Geological Time Scale

The geological time scale is a system of division of geological time, generally presented in chart form (see Table 1). The time scale is a scientific statement, and as such is constantly subject to refinement and correction, as new dates are obtained for division boundaries, finer subdivisions are recognized, and rock layers in different regions of the earth are matched up more accurately.

The idea that the earth has had a long history, and that the sequence of events that have led to the present state of the earth can be studied scientifically, is only a few centuries old. The Danish physician Niels Stensen (known as "Steno"; 1638-1686) formulated two laws pertaining to the understanding of sedimentary rocks and their relative ages. Steno saw that sedimentary rocks are formed by the cementing together of particles of sand, clay, or lime, and thus they are formed from the

Table 1

Geological Time Scale

Table 1

Geological Time Scale

Eras

Periods

Epochs

Duration*

Cenozoic

Quaternary

Recent

Present-0.01

Pleistocene

0.01-1.6

Tertiary

Pliocene

1.6-5.3

Miocene

5.3-23.7

Oligocene

23.7-36.6

Eocene

36.6-57.8

Paleocene

Cretaceous

Jurassic

Triassic

66.4-144 144-208 208-245

Mesozoic

Paleozoic

Cretaceous

Jurassic

Triassic

66.4-144 144-208 208-245

Precambrian

Permian 245-286

Pennsylvanian i 286-325

Mississippian I 325-360

Devonian 360-408

Silurian 408-438

Ordovician 438-505

Cambrian 505-570 570-4500

*Approximate time in millions of years before present

Source : Eldredge, Niles. 1999. The Pattern of Evolution. New York:

W. H. Freeman and Company.

Note: A simplified chart of geological time for the most recent million years, emphasizing the nested, hierarchical structure of the divisions of geological time: epochs are parts of periods, which are divisions of geological eras.

same kinds of sediments that are accumulating today in lakes, oceans, and sand dunes. He deduced that (1) the layers of sedimentary rocks were initially formed in horizontal beds (so that layers that are no longer horizontal must have been secondarily tilted by forces within the earth—the "Law of Original Hor-izontality"), and (2) in a sequence of layered rocks, those on the bottom of the pile must have been deposited first (the "Law of Superposition"). It was especially the Law of Superposition that allowed geologists to see a connection between a deposit of sedimentary rocks and the passage of geological time.

Even well into the first half of the nineteenth century, it was still generally supposed that the earth was no older than the approximate date of 10,000 years—a date based on an analysis of the ages of the ancient men

(such as Methuselah) as recounted in Genesis, the first book of the Bible. But early geologists of the late eighteenth and early nineteenth centuries (all of whom were amateurs, including some clergymen, as the science of geology was just being developed by these early practitioners) were already hard at work deciphering the layers of rock and noting in particular the fossils contained in them. Baron Georges Cuvier and his collaborator Alexandre Brongniart produced a map of the Paris region (the so-called Paris Basin); across the English Channel, William Smith produced England's first geological map in 1815.

Smith was a surveyor, and he was mapping the countryside in connection with the building of canals in the early days of Britain's Industrial Revolution. As he climbed the hills to set up his surveying equipment, Smith noticed that the fossils exposed along the way always occurred in the same order. He reasoned that the same fossils collected on two separate hillsides must have been living in the same seaway at the same time (Smith's fossils were ammonoids and other marine mollusks—see Paleontology). Further up the hill, he would observe a somewhat different group of fos-sils—also found elsewhere. On hillsides further away, he would perhaps not find the lower assortment of fossils, but would find the higher one—plus yet another different assortment of fossils above that one (see Figure 1). Thus Smith saw that (1) not all layers of the earth are exposed at one place (not even in the Grand Canyon!), but that (2) by careful comparison of the layers—and especially their fossil content—from place to place, geologists could work out the overall sequence of rock layers of an entire region.

With these principles in mind, geologists rapidly began to map all the rocks exposed in streams, road cuts, and hillsides around them. Most famous were the joint expeditions of

Figure 1

Using Fossils to Correlate Rocks

Figure 1

Source: Based on Eldredge, Niles. 1999. The Pattern of Evolution. New York: W. H. Freeman and Company.

Note: Geologists use patterns of fossil occurrence to determine age equivalency in isolated bodies of rock. This figure shows three outcrops: two that contain a Devonian trilobite species, and two that contain a Mesozoic ammonite species. In general, many species are used to establish correlations between rock strata.

Roderick Impy Murchison (who later discovered Murchison Falls in Africa) and Adam Sedgwick, rector of the cathedral in Cambridge and, coincidentally, the closest thing to a scientific mentor that Charles Darwin had before he embarked on his fateful voyage on the HMS Beagle in 1831.

Friends at first, Sedgwick and Murchison set out during the summer months over successive years in the 1820s to examine the sequence of rocks of western England and Wales. Dividing up the territory, Sedgwick worked in Wales while Murchison worked in England. Sedgwick called his rocks "Cambrian" (the old Roman name for Wales was "Cambria"), while Murchison was working in a sequence he called the "Silurian" (the Silures were a primordial tribe of native Britons). Sedgwick was working up the sequence—realizing that his rocks were the oldest in the region to have fossils; Murchi-son was working downward—and soon they discovered that the rocks near the top of Sedgwick's Cambrian sequence were the same as those mapped by Murchison as his lower Silurian sequence. Their friendship was ruined over this early geological squabble over how to name rocks; later, another geologist (Charles Lapworth) solved the problem by naming the rocks in dispute the Ordovician System (the Ordovices were still another ancient tribe of the region).

