Every so often in a Victorian novel or the biography of someone of that era, you will come across a mention of “Lyell on geology” that often implies something momentous and perhaps a bit mind-boggling for the person grappling with it. Or it might be to someone so old-fashioned as to have been unaffected by the challenge. It evinces an odd image of ladies and gentlemen in their drawing rooms, wearing heavily starched clothing and excited, or distressed, by something involving rocks.
At issue was the three-volume Principles of Geology by Charles Lyell -- an international best-seller published in the early 1830s and still much discussed upon the author’s death in 1875. While hardly the first natural philosopher to challenge the literal truth of the Book of Genesis, Lyell made the most far-reaching and cogent argument that the earth’s features (mountains, gorges, the course of rivers, etc.) could be explained by slow changes over extremely long periods of time. Among Lyell’s readers, no surprise, was the young Charles Darwin, who studied the Principles while voyaging on the Beagle.
One way to put it is that Lyell sank Noah’s Ark. But the damage to orthodox religious belief was only part of the Principles’ impact. There was also the strain of imagining the scale of time implied by “Lyell on geology” -- a phrase we should probably read as implying more than the replacement of “catastrophism” by “uniformitarianism” (terms introduced as the accepted explanation for environmental change). For it was also the moment when human history shrank to an almost inconceivably tiny aspect of natural history, like a speck of dirt atop a mountain.
Reading Paul B. Wignall’s The Worst of Times: How Life on Earth Survived Eighty Million Years of Extinction, from Princeton University Press, can induce something of that perturbed feeling. It did in me, anyway, as I tried every so often to picture a timeline of the catastrophic events that Wignall and his colleagues have reconstructed. (The author is a professor of paleoenvironments at the University of Leeds.)
The geologically unsophisticated layperson will probably anticipate new ideas or evidence about what killed the dinosaurs. But that’s an index of how limited an impact Lyell has had. We still imagine change on too constricted a scale. The rise and fall of Jurassic wildlife are, for Wignall, something like last week’s news might seem to an ancient historian: interesting enough, sure, but the author would really prefer to stay focused on the past and not get sidetracked chattering about recent trends.
The catastrophic events covered in The Worst of Times affected life on Pangaea, the vast landmass that took shape 300 million years ago and disintegrated into pieces that drifted across the globe to form the continents we have now.
Two mass extinctions -- defined as “geologically brief intervals when numerous species go extinct in a broad range of habitats, from the ocean floor to forests, and all latitudes, from the Equator to the poles” -- had already taken place in very distant pre-Pangaean times, but the formation of the super-continent seems to have accelerated the pace of disaster: in the period between 260 and 180 million years ago, two of Earth’s five known mass extinction events took place, along with four other extinction episodes of smaller scale or impact.
That leaves one mass extinction unaccounted for: the crisis following the impact of a giant meteor hitting what is now the Yucatan Peninsula, 65 million years ago, ending the reign of the dinosaurs, among other species. That was a good 100 million years after Pangaea’s fragmentation got well underway, and the continents that existed during the fifth mass extinction event are recognizable in their current form, if not location, from one of the maps on the U.S. Geological Survey's website.
The very idea of Pangaea has always fascinated me (insert nerd emoji here) yet the evidence suggests it was a difficult place for evolution to happen. In fact, that is an understatement: Wignall’s reconstruction of the deep history suggests that Pangaea was not just the scene of disasters but also a major factor in their scale and frequency.
The issue was volcanoes, and not just the piddling sort of modern times that could wipe out a city or two. The biggest volcano of the last thousand years produced about 30 cubic kilometers of magma, while a given Pangaean volcano (one of an untold number) threw out millions of cubic kilometers. The eruptions filled the atmosphere with carbon dioxide while also setting off chain reactions that created hot, de-oxygenated, acidic oceans, killing off much marine life.
The fracturing of Pangaea did not mean a complete end to monster volcanoes and their sundry terrible side effects (including periods of climate change, up and down in temperature). But Wignall suggests that the supercontinent’s eventual dispersal into smaller landmasses created better conditions for evolution -- and even for simple survival.
“A huge continent has vast areas in the interior that are too far away from the sea to receive much rain,” he writes. “In contrast, smaller, more fragmented continents receive precipitation over a greater area …. Continental runoff also supplies nutrients to the oceans, which stimulate plankton growth that removes more carbon dioxide, which gets buried as organic carbon in marine sedimentary rocks.”
Which, in turn, makes for relatively more moderate short-term changes in climate. One benefit of studying volcanic activity of the Pangaean era is that it created “effects that may be akin to modern anthropogenic activity, such as the emission of huge amounts of carbon dioxide into the atmosphere.” The author avoids speculating on the more dire potential implications, but indicates that having dispersed continents now is at least some advantage.
As a side note, let me mention the surprise at seeing a geologist use the word “catastrophism” in a neutral way. Evidently uniformitarianism is no longer quite the bedrock principle that it seemed in the wake of “Lyell on geology” -- a development that believers in Noah’s Ark seem to find encouraging. But catastrophism as Wignall and his colleagues understand it means recognizing that the unimaginably vast stretches of time in which the earth changes slowly have been punctuated, on occasion, by cataclysmic events lasting a few hundred thousand years. On the grand scale, that counts as sudden change. But 40 days and 40 nights it isn’t.
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