Michael R Rampino. American Scientist. Volume 105, Issue 4. July/August 2017.
[The principles of reasoning in geology] are… that no causes whatever have from the earliest time to which we can look back, to the present, ever acted, but those now acting; and that they never acted with different degrees of energy from that which they now exert.
~ Charles Lyell, in a letter to Roderick Murchison, 1829
Modern geology derives its core ideas from the British geologist Charles Lyell’s seminal text Principles of Geology: Being an Attempt to Explain the Former Changes of Earth’s Surface, by Reference to Causes Now in Operation. This magnum opus came out in three volumes from 1830 to 1833 and sold well, making money for its author. Knighthood, followed by a baronetcy, contributed to his reputation as the most influential geologist of the mid-19th century. Lyell’s prosperous family, close friendship with Charles Darwin, and prestigious position at King’s College further cemented his outsize reputation.
Principles of Geology outlined three fundamental ideas, which defined the science of geology for a long period after, and still have a strong hold on the field. Lyell described an Earth shaped by gentle, slow-moving change, employing the same forces that we can see at work in the world today. He discounted the catastrophist views of his contemporaries, ridiculing their need to invoke cataclysms from the sky to explain the major geologic and biological events that have taken place in Earth’s history.
A large and growing body of evidence shows that Lyell was fundamentally wrong, however. The discovery in the 1980s that the dinosaurs were killed off by a cataclysmic event threw the whole gradualist view into question. Since then, geologists have come to realize that catastrophes caused other mass extinctions and abrupt, major changes that we see in the geologic record. Most unexpected of all, a group of colleagues and I have found indications that these catastrophic events may happen in regular, repeating cycles. These discoveries have given us an entirely new way to understand the progression of Earth’s geological and biological history.
Lyell was a Scottish lawyer, but his private funds allowed him to pursue his passion for geology. He honeymooned among the rocks in Switzerland and Italy, and later published two popular geological travel guides based on his visits to North America. Through his world travels and astute geologic observations, he established himself as an authority within the field.
Lyell’s experiences allowed him to generate strong opinions regarding the theories that attempted to explain how the Earth formed. The most unacceptable kind of theory to Lyell involved cataclysmic events. He explained his objections in a letter written in 1830, in which he maintained that significant changes of climate can occur “without help from a comet, or any astronomical change.” This jibe was aimed at the musings of scholars such as William Whiston, the Lucasian Professor of Mathematics at the University of Cambridge, who attributed events in the geological record to collisions with comets.
In 1696, Whiston published A New Theory of the Earth, in which he proposed a cosmogony in which our planet originated when a comet was transformed into an ideal world, with a circular orbit, without tilt or rotation. Later, according to Whiston’s account, God sent another comet toward Earth, and its collision changed the planet’s orbit and started it rotating. This great impact was supposed to have cracked the Earth’s crust, releasing the waters of the biblical flood, while the vapors of the comet’s tail condensed into torrential rain. Comets were thus transformed from harbingers of local calamities, caught up in ancient superstitions, into natural causes of global cataclysms. Even Edmund Halley (the * astronomer who determined the orbit of his namesake comet) and Isaac Newton surmised that historical, sacred, and geological periods were punctuated by cometary catastrophes.
Lyell worried that such ideas threatened to lead geology into areas of wild speculation. Instead, he followed in the footsteps of late 18th-century Scottish scientist James Hutton, one of the first to understand the meaning of “deep time”-the realization that long ages were required to explain the production of great geological changes by the slow forces of erosion, uplift, and deformation of rocks. Lyell believed that any and all factors affecting the formation of geologic features and the history of life were inherent to the Earth. In this way, he sought to keep geology focused on local processes that could be studied, tested, and understood. In his text, he described a world changing at a steady pace with three simple rules.
First, geologic change is the product of slow and gradual processes that we can observe today, acting over long periods of time. Lyell mocked the idea that catastrophic changes had occurred in Earth’s history, and he railed against the zealous geological catastrophists who “told of general catastrophes and a succession of deluges, of the alteration of periods of repose and disorder, of the refrigeration of the globe, of the sudden annihilation of whole races of animals and plants.”
