Letters From the Grand Canyon:
Nuts and Bolts, Part I

It's About Time
No discussion of geologic history, least of all that of the Grand Canyon, can be undertaken without first touching on subjects that are not part of routine human experience. These subjects need to be converted into something that can be grasped in everyday terms. Human experience is far too limited in time and space to visualize directly, instinctively, the stately succession of events, landscapes, and lifeforms that have followed each other in geologic time. It is precisely this traveling through time, this going far beyond what any of us can actually experience, this reconstructing things stranger even than the imaginings of science fiction, that is the true pleasure of a historical science such as Geology, a science whose home is Time.

Rivers and Rocks
A common impression is that the Grand Canyon and the rocks through which it is cut are closely related, perhaps by being born at the same time. But nothing could be farther from the truth, whether for the Grand Canyon, the Colorado River, or nearly any river. The rocks are merely the stage upon which rivers carve their will.
As we shall see, much of this “stage” is up to ten times older than the earliest possible age of the Colorado River, and is more than 100 times older than the carving of most of the Grand Canyon.
As the “stage” was being set, the landscape of the region had no resemblance to what we see today on the Colorado Plateau. Instead of tree covered uplands and grass- or scrub-covered flats, one might have seen something rather like the Bahama Banks: a warm, shallow sea teeming with life. Later, in Mesozoic time, the area was alternately shallow seas and dry land. Only when the Colorado River was born (probably some 60 million years ago), when the seas were completely gone, did the landscape start evolving recognizably toward what we see today. So the Colorado River merely flows through, and cuts into, rocks deposited long before its birth. The only connection between river and rocks is that these rocks, some tough, others easily eroded, all mostly flatlying, give the Grand Canyon its characteristic architecture of horizontal entablatures and vertical cliffs, and the Colorado Plateau its tablelands and great escarpments.

A Matter of Time
For a geologist, one million years is small change, the unit of currency, a dollar payable at the treasury of Time. It is not so hard to get a good feeling for what one dollar represents—little enough nowadays—but one million years? How do we fathom a span of time that so greatly exceeds our life span, the stretch of our experience? The only way is by playing games of analogy.
Consider one centimeter. Now let us envision a great fault, like the San Andreas, along which rocks move horizontally. Let us then make the reasonable assumption that rocks on one side of the fault move past those on the other side at an average rate of one centimeter per year. After one million years, points that initially were right next to each other across the fault line will be one million centimeters apart. This is equal to ten kilometers (six miles). Respectable. Rocks displaced at the same rate along a fault that moves vertically would form a mountain comfortably higher than Mt. Everest in the same time interval.
Many people find a biological analogy more workable than a physical one, because biology relates more directly to our human experience. So, think of a human generation as being twenty years, which is not far off the mark. How many such generations are there between us and the time Christ was born? One hundred, that is all. And how many between now and the time—about 1300 bc—when Helen's face launched a thousand ships to besiege Homer's Bronze Age Troy, home to kingly Priam and lugubrious Cassandra? One hundred sixty-five. And how many in one million years? Fifty thousand. Perhaps the stately pace of geologic change begins to make sense after all: there is so much time!
Let us now establish an analogy between the space of one centimeter and the time of one million years. We can use this yardstick to map out the time entombed in the rocks of the Grand Canyon, which reaches back some 1,750 million years (give or take a few million). At one centimeter per million years, Grand Canyon time measures nearly 17 1/2 meters (58 feet). And in this 1,750 million years, our Mt. Everest, still rising at the rate of one centimeter per year, would reach the notable altitude of 17,500 kilometers, (10,200 miles) while 85 million generations would be born and die. Impressive stuff, but it is well to remember that Grand Canyon rocks are pretty young as Earth goes: rocks elsewhere are twice as old, some even more. Just to keep things in perspective.

