Earth Science: Gradualism and Catastrophism
Earth Science: Gradualism and Catastrophism
Gradualism and catastrophism were schools of thought in the earth sciences that explained the major features of Earth's surface and lifes history by appealing to different sorts of causes. Gradualists explained geological features as the result of slowly acting processes such as erosion, while catastrophists argued that Earth had been shaped mainly by a series of violent events or catastrophes, whether over a relatively short time (6,000 to 10,000 years) or over many millions of years. In the early nineteenth century, gradualism seemed to win out completely over catastrophism, but in the late twentieth century scientists discovered that catastrophic events have also played a major role in Earth's history. For example, Earth's moon was probably formed by the collision of the early Earth with a Mars-sized object, and it is now known that the history of life has been repeatedly shaped by impacts of asteroids or comets that wiped out large numbers of species, allowing new ones to evolve—including ourselves. Today, the dispute between gradualism and catastrophism is largely over, as scientists recognize that both gradual and sudden processes have shaped the surface of Earth and the course of biological evolution.
Historical Background and Scientific Foundations In Europe during the Middle Ages and Renaissance, theories about Earth's history and of the origin of life tended to follow a literal reading of the Bible, especially the book of Genesis, which describes the creation of Earth and a world-covering flood survived by Noah and the other passengers on his ark. Genealogies in the Bible that linked one generation to another and gave lifespans in years seemed to show that Earth could be no more than about 10,000 years old, as young-Earth creationists still believe today. Given such a short span of time, it seemed reasonable to suppose that Earth's features had either been created as they are today or shaped mainly by brief, violent events—catastrophes. As for plants and animals, they were believed to have all appeared suddenly, as described in Genesis.
In the eighteenth and early nineteenth centuries, scientists began to gather evidence that Earth is far older than 10,000 or so years—perhaps millions or thousands of millions of years old. (We now know that it formed about 4.5 billion years ago; the universe itself formed about 13.7 billion years ago.) Some scientists, such as the French naturalist Georges Cuvier (1773–1838), formulated a non-biblical, scientific version of catastrophism that accommodated this new vision of an ancient Earth. Cuvier showed in the 1790s that some species of animals known only from fossils were truly extinct, overturning earlier assumptions that all animals present at the Creation must still survive somewhere on Earth. Moreover, he argued that extinctions occurred in waves caused by great disasters or catastrophes. Other theorists, such as German geologist Abraham Werner (1749–1817), proposed that most features of Earth's crust could be explained by the recession of waters from a single, great, world-covering flood, possibly the biblical Noachian flood. These were the doctrines of the catastrophist school of thought.
However, in the late 1780s the Scottish geologist James Hutton (1726–1797) proposed a different view. He suggested that Earth's history was cyclic, with uplift (raising of land) and erosion (wearing away of land) going on continuously, uniformly, and gradually. Erosion removed particles and washed them to places where they were deposited, eventually forming layers of sedimentary rock; uplift caused by Earth's internal heat raised these sedimentary layers so that they could be eroded once again. Uplift, erosion, deposition; uplift, erosion, deposition; an eternal cycle. He proposed that in Earth's cyclic system there was no vestige of a beginning,—no prospect of an end. Time, he taught, is to nature end-less and as nothing (quoted in Gould, 1987).
For decades, both catastrophism and Huttons slowly cycling world had scientific supporters. But in 1830 through 1833, Scottish geologist Charles Lyell (1797–1875) published his persuasive work Principles of Geology, in which he developed Huttons ideas further. Lyell argued for a strict form of what was quickly labeled uniformitarianism, the doctrine that all past events in Earth's history can be explained by causes still in operation today: volcanic eruptions, erosion and deposition of sediments, uplift. Like Hutton, Lyell asserted that Earth had always looked much as did today and always would, as eternally-operating causes recycled its surface. Moreover, Lyell taught, these causes were gradual, not catastrophic: They worked at a slow, steady pace through deep time.