Second, geological forces are intrinsic to Earth. Comets or other extraterrestrial bodies are not to be invoked to explain geological history. For Lyell, astronomical catastrophism was a distraction that “induc[ed men] to waste time in speculations on the power of comets to drag the waters of the ocean over the land-on the condensation of the vapors of their tails into water, and other matters equally edifying.”
Third, the geologic record does not contain regular repeating patterns influenced by celestial cycles, an idea that smacked of predestination to Lyell. Could God allow the stars to influence his orderly world? Lyell ridiculed such theories proposed by naive astronomers who compared the course of events on our globe to astronomical cycles. Of them he said, “not only did they consider all sublunary affairs to be under the influence of celestial bodies, but they taught that on Earth, as well as in the heavens, the same identical phenomena recurred again and again in perpetual vicissitude.”
Other geologists had espoused similar gradualist ideas before Lyell, but such ideas were amplified and codified in his Principles, and then passed on through generations of textbooks. Because Lyell was trained in law, Principles of Geology can really be considered a legal brief. It begins with a particular theological and philosophical view of the geologic record and is constructed as a long argument, with examples specifically chosen to support this view. As an example of a tight a priori argument, its conclusions seem unassailable. Principles of Geologi, as a brief, proved to be extremely effective in the court of scientific opinion, and the doctrine it put forth, now known as uniformitarianism, is considered Lyell’s great contribution to the geosciences. Uniformitarianism influenced Charles Darwin to consider that slow and steady processes changed the types of life on Earth as well. Nevertheless, some early dissenters remained unswayed.
An early version of a catastrophist theory of geological change was advocated for by Baron Georges Cuvier in the early 19th century. Cuvier was a brilliant comparative anatomist, often referred to as the “Father of Vertebrate Paleontology.” With his colleagues, he studied the fossils and layers of rock in the Paris Basin, and reported empirical evidence for episodic, catastrophic, and sweeping changes. They found that the geologic record gave evidence of long periods of quiet alternating with brief times marked by the sudden disappearance of fossil species-what we now know as mass extinctions of life. Cuvier attributed these mass extinctions to unknown cataclysmic forces. He argued that “we shall seek in vain among the various forces which still operate on the surface of our earth, for causes competent to the production of those revolutions and catastrophes of which its external crust exhibits so many traces.”
Lyell’s reply to the findings of Cuvier and the French school of catastrophists was that the apparent cataclysms and sudden geological and biological changes resulted from a grossly imperfect geologic record, which resembled a book that could be read, but had many pages missing because of erosion or nondeposition. Therefore, according to Lyell, we cannot trust our own observations, especially when our observations run counter to the “plan of Nature,” which we are a priori able to deduce.
Lyell convinced his readers that he was taking the only reasonable course, and in the end, gradualism carried the day. A triumphant Lyell proclaimed that “all theories are rejected which involve the assumption of sudden and violent catastrophes and revolutions of the whole Earth, and its inhabitants.” The motto of geology became “the present is the key to the past”-in other words, one must study the gradual geological processes now in operation and extend those same slow-acting processes over vast geologic time to help explain the origins of geologic features.
It makes sense that gradualism has been the accepted doctrine in geology for the past two centuries. We can observe gradual processes in action today, and for many geologists it was only logical to assume that the gaps in an incomplete róck record could be filled by those same familiar processes. Even the revolutionary shift in the field that brought about the acceptance of plate tectonics 50 years ago did little to change the dominance of Lyellian views in geology. Indeed, the machinelike Earth of plate tectonics is the very model of a slow and orderly system, driven by invisible internal forces.
It’s clear, however, that Hutton and Lyell did not comprehend the full meaning of the “deep time” they championed. If they had, they could never have portrayed the history of Earth as dependent primarily on gradual processes. Events of many kinds tend to follow a particular inverse relationship between frequency and magnitude: Small magnitude events tend to happen much more often than large magnitude events. For example, for seismicity, small earthquakes are common, larger earthquakes happen less often, and the largest earthquakes are by far the most infrequent-yet these are the times when geological changes can most readily be seen. The same holds true for volcanic eruptions and also for meteorite impacts. In the case of impacts, small meteorites are generated by collisions between asteroids in the asteroid belt, which tend to produce lots of little fragments and fewer big ones. For volcanic eruptions, the largest events-massive outpourings of lava called flood-basalt eruptions-develop during rare episodes when huge, rising plumes of hot rock from Earth’s interior impinge on Earth’s crust.
