ENORMOUS TIME AND DYNAMIC EARTH
Framework of time
One of geology's greatest philosophical contributions has been the demonstration of the enormity of geologic time. Astronomers deal with distances so great that they are almost beyond understanding; nuclear physicists study objects so small that we can hardly imagine them. Similarly, the geologist is concerned with spans of time so immense that they are scarcely comprehensible. Geology is a science of time as well as rocks, and in our geologic story of the Teton region we must refer frequently to the geologic time scale, the yardstick by which we measure the vast reaches of time in earth history.
Rocks and relative age
Very early in the science of geology it was recognized that in many places one can tell the comparative ages of rocks by their relations to one another. For example, most sedimentary rocks are consolidated accumulations of large or small rock fragments and were deposited as nearly horizontal layers of gravel, sand, or mud. In an undisturbed sequence of sedimentary rocks, the layer on the bottom was deposited first and the layer on top was deposited last, All of these must, of course, be younger than any previously formed rock fragments incorporated in them.
Igneous rocks are those formed by solidification of molten material, either as lava flows on the earth's surface (extrusive igneous rocks) or at depth within the earth (intrusive igneous rocks). The relative ages of extrusive igneous rocks can often be determined in much the same way as those of sedimentary strata. A lava flow is younger than the rocks on which it rests, but older than those that rest on top of it.
An intrusive igneous rock must be younger than the rocks that enclosed it at the time it solidified. It may contain pieces of the enclosing rocks that broke off the walls and fell into the liquid. Pebbles of the igneous rock that are incorporated in nearby sedimentary layers indicate that the sediments must be somewhat younger.
All of these criteria tell us only that one rock is older or younger than another. They tell us little about the absolute age of the rocks or about how much older one is than the other.
Fossils and geologic time
Fossils provide important clues to the ages of the rocks in which they are found. The slow evolution of living things through geologic time can be traced by a systematic study of fossils. The fossils are then used to determine the relative ages of the rocks that contain them and to establish a geologic time scale that can be applied to fossil-bearing rocks throughout the world. Figure 18 shows the major subdivisions of the last 600 million years of geologic time and some forms of life that dominated the scene during each of these intervals. Strata containing closely related fossils are grouped into systems; the time interval during which the strata comprising a particular system were deposited is termed a period. The periods are subdivisions of larger time units called eras and some are split into smaller time units called epochs. Strata deposited during an epoch comprise a series. Series are in turn subdivided into rock units called groups and formations. Expressed in tabular form these divisions are:
The time scale based on the study of fossil-bearing sedimentary rocks is called the stratigraphic time scale; it is given in table 1. The subdivisions are arranged in the same order in which they were deposited, with the oldest at the bottom and the youngest at the top. All rocks older than Cambrian (the first period in the Paleozoic Era) are classed as Precambrian. These rocks are so old that fossils are rare and therefore cannot be conveniently used as a basis for subdivision.
The stratigraphic time scale is extremely useful, but it has serious drawbacks. It can be applied only to fossil-bearing strata or to rocks whose ages are determined by their relation to those containing fossils. It cannot be used directly for rocks that lack fossils, such as igneous rocks, or metamorphic rocks in which fossils have been destroyed by heat or pressure. It is used to establish the relative ages of sedimentary strata throughout the world, but it gives no information as to how long ago a particular layer was deposited or how many years a given period or era lasted.
The measurement of geologic time in terms of years was not possible until the discovery of natural radioactivity. It was found that certain atoms of a few elements spontaneously throw off particles from their nuclei and break down to form atoms of other elements. These decay processes take place at constant rates, unaffected by heat, pressure, or chemical conditions. If we know the rate at which a particular radioactive element decays, the length of time that has passed since a mineral crystal containing the elements formed can be calculated by comparing the amount of the radioactive element remaining in the crystal with the amount of disintegration products present.
Three principal radioactive clocks now in use are based on the decay of uranium to lead, rubidium to strontium, and potassium to argon. They are effective in dating minerals millions or billions of years old. Another clock, based on the decay of one type of carbon (Carbon-14) to nitrogen, dates organic material, but only if it is less than about 40,000 years old.
The uranium, rubidium, and potassium clocks are especially useful in dating igneous rocks. By determining the absolute ages of igneous rocks whose stratigraphic relations to fossil-bearing strata are known, it is possible to estimate the number of years represented by the various subdivisions of the stratigraphic time scale.
The yardstick of geologic time
Recent estimates suggest that the earth was formed at least 4.5 billion years ago. To visualize the length of geologic time and the relations between the stratigraphic and absolute time scales, let us imagine a yardstick as representing the length of time from the origin of the earth to the present (fig. 19). On one side of the yardstick we plot time in years; on the other, we plot the divisions of the stratigraphic time scale according to the most reliable absolute age determinations.
We are immediately struck by the fact that all of the subdivisions of the stratigraphic time scale since the beginning of the Paleozoic are compressed into the last 5 inches of our yardstick! All of the other 31 inches represent Precambrian time. We also see that subdivisions of the stratigraphic time scale do not represent equal numbers of years. We use smaller and smaller subdivisions as we approach the present. (Notice the subdivisions of the Tertiary and Quaternary in table l that are too small to show even in the enlarged part of figure 19). This is because the record of earth history is more vague and incomplete the farther back in time we go. In effect, we are very nearsighted in our view of time. This "geological myopia" becomes increasingly evident throughout the remainder of this booklet.
Last Updated: 19-Jan-2007