Historical Geology - Real-life applications
Uniformitarianism
Late in the eighteenth century, the Scottish geologist James Hutton (1726-1797) put forward an idea that transcended the debate over Earth's origins. Rather than speculate as to how Earth had come into being, Hutton analyzed the processes at work on the planet in his time and reasoned that they must be a key to understanding the means by which Earth was shaped. This was the principle of uniformitarianism, which is still a key concept in the study of Earth. Thanks to his introduction of this influential idea, Hutton today is regarded as the father of modern scientific geology.
Uniformitarianism, in general, is the idea that the geologic processes at work today provide a key to understanding the geologic past. This means that the laws of nature have always been the same. The uniformitarianism promoted by Hutton and his fellow Scottish geologist Charles Lyell (1797-1875), however, has undergone some modification, namely, by the addition of the qualifying statement that the speed and intensity of those processes may not always be the same at any juncture in geologic history. For instance, land does not erode today at the same rate that it did before plants existed to hold rocks and soil in place.
GOULD'S FOUR UNIFORMITIES.
In the late twentieth century, the American paleontologist Stephen Jay Gould (1941-2002) identified four different meanings of uniformity in science, not all of which are equally valid. Gould's listing and analysis of these four meanings is as follows:
- Uniformity of law: The assumption that natural laws do not change over time. This idea governs all sciences.
-
Uniformity of process: The idea embodied in the most well-known
definition of geologic
uniformitarianism, "The present is key to the past."
M ETEOR C RATER , A RIZONA . M OST NOTABLE AMONG CATASTROPHE THEORIES OF E ARTH ' S FORMATION IS THE COLLISION OF METEORITES . (© Francois Gohier/Photo Researchers. Reproduced by permission. ) - Uniformity of rate: The incorrect assumption that the rate at which processes occur presently is the same as the rate at which they occurred in the past.
- Uniformity of state: The incorrect assumption that the state of the universe always has been as it is today.
As noted, uniformity of law is essential to all sciences. For instance, there is every reason to believe that the conservation of energy (a law stating that the total amount of energy in the universe remains constant) always has been the case. If the contrary were true, the conservation of energy could no longer properly be called a law, because it might cease to be the case at some time in the future.
The statement "The present is key to the past" was formulated by yet another Scottish geologist, Sir Archibald Geikie (1835-1924). Geikie's statement often has been criticized as an oversimplification, because processes that occurred in the past may not necessarily be occurring now, or vice versa, even though they could occur again. This idea has required modification of uniformitarianism, as noted earlier, to take into account the fact that the speed and intensity of processes may not always be the same. Part of this modification has involved acceptance of a form of catastrophism, discussed later in this essay.
Variations in the speed and intensity of processes also were addressed by Gould, with his observation that "uniformity of rate" is a fallacy. So, too, is "uniformity of state," which is one of the few areas on which adherents of creationism (a strict interpretation of the Genesis account) would agree with their opponents. Even the Bible, after all, says "In the beginning … the earth was without form, and void."
Catastrophism
In Theory of the Earth (1795), Hutton suggested that the weathering effects of water produced the sedimentary layers of Earth. Based on observation of river flow and mud content, he realized that this process would require much longer than 6,000 years. So, too, did Lyell, author of the highly influential Principles of Geology, which appeared in 12 editions from 1830 to 1875 and which presented a strict version of uniformitarianism.
Aqueducts and other structures erected by the Romans had stood for a good one-fourth to one-third of the entire history of Earth, assuming that it was as young as Ussher's biblical interpretation implied. Yet these Roman constructions had experienced very little weathering and certainly much less than mountains would have had to experience to leave behind the sediments observed by geologists. Surely, then, Earth must be millions upon millions of years old, not just a few thousand.
CUVIER'S CATASTROPHIC THEORY.
Not so, countered adherents of a movement known as catastrophism, which arose in opposition to uniformitarianism during the late eighteenth and early nineteenth centuries. Catastrophism associates geologic phenomena with sudden, dramatic changes rather than ongoing and long-term processes, as in uniformitarianism. The leading proponent of catastrophism was the French geologist Baron Georges Cuvier (1769-1832), who used this theory to explain unconformities. These apparent gaps in the geologic record, revealed by observing rock layers, or strata, are discussed in the essay, Stratigraphy.
Whereas Cuvier's countryman (and fellow French noble) Buffon had asserted that Earth was 75,000 years old while actually believing that it was much older, Cuvier maintained that the planet is just 75,000 years old. The formation of mountains and other landforms, which should have taken millions of years, could be explained by sudden, violent changes, an example of which was Noah's Flood in the Book of Genesis. As the ocean waters receded, they moved rocks far from their sources, carved out valleys, and left behind lakes and other bodies of fresh water.
CATASTROPHISM TODAY.
As more and more evidence for a very old Earth began to accumulate during the nineteenth century, catastrophism fell into disfavor. Discoveries from the 1970s onward, however, influenced a new look at catastrophism, and, as a result, the idea has received new attention in later years.
This has not led to a wholesale endorsement of creationism; rather, scientists have come to understand that the generally steady pace of processes on Earth periodically is broken by catastrophic events. Most notable among types of catastrophe is the collision of a meteorite with Earth, a remarkable example of which apparently occurred some 65 million years ago. That dramatic event seems to have forced so much dust and gas into the atmosphere that it blocked out the Sun, leading to the ultimate extinction of the dinosaurs.