Other geologists soon followed suit. The Devonian was named from rocks mapped in Devon-shire—though even better exposures in New York state almost caused their name to be the "New Yorkian." The Permian Period was named after the Perm district in Russia, while the Carboniferous was named for the famous "coal measures" that were being mined so extensively in Britain during the Industrial Revolution.

The Triassic derives its name from a threefold division of rocks recognized in Germany; Jurassic is named for the Jura Mountains of France, where these rocks are extensively exposed. The Cretaceous comes from the Greek word kreta, meaning "chalk"—many chalk deposits, including the white cliffs of Dover, are of this age. The Tertiary and Quar-ternary were divided into epochs by Charles Lyell, who produced the forerunner to the modern classification of Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Holocene. Cene means "recent," and the divisions are intermediates between "ancient recent" (Paleocene) and "completely recent" (Holocene).

The sequence of these basic divisions of geological time (the so-called geological periods) had been established by Steno's laws and the use of fossils to correlate rocks (meaning the principle that the same or closely similar fossils in two different bodies of rock imply that the rocks are very similar in age). By the 1840s, it had also been recognized that the fossils in rocks from Cambrian through Permian age, though they may look quite different up through the sequence of rocks, have an overall similarity; trilobites, for example, are found in all these rocks from the Cambrian through the Permian periods. They are not found in younger rocks. Similarly, certain kinds of corals are found only in rocks of Cambrian through Permian age; and in all these rocks, bra-chiopods usually predominate over all other forms of shelled invertebrate life. Thus geologists readily accepted the suggestion that these rocks might be classified together as the Paleozoic Era (meaning the era of ancient life; sometimes this division of geological time is called the "Age of Invertebrates").

Similarly, fossils of a certain type—especially the dinosaurs, but including many groups of marine animals, such as ceratite and ammonite ammonoids (see Paleontology)— are characteristic of Triassic, Jurassic, and Cretaceous rocks and are not found in the younger rocks of the Tertiary. So these three periods were lumped, logically enough, into the Meso-zoic Era (for "middle life"—also known informally as "The Age of Dinosaurs"). The final division, consisting mostly of rocks of Tertiary age, was called the Cenozoic Era (meaning "recent life"), and it is sometimes informally called the "Age of Mammals."

When radioactivity was discovered at the end of the nineteenth century, geologists were quick to realize that radioactive elements occur naturally in minerals in the earth's crust. The idea is that, if we know the rate at which an element decays from its initial state to its final state (sometimes called the "parent" and "daughter" states), we can estimate the age of a sample of rock by measuring the ratio of parent and daughter elements; for example, various forms ("isotopes") of uranium decay into different isotopes of lead at known rates. It is important for these calculations that we can be sure that the initial formation of the rock had 100 percent of the "parent" isotope. Sedimentary rocks, composed of grains weathered from other rocks, are therefore poor candidates for this so-called radiometric dating. But igneous rocks (formed from a hot melt, such as the lavas of volcanoes), and even metamorphic rocks (which are formed from other rocks through heat and pressure), do lend themselves to radiometric dating.

Thus to add numbers to the geological time scale—and to date parts of the earth's crust before there were fossils both abundant and well-preserved enough (see Evolution and Paleontology)—geologists rely on igneous and metamorphic rocks. Fossils do not generally occur in these rocks, but igneous rocks in particular, whether ancient lava flows or layers of granite injected into a sequence of sedimen tary rock, can be radiometrically dated. The way geologists have been able to match up the sedimentary sequences, recognized and named by their relative position and fossil content (for example, Middle Devonian), with the actual age in years since they were formed (the so-called absolute ages of the rocks), is by finding, for example, rocks classified as Middle Devonian based on their fossils and measuring the age of the volcanic rocks sometimes found intruding the sediments. The ages measured radiometrically are always in the same order as the sedimentary sequence—meaning, for example, that the rocks dated as Middle Devonian based on their fossils always are dated somewhere around 380 million years old, no matter what the technique used, and no matter where in the world the samples come from; uppermost Cretaceous rock always dates to between 70 and 65 million years (the last great mass extinction occurred 65 million years ago); and rocks considered uppermost Permian in age—the very end of the Paleozoic Era and the time of the greatest mass extinction to have struck the earth so far—always come out to be 248 million years old.

The great depth of time of the Precam-brian—just over 4 billion years of the earth's 4.65-billion-year history—has been analyzed mostly by radiometric dating, supplemented by studies of fossil bacteria and other forms of microbial life. The oldest rocks so far discovered and dated are just over 4 billion years old. We calculate the age of the earth as 4.65 billion years based on the ages of meteorites and the oldest moon rocks—and the certain knowledge that the restive crust of the earth has long since obliterated all traces of the original rocks, through the ravages of erosion and the swallowing of crustal plates back into the earth in the processes of plate tectonics.

—Niles Eldredge

See also: Evolution; Hutton, James; Lyell, Charles; Paleontology; Plate Tectonics

Bibliography

Levin, Harold L. 1996. The Earth through Time. Fort Worth: Saunders College; Press, Frank, and Raymond Siever. 1998. Understanding Earth. New York: W. H. Freeman; Stanley, Steven M. 1998. Earth System History. New York: W. H. Freeman.

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