This predictable pattern means that any study of Earth that includes the notion of deep time must take into account the fact that the greatest magnitude events should not happen often; indeed, there could be millions of years between the largest events. This being the case, the true meaning of deep time is that even though we expect massive, Earth-altering events to be rare, the long geologic time scale virtually guarantees that such events will happen from time to time, and these energetic events are so extreme that they could well be the dominant factors in creating the geologic record. Studying Earth history with the uniformitarian maxim “the present is the key to the past” ignores the very existence of deep time. Insofar as major events will occur at very long intervals compared with the brief period of our own observations, the likelihood that we will see them happen is low.
Although most geologists seem unaware of it, Lyell’s edicts were built in part on a distinctly nonscientific assumption that we inhabit a planet designed for human occupancy and that we enjoy a geologic history that represents the unfolding of a calm and methodical process leading to the present world. The further assumption is that change, when it happens, is always slow and gradual. According to Lyell, this was God’s plan for Earth. Lyell wrote, “in whatever direction we pursue our researches, whether in time or space, we discover everywhere the clear proofs of a Creative Intelligence and of His foresight, wisdom and power.”
A New Geology
The traditional focus of geology has been limited to the local and small scale, and observations about Earth were for a long time circumscribed in that way. To many geologists, the stars and planets might as well be points of light on a distant dome, with little apparent effect on the events that take place on the surface of our special world. We now have evidence, however, that the history and evolution of Earth and life have an inextricable link with the larger universe.
During the late 19th century, while geology was moving away from studies designed to explain biblical catastrophes and more toward Lyell’s uniformitarian view, astronomy was changing from descriptions of the movements of the heavenly bodies to hypotheses about the evolution of Earth, the Moon, the Solar System, and the universe at large. Even so, it wasn’t until the middle of Öre 20th century that astronomy began to impinge on geology as more and better data were obtained regarding the Moon and planets, making it possible to apply geological studies to those bodies.
The investigation of the inner rocky planets by the Mariner missions and the exploration of the Moon by the Apollo missions during the 1960s and 1970s made it clear that impact cratering and volcanism are two of the most important geologic processes that have affected other bodies in the Solar System. Planetary geology and planetary astronomy were soon seen to be overlapping fields-together, they contributed to a new geology of the Solar System, and with it new support for the old alternative to Lyell. After all, why should the Earth be spared the cosmic disasters so evident on other planets?
Evidence uncovered late in the 20th century began to suggest that there were kernels of truth in Cuvier’s longdiscarded catastrophist theory. The fall of the dinosaurs 66 million years ago at the end of the Cretaceous period was an oft-debated and mysterious event in Earth’s history until it was proposed that the extinction was triggered by a powerful asteroid or comet impact, a hypothesis suggested by the father-son team of Luis and Walter Alvarez in 1980.
The Alvarez hypothesis sent a shock wave through the geological sciences. It was produced by a maverick group of scientists consisting of a physicist (Luis), a geologist (Walter), and two experts in detecting trace elements (Frank Asaro and Helen Vaughn Michel), and it violated two of Lyell’s laws: It was catastrophic, and it involved forces from outside Earth. Originally, the impact hypothesis was supported primarily by analyses of trace amounts of the element iridium, which most geologists knew nothing about. It took a while for the geological community to accept the Alvarez hypothesis and the idea that an impact started a chain reaction ending in a mass extinction, but the discovery of the Chicxulub crater in the Yucatán in 1991 ended much of the resistance to it, although even now some dinosaur experts still refuse to believe that their beasts were killed off by a rock from space.
Not long after the initial hypothesis was put forth, David Raup and Jack Sepkoski of the University of Chicago introduced a surprising new elaboration on the theory with the 1984 publication of a paper that provided new evidence for an apparent 26-million year cycle in the timing of the mass extinctions. This paper motivated geologists to investigate whether or not all of these extinctions correlated with a large impact event, and it started an active debate on the existence of cycles in the geologic record.