The Ticking of Clocks
We've been tossing around millions of years with much abandon, but the hard-eyed reader may well ask: how do you know something happened 500 million years ago, not 200, or 900? In other words, what sort of clock do geologists use to measure these times? And how reliable and accurate is this clock? Fair questions.
Traditionally, geologists have used a time scale that tells us that one rock layer is older than another—a relative time scale. This scale is based chiefly on superposition (in a stack of rock layers, the youngest layers are on the top, and the oldest ones are on the bottom), and on the orderly succession of gradually changing life forms. The result is that one can state, for example, that rocks from anywhere in the world containing fossils of certain large, carnivorous, lizard-like reptilians are of Jurassic age, but one cannot tell precisely when the Jurassic Period began and ended. Getting precise numbers depends on finding a clock that can measure the vastness of geologic time. This is a tall order, whose filling had to wait for the deeper understanding of nuclear chemistry and physics. We now know that certain elements are radioactive and decay into other (daughter) elements at a rate that is absolutely fixed and unaffected by anything that might happen to the parent element, including melting. So, all one needs do to read the clock is, measure the amounts of the parent and daughter element contained in a rock. If one knows the rate at which parent decays into daughter, one then knows how long it took to generate the amount of daughter present, which in turn gives the date of when the rock was formed. Simple, yes? Well, not quite. In the first place, these elements are often present in concentrations of only parts per billion, which makes them difficult to measure, and raises the possibility of contaminated rock samples. One must also establish the decay constant very accurately, which is not easy. Finally, one must make sure that no daughter element was present in the rock when the rock was formed, and none was lost since then. But all this has been worked out by refining techniques and learning what chemical systems work best for specific applications. The consequence is that we can now determine the age of a rock with a gratifying degree of precision if we are careful, select a suitable sample, and encounter no undue difficulties. Many people have read about K-Ar (Potassium-Argon), Ar-Ar (Argon-Argon), U-Pb (Uranium-Lead), 14C (Carbon 14), 10Be (Beryllium 10), 26Al (Aluminum 26) and the like. All are chemical systems that make up clocks based on the concepts outlined above and in general use today. The result is that we now know rather well when rocks were formed; in turn this gives us good insights into geologic history, and the rate at which many geologic processes take place.

Canyon Calendar
Winston Churchill divided his wonderful “A History of the English-Speaking Peoples” into “books”— “The Birth of Britain”, “The New World”, and so on—each corresponding to a particular set of historic circumstances, each covering a specific interval of time. Similarly, the development of the Grand Canyon is divided into books that correspond to major geologic scenarios succeeding each other in time. Starting with the oldest:
1840–1660 million years ago (m.y.): Outward building of the growing continent's edge through deposition, then metamorphism (change) resulting from heat, pressure, chemical agents, and intrusion of molten rock. Rocks of the Inner Gorge.
1400 (?)–1250 (?) m.y.: Erosion.
1250–850 m.y.: Deposition of sediments in shallow sea that occupied sinking trough; intrusion of basaltic magma (sills); eruption of basaltic volcanos. Very minor metamorphism due to burial. Grand Canyon Supergroup. Best exposed at the bottom of the eastern Grand Canyon between the Little Colorado River and Hance Rapid (between river miles 61.5 and 76.5).
850–570 m.y.: Tilting of blocks of the Grand Canyon Supergroup, erosion.
570–250 m.y.: Deposition in shallow seas or on land close to sea level, interspersed with intervals of erosion. Paleozoic rocks: the conspicuous horizontal layers that form most of the Grand Canyon walls visible from the rim.
250–50 m.y.: Deposition mostly on land; inland seas, gradually retreating and shrinking into residual lakes at end of time interval. Broad areas of sand dunes common. Erosion intervals. Mesozoic and early Tertiary rocks. Absent from Canyon, but common nearby (Vermilion Cliffs, Echo Cliffs, Painted Desert, etc.) and throughout Colorado Plateau.
50–5.5 m.y.: Some uplift; erosion. Establishment of ancestral Colorado River.
5.5 m.y.–present: Uplift, deep erosion, canyon cutting. Establishment of present course of Colorado River.
Dr. Ivo Luchitta
This is the second in a series of “Letters from Grand Canyon” by Ivo Lucchitta, that will appear in future issues of the bqr.