When science in the modern sense was first emerging in the seventeenth and eighteenth centuries in Europe, those who studied rocks, landforms, and fossils—studies that would evolve into geology and paleontology—believed that Earth’s past had been shaped by sudden and violent catastrophes, chief among them being Noah’s Flood. These people were strongly influenced by the Bible, but as Christianity fell into disfavor among many of the intellectual elites, catastrophism came to be rejected in favor of uniformitarianism, the belief that the past was shaped by the same processes we can observe today—slow climatic change, the gradual advance and retreat of glaciers, the almost imperceptible rise of mountain ranges, and the equally slow process of erosion, leveling those ranges and carving out valleys. By the late eighteenth century, the proponents of uniformitarianism, chiefly James Hutton and Charles Lyell, dominated the earth sciences. There was a brief rebellion in the early nineteenth century, led by Georges Cuvier, who, although he did not rely on any Biblical accounts, believed that there had been truly catastrophic floods and earthquakes in the past. But his rebellion failed, and uniformitarianism became a dogma, not to be questioned.
But all along there has been a major problem with uniformitarianism, ignored by scientists who should have known better—or been more honest. For geologists and paleontologists have divided Earth’s prehistoric past into Eons, the Eons into Eras, then Periods—all of widely varying lengths. If no catastrophes had occurred, they would more logically be of the same length, making everything simpler, with perhaps billion-year Eons, 100 million-year Eras, and so forth. The varying lengths of the divisions is a tacit admission that something happened at the end of each one.
When, in the 1950s, the maverick scientist and psychoanalyst Immanuel Velikovsky wrote Worlds in Collision, proposing an extreme theory of catastrophism, he as savagely attacked by the scientific establishment. Velikovsky postulated terrifying events in the remote past and even in relatively recent historical times, caused, he said, primarily by the eruption of a massive “comet” from Jupiter, which careened around the Solar System for quite some time, narrowly missing the Earth, upsetting the orbit of Mars, and finally settling into a stable orbit as the planet Venus. It is easy, now, to detect the flaws in his theory. A comet is, by definition, a very small body, not, as the critics point out, one as massive as Venus. Venus is mostly made of silicates and nickel and iron, but Jupiter, save perhaps for its innermost core, is mostly hydrogen (like the Sun). There is no known force, we are told, that could eject a planetary-sized mass even from the outer layers of Jupiter, let alone from the core; and if there was some sort of eruption from the core, it would be slowed and diffused trying to pass through the thousands of miles of densely compressed gases above it. Any energy sufficient to accelerate a huge mass to Jupiter’s very high escape velocity would generate so much heat that the mass, as it escaped from Jupiter, would simply explode. Velikovsky was, it seems clear, wrong. But perhaps he was not completely wrong.
There is, strangely, an actual connection between Venus and a large comet or asteroid. Venus rotates on its axis, seen from the north, in a clockwise direction. Earth and most other planets rotate counter-clockwise, like their orbits around the Sun, which causes the axial rotation. But not only does Venus rotate the wrong way, defying the recognized laws of physics, but its entire crust is less than a billion years old, perhaps a great deal less. There is only one explanation for all of this, and perhaps for the dense carbon dioxide atmosphere as well: at some point in the last billion years, Venus was struck by a massive planetesimal or asteroid at such an angle that it reversed Venus’ axial rotation and the energy of the impact melted the planet’s crust. There may even be a Jupiter connection, since Jupiter’s gravity perturbs the orbits of any comets or asteroids that come even relatively near it. On top of all that, Velikovsky claimed that electromagnetism played a major role in the universe; since then, we have learned a great deal about the Solar Wind and the Earth’s magnetosphere, and some maverick physicists and astronomers even propose that much of the Sun’s energy may be caused by a flow of cosmic electricity. The science is not settled—but then, science is not supposed to be settled.
Discoveries of the last few decades are reviving a belief in catastrophism. Of course, the slow, gradual processes of erosion and climatic change are also happening, and always have. It’s just that the gradual processes are interrupted from time to time by sudden and devastating events.
The craters on our Moon have been observed for centuries, and astronomers had long suspected that most of them were caused by huge comet and asteroid impacts; this has been pretty much confirmed now that we have been there and brought back samples to study. Our space program has shown that other planets in our Solar System, such as Mars and Mercury, are also heavily cratered, as are some of the moons of the outer planets, and even the asteroids and comets themselves. So it was obvious that Earth had to have been struck many times in the past, and now aerial and satellite photos have shown the eroded remnants of huge impact craters on our home planet. These are just the ones not destroyed by plate tectonics, glaciation, and erosion, or covered by oceans and buried in sediment. In fact, we are beginning to realize that smaller impacts, equivalent to Hiroshima-size bombs or even greater, are terrifyingly common, although many of these explode so high in the atmosphere that they do little or no damage at ground level. A few of the better-known craters are the Vredefort Ring in South Africa dating back two billion years, the Sudbury, Ontario crater about one billion, 800 million years old, and the Manicougan crater in Quebec created 215 million years ago. Most of these still exist because they are on very old sections of continental crust, like the Canadian Shield.