I was studying volcanism and its effects on climate at NASA’s Goddard Institute for Space Studies when the impact story broke. I realized that this was going to be one of the most exciting areas of research, and I wanted to be a part of it. So I switched my research program to a consideration of the role that impacts have played in Earth history. Richard Stothers, an astrophysicist from NASA and an expert on Greek and Roman classical literature, worked just down the hall from me at the institute, and we were already collaborating on the record of early historical eruptions. As I shifted my research focus to investigating cataclysms from space, it proved serendipitous having an astrophysicist next door. My new work with Stothers analyzing worldwide impact crater ages found a cycle similar to the one identified in mass extinctions by Raup and Sepkoski, of about 30 million years. It seemed, from the evidence, that periodic impacts could be causing periodic extinctions. Now all three of Lyell’s laws were being challenged-gradualism, internātiem, and noncyclirity.
Even among supporters, this new, paradigm-shaking hypothesis led to some heated debates about the various proposed mechanisms for the periodicity. Scientists presented several astronomical hypotheses that Earth could have been bombarded by comet showers as a result of perturbations of the distant Oort cloud, the thick halo of comets and other icy debris circling far beyond the orbit of Neptune. My own version of this hypothesis held that comets could be perturbed by the cyclic motion of the Solar System swinging through the disk of our Milky Way galaxy. Others proposed comet disturbances by a theoretical small companion star to the Sun, named Nemesis, orbiting beyond the Oort cloud, or by an undiscovered planet in our Solar System, dubbed Planet X.
For the galactic model, Stothers and I envisioned that as the Solar System passed through the crowded midplane of the galaxy every 30 million years or so, the concentration of stars and clouds of gas and dust (now including a proposed invisible disk of dark matter, as recently suggested by astrophysicist Lisa Randall and her colleagues at Harvard University) shook the Oort cloud at the fringes of the Solar System, sending a barrage of comets toward Earth.
Meanwhile, other geologists pointed out that the end of the Cretaceous was also the time of the catastrophic eruption of the Deccan flood basalts of India, where more than 1 million cubic kilometers of lava, covering one-third of the Indian subcontinent, erupted over a relatively brief period of tens to hundreds of thousands of years. This event suggested that cataclysmic volcanism was also involved in the end Cretaceous and other disasters.
The plot thickened when Stothers and I realized that a number of the giant flood basalt episodes in Earth’s past correlated with times of extinction, and also with times when the oceans became stagnant and severely depleted of dissolved oxygen. Several research groups have argued that the environmental effects of such cataclysmic flood basalt eruptions might be severe enough to trigger mass extinctions. So destruction might come from above or below. Perhaps mass extinctions are most intense when different types of catastrophic events coincide. It has been suggested that large impacts can in some way trigger increased volcanism, or that there might be an even deeper connection.
Back in the 1980s, when I was working for NASA, I noticed that various kinds of geologic events-such as massive volcanism, mountain building, creation of volcanic hotspots, and fluctuations in sea level and climate (all related through plate tectonics)- seemed to be happening on a similar 30-million-year schedule. This apparent periodicity had been pointed out by some geologists in the early 20th century but was largely ignored by a scientific community steeped in Lyell’s uniformitarianism. Although these disparate events seem to have similarly disparate causes, a number of researchers have wondered whether they might be connected. It is possible to develop a speculative scenario based on astrophysical theory and dark matter. Theorized to be distinct from the particles that make up visible matter, dark matter has been called upon to explain a number of strange features of our universe, including gravitational anomalies and, now, periodicity of cataclysms. We are exploring the possibility that dark matter concentrated in clumps near the midplane of the Milky Way galaxy would be captured in the Earth’s core as the Solar System swung through. The dark matter would undergo self-annihilation in the Earth’s core, and could release a great deal of energy. Because Earth’s mantle is unstable, excess heat from the core will raise the temperature of the boundary layer between the core and mantle. Such a pulse of heat could create a mantle plume-a rising column of hot mantle rock. When such plumes impinge on Earth’s crust, they create a volcanic hotspot, initiate flood-basalt volcanism, and lead to continental fracturing and the initiation of a new phase of seafloor spreading.