Understanding Geologic Time
So just how old is Earth? Modern earth scientists working in the realm of historical geology, and specifically geochronology, estimate its age at about 4.6 billion years. (The dating techniques used to determine the age of the planet are discussed in the essay Stratigraphy.) Such a vast span of time is more than a little difficult for humans to comprehend, given the fact that our lives last 70-80 years, on average, and the entire history of human civilization is only about 5,500 years long.
For this reason, it is helpful to use scales of comparison, such as that offered at the Web site listed under the title Comprehending Geologic Time. Suppose that the entire geologic history of Earth were likened to a single year of 365.25 days, starting with the formation of the planet from a cloud of dust and ending with the present. More than two months would have been required simply for the accretion of Earth from a gas cloud to a planetesimal to something like its present form, but by about March 5 this evolution would have been accomplished.
The entire spring would be analogous to a long, long period of time in which Earth was pounded by meteor showers and the oceans began to form. Not even the oldest known rocks date back this far, and many of our ideas about this phase in Earth's history are based on conjecture. Much more is known about the second half of geologic history, beginning with the origins of the first single-cell life-forms on June 16.
FROM SINGLE CELLS TO DINOSAURS.
We are now almost halfway through the year and still a long, long way from any sort of complex living beings. This is not surprising, given the fact that the formation of the continental plates and the development of oxygen in the atmosphere would have occurred only by about August 26. Even in the week after Thanksgiving, the most complex organisms would have been snails. Finally, a few days before the beginning of December, creatures would have begun to invade the land.
We tend to associate the dinosaurs with the early phases of Earth's history, but this only illustrates our distorted view of geologic time. In fact the Jurassic period, when dinosaurs roamed Earth, would be parallel to a period of about five days, from December 15 to 20. By Christmas Day, the meteorite referred to earlier would have hit Earth, and the dinosaurs would be headed toward extinction, their dead bodies eventually forming the fossil fuels that have powered much of human civilization.
THE SHORT SPAN OF HUMANITY'S EXISTENCE.
By this point, we are within a few days of the year's end, and yet nothing remotely resembling a human has appeared. Our own species, Homo sapiens, would not have come on the scene until the last 0.16 days of the year—that is, at a few minutes after 8:00 p.m. on December 31. The New Year's Eve countdown would be nearing by the time human civilization began, at about 42 seconds before midnight.
Now we have come to a period about 6,000 years ago, or the point at which, according to Bishop Ussher, Earth was created. No wonder many people wanted to believe in a young Earth, and some even hold on to that belief today: when viewed against the backdrop of the planet's true age, humanity seems very insignificant indeed. Christ's birth would have occurred at about 14 seconds before midnight, and the final 10-second countdown would begin about the time the Roman Empire fell. The life span of the average person would correspond to about half a second or less.
How Do We Know Earth's Age?
What we know about Earth's age comes, of course, not from direct observation but from the study of materials. One of the most important techniques for determining the age of samples taken from the earth is radiometric dating, discussed in more detail in Geologic Time. Radio-metric dating involves ratios between two different kinds of atoms for a given element: stable and radioactive isotopes. Because chemists know how long it takes for half the isotopes in a given sample to stabilize (a half-life), they can judge the age of the sample by examining the ratio of stable to radioactive isotopes. In the case of uranium, one isotopic form, uranium-238, has a half-life of 4,470 million years, which is very close to the age of Earth itself. Use of uranium dating has detected rocks of an age between 3.8 and 3.9 billion years old, as well as even older crystal formations that suggest the earth had solid ground as early as 4.2 billion years ago.
A rock discovered in the Australian desert during the early 1980s appears to be the oldest rock sample in the world, according to data originally reported in Nature and included on the Scientific American Web site in early 2001. This zircon crystal, according to Simon Wilde of Curtin University in Western Australia, is 4.4 billion years old. Wilde and associates reported that extensive study of the sample suggested that at the time of its formation, Earth was already covered in water—something that had supposedly happened many millions of years later. If this was the case, it could suggest the possibility that life appeared much earlier than has previously been supposed, and perhaps even that life disappeared and reappeared several times before finally taking hold.
WHERE TO LEARN MORE
Bishop, A. C., A. Woolley, and A. Hamilton. Cambridge Guide to Minerals, Rocks, and Fossils. New York: Cambridge University Press, 1992.
Boggy's Links to Stratigraphy and Geochronology (Web site). <http://geologylinks.freeyellow.com/stratigraphy.html> .
Comprehending Geologic Time (Web site). <http://www.athro.com/geo/hgfr1.html> .
Harris, Nicholas, Alessandro Rabatti, and Andrea Ricciardi. The Incredible Journey to the Beginning of Time. New York: Peter Bedrick Books, 1998.
"Historical Geology." Georgia Perimeter College (Web site). <http://www.dc.peachnet.edu/~pgore/geology/geo102.htm> .
Lamb, Simon, and David Sington. Earth Story: The Shaping of Our World. Princeton, NJ: Princeton University Press, 1998.
MacRae, Andrew. Radiometric Dating and the Geological Time Scale (Web site). <http://www.talkorigins.org/faqs/dating.html> .
Reeves, Hubert. Origins: Cosmos, Earth, and Mankind. New York: Arcade, 1998.
Spickert, Diane Nelson, and Marianne D. Wallace. Earthsteps: A Rock's Journey Through Time. Golden, CO: Fulcrum Kids, 2000.
UCMP (University of California, Berkeley, Museum of Paleontology) Web Time Machine (Web site). <http://www.ucmp.berkeley.edu/help/timeform.html> .