Lyells view triumphed, and by the end of the nineteenth century catastrophism had been abandoned, thereafter to be reviled as superstitious nonsense. So strong was this rejection that later writers of histories and textbooks often mistakenly spoke of catastrophist scientists such as Cuvier as biblical literalists who clung to an untenable reading of sacred texts rather than looking at the evidence of geology. This is an inaccurate depiction of Cuvier and his colleagues, who were skilled scientists. But throughout much of the twentieth century, catastrophist ideas were literally unthinkable in the earth sciences. Fossil evidence accumulated showing that Cuvier had been right about episodes of mass extinction in the deep past, but it was taught that gradual causes such as continental drift, not catastrophes, must be responsible for such changes.
This changed dramatically beginning in 1980, when Walter Alvarez (1940) and his father, Luis Alvarez (1911–1988), published evidence showing that the great mass extinction that occurred at the geological moment dividing the Cretaceous and Tertiary periods, known as the K-T extinction event, was caused at least in part by the impact of a comet or asteroid some 6 miles (about 10 km) in diameter. Shock waves and dust clouds from the impact had, the Alvarezes argued, killed off the last of the dinosaurs along with 65–70% of all other plant and animal species alive at that time.
Evidence for the Alvarez hypothesis came not only from the sudden nature of the extinctions, as shown in the fossil record, but from a thin layer of particles found all over the world at the boundary between the Cretaceous and Tertiary sediments, dust that must have been deposited at the time of the extinctions, 65 million years ago. The Alvarezes showed that this dust is rich in iridium, an element more common in meteorites than on Earth. The best explanation for this iridium-rich layer was the impact of a gigantic asteroid. The Alvarezes even identified the impact crater, the 40-mile-wide (70 km) Chicxulub crater on the Yucatan peninsula in Mexico, which had been discovered by petroleum geologist Glen Penfield in 1978. The crater was the right size and age to have caused the K-T extinction.
Some scientists found the new theory distasteful because it was a revival of catastrophism. However, evidence mounted, and Earth scientists were forced to re-admit catastrophic events into their toolkit of explanatory possibilities. In 2007, the editors of the journal Nature, looking back on the whole episode, said that the effect of the Alvarez hypothesis was to change how scientists and others see the world, and reintroduce catastrophism to the Earth sciences. Later, scientists would marshal evidence that the Chicxulub impact probably only finished off a process of extinction that had already begun due to vast volcanic eruptions in the area that is now India.
Today, gradualism or uniformitarianism blends with catastrophism as it is recognized that catastrophes, though occasional, may still occur, just as they did in the past. Catastrophes can be seen as one of Lyells ever-present causes working over long spans of time. This was demonstrated spectacularly in 1994, when comet Shoemaker-Levy 9 collided with Jupiter, releasing at
least 750 times more energy than all the nuclear weapons on Earth exploded at once. Telescopes revealed the white flash of the impact and the dark scars that lingered in Jupiters atmosphere for days, a dramatic proof that cosmic catastrophes can still happen.
Modern Cultural Connections
Catastrophism, especially in the form of cosmic collisions, is now taken seriously as a possible threat to human life, mostly as a result of the discovery that such collisions have occurred repeatedly in Earth's past. In the 1980s and 1990s, a number of projects were begun by astronomers to discover and track asteroids that might impact Earth someday; many were commenced even before Shoemaker-Levy hit Jupiter. In 1992, the U.S. Congress ordered the National Aeronautics and Space Administration (NASA) to identify 90% of large asteroids that approach Earth closely by 2002. This goal was later revised to include the discovery of at least 90% of all near-Earth objects with diameters greater than .62 miles (1 km) by 2008. In 2005, Congress ordered NASA to detect 90% or more of near-Earth objects with diameters of 490 feet (150 meters) or more by 2020. In early 2008, NASA announced that it would meet the goal of detecting most 1-km objects by the end of that year.
Various schemes have been proposed for deflecting any asteroid found to be on a catastrophic collision course with Earth, including exploding nuclear weapons near its surface, darkening part of its surface so that light pressure changes its course, using a spacecrafts gravitational attraction to an asteroid to tow it slightly off course, and more. The farther away an asteroid is when deflection efforts begin, the smaller the deflection that would be required to make it miss Earth, thus preventing a repeat of the disaster that extinguished the dinosaurs.
Gould, Stephen Jay. Times Arrow, Times Cycle. Cambridge, MA: Harvard University Press, 1987.
The Editors of Nature. The Big Splash. Nature 449 (2007): 1–2.