In this way, we might see pulses of geologic activity and volcanism with the same 30-million-year cycle that we see in impacts and mass extinctions, all from a common extraterrestrial cause. Both dark matter and catastrophic cycles on Earth are now extremely active areas of research. The Alvarezes’ discovery and the studies that followed could mark the beginnings of a new geology for the 21st century, a cataclysmic geology that takes into consideration the effects on our planet of the wider Solar System and galaxy. The idea that astronomical factors could be controlling Earth’s long-term geologic processes would be truly revolutionary.
Rethinking Natural Theology
The past 35 years of discoveries in the Earth sciences suggest that all three of Lyell’s laws that anchor modem geology may be mistaken. At least two of them are clearly wrong. First, geological changes are not always slow and gradual. Asteroid and comet impacts and massive flood-basalt eruptions cause rapid and irrevocable changes in Earth’s environment. Second, all the forces that govern the planet’s biological and geological evolution may not be terrestrial, considering the evidence for asteroid impacts and dark matter interactions. Third, and more speculative, there may be grand geological cycles driven by astronomical circumstances, as evidenced by the 26-million-year extinction cycle seen in the fossil record.
From this new perspective, it appears that Lyell advanced a largely theoretical picture of nature organized by a divine order despite the evidence for sudden changes in the geologic record. Lyell and those who have since followed his approach argued in error that rates of known geologic processes have not varied. The evidence shows otherwise. We see that violent earthquakes, cataclysmic volcanic eruptions, flash floods, and tsunamis are all inscribed in the geologic record. Uniformity of process does not imply uniformity of rates.
There is an important theme in geology that runs back to the early days of the science. It is a theme that has its roots in the natural theology of our predecessors, one we overlook or that is present only in an unconscious form. Lyell’s religious bent is usually not mentioned when we recognize him as among the greats of geology, but there is no question that theology colored his advocacy for uniformitarianism. This same kind of notion may be a part of the reason that some geologists are still reluctant to appeal to catastrophic events, even when the geologic evidence points that way. It is too soon to say whether the periodic hypothesis is correct, but at last we are at least looking openly to find out whether it is.
Many geologists are blinded by the fact that Earth is our home. The study of Earth is the study of our own planet, so it is difficult to maintain an objective approach when examining its history. To the founders of geology, steeped in natural theology, Earth was a place created by a beneficent deity to be the orderly and overall peaceful residence of humankind. Why should the planet be forced to experience periodic catastrophes in which many kinds of life disappear? This viewpoint would betray the central idea of God’s gift of Earth as the abode for humans. Geologic change could not be “the work of accident, or effect of an occasional transaction” such as encounters with comets or other extraterrestrial bodies, or volcanic cataclysms of global import, to quote Hutton.
Modem geology, it turns out, still retains some aspects of that old natural theology. Many geologists experience a kind of spirituality through their study of Earth. We feel close to nature, and we ponder the great expanse of geologic time. Are geologists to give up that comfortable feeling, and face an Earth history full of catastrophic and cyclical events? For most geologists, contemplating the overall benign effects of slow grain-by-grain erosion and deposition in altering the landscape, and envisioning how mountains could be built over time by uplift, and then disappear bit by bit through erosion over great periods of time, gives subtle assurance that the world, if not made for humans, is at least compatible with their survival. Extinctions, when they happened, came as a result of a fair game of competition among species, in which the better-adapted organisms survived.
The new findings in catastrophic geology suggest that our serene feeling of being at home on an Earth governed by Lyell’s laws of gentle and gradual change is in need of revision. But looking to the cosmos to explain extraordinary events in our history need not detract from, and indeed should heighten, our feelings of awe at the grandeur of Earth. Modem geologists who focus only on terrestrial causes for geologic events, and who stress the importance of gradual processes, may be missing a significant connection between the geological and astronomical sciences, and thereby depriving themselves of a broader understanding of our vibrant field of study and the Earth’s place in the universe.