In addition to impacts, Earth has repeatedly suffered supervolcano eruptions that have instantly devastated large areas and triggered ice ages. Some of these are immense explosions of ash and gases from places like Yellowstone or California’s Long Valley Caldera. Others are eruptions of lava from long fissures; these can be observed happening today in places like Hawaii and Iceland. But in the past some were much larger, with multiple fissures many miles in length erupting simultaneously over vast areas. As old fissures quieted down, new ones formed, so eruptions could be almost continuous for thousands of years or even longer. Today, these ancient flood basalts, eroded for millennia, are called “traps,” a geological term referring to the staircase-like ridges that often form. America’s Snake River and Columbia Plateau regions were covered by such eruptions, and the Siberian Traps and India’s Deccan Traps are other well-known examples. The Siberian eruptions, which continued for about one million years, probably caused the Permian Extinction some 250 million years ago, the greatest biosphere destruction known; some geologists believe that the lava set fire to coal seams in the crust, releasing truly massive amounts of carbon dioxide and heating the entire planet.
Geologists believe that these massive eruptions occur at hot spots on the earth, triggered by mantle plumes of hot magma. Initially, it was theorized that these were relatively narrow and originated near the core-mantle boundary very deep within the earth. Now that theory has been revised, and many geologists believe that the hot spots are caused by relatively shallow and somewhat broader convection currents in the asthenosphere of Earth’s mantle. The lithosphere is composed of the crust and the solid upper mantle (the rocks of the mantle chemically differ from those in the crust). The asthenosphere, deeper and hotter, is more nearly molten, more plastic than the lithosphere. These convection currents, caused as much by the descent of cooler and denser material as by the upwelling of hotter and less dense magma, also play a major role in plate tectonics.
In addition to all these disasters, we also have to worry about megatsunamis, which can be triggered by volcanism, impacts, underwater landslides, or various combinations of these. Then there are gamma-ray bursts originating many light-years from Earth; and, on top of all that, we now realize that climate change can be quite sudden, with ice ages beginning and ending within just a few decades, causing sea levels to fall and rise just as abruptly. And there are the catastrophic floods caused by the warming as ice ages end, when lakes of glacial meltwater are suddenly released by the melting of the ice dams that contain them. As they rush down to the sea they destroy everything in their path, and then cause megatsunamis that can travel thousands of miles. The continuance of our civilization, and, even our survival as a species, is by no means assured.
Of all the asteroid impacts we have suffered, the one that may have finished off the large dinosaurs is the most famous. Physicist Luis Walter Alvarez first proposed that a layer of (relatively) iridium-rich soil dating back to about 65 million years BP (before the present) was probably caused by an impact. Petroleum geologists searching for oil north of Mexico’s Yucatan Peninsula found the crater, now named “Chicxulub,” buried under sediments and no longer visible to us laymen, and it seemed to date to about the right time. In addition, tektites (glassy silicate rocks thrown out by impacts) were found as far away as Texas and these, too, seemed to date to the right time—and above the iridium layer no dinosaur fossils have been found. The Boltyish crater in the Ukraine is thought to date to about the same time, and a feature in the shallow North Sea called “Silverpit” may also be an impact crater from that period. It is possible that these were associated with the Chicxulub event, with at least three asteroids traveling close together hitting Earth at once. Critics were quick to pounce on this theory, claiming that dinosaurs were already slowly dying out, and postulated climate change and the spread of disease as the likely cause of their demise. People tend to get obsessive about one theory or another and forget that several events combined can contribute to an extinction. The Chicxulub impact may have simply finished off a great many species already in decline.
But Chicxulub has another competitor. At about the same period, the Deccan Traps were formed by immense fissure eruptions, and these might have caused a sudden and perhaps temporary climate change. The problem is that all the dating for these events is only approximate. Did the eruptions happen first and wipe out the dinosaurs? Or did they wipe out most of them, and the Chicxulub impact finished them off? Or did Chicxulub wipe them out and the eruptions happened later? Or did the impact and the eruptions happen at the same time? Could the impact somehow have triggered the eruptions, or helped to trigger them?
On the planet Mercury there is a huge impact crater, the Caloris Basin. Within the crater itself there are flood basalts, just as in the “seas” on our Moon. This makes sense, as an impact generates a great deal of heat. But almost exactly 180 degrees from the Caloris Basin is an area of strangely jumbled terrain—the theory is that shock waves from the impact converged at this antipodal point. There are no flood basalts in the area, but Mercury is smaller than Earth, and its interior appears to have cooled and solidified much sooner.
And the Deccan Traps were almost exactly antipodal to Chicxulub. About 65 million BP the landmass that would become the Indian subcontinent was drifting northeast, having separated from Africa around 140 million BP. Some geologists have theorized that the Deccan eruptions were triggered by the impact on the other side of the planet. The problem is that it is hard to find any other impact craters antipodal to flood basalts—but there are difficulties in dating with real precision, and the movement of landmasses due to plate tectonics in the past is only approximately mapped. The Siberian Traps eruptions began around 251 to 250 million BP and there are what appear to be two or more impact craters deep under the east Antarctic ice cap, an area believed to have been antipodal to the Siberian Traps at the time the eruptions began, although there is no way to date the supposed craters. So the jury is still out. Perhaps the impacts can weaken the earth’s crust at the antipodal point, and if there is already an upwelling of magma there, convert what would have been, at most, a small eruption into a truly massive one.
Whatever the case, climate continues to change naturally, and volcanoes continue to erupt. A Hawaiian or Canary Island landslide could trigger megatsunamis at any time. And, judging by the small asteroid that narrowly missed a Siberian city not long ago, we are still living in a cosmic shooting gallery.