it. Then in 1906, an Irish geologist named R. D. Oldham, while examining some seismograph readings from an earthquake in Guatemala, noticed that certain shock waves had penetrated to a point deep within the Earth and then bounced off at an angle, as if they had encountered some kind of barrier. From this he deduced that the Earth has a core. Three years later a Croatian seismologist named Andrija Mohorovii?c was studying graphs from an earthquake in Zagreb when he noticed a similar odd deflection, but at a shallower level. He had discovered the boundary between the crust and the layer immediately below, the mantle; this zone has been known ever since as the Mohorovii;c discontinuity, or Moho for short.
We were beginning to get a vague idea of the Earth’s layered interior-though it really was only vague. Not until 1936 did a Danish scientist named Inge Lehmann, studying seismographs of earthquakes in New Zealand, discover that there were two cores-an inner one that we now believe to be solid and an outer one (the one that
