Explain how rocks of similar age that are in different places can be matched up.
To develop a geologic time scale that is applicable to the entire Earth, rocks of similar age in different regions must be matched up. Such a task is called correlation. By correlating the rocks from one place to another, a more comprehensive view of the geologic history of a region is possible. Figure 1, for example, shows the correlation of strata at three sites on the Colorado Plateau in southern Utah and northern Arizona. No single locale exhibits the entire sequence, but correlation reveals a more complete picture of the sedimentary rock record.
Correlation Within Limited Areas
Within a limited area, geologists can correlate rocks of one locality with those of another simply by walking along the outcropping edges, but this may not be possible when the rocks are mostly concealed by soil and vegetation. Correlation over short distances is often achieved by noting the position of a bed in a sequence of strata. Or a layer may be identified in another location if it is composed of distinctive or uncommon minerals.
Many geologic studies involve relatively small areas.
Although they are important in their own right, their full value is realized only when they are correlated with other regions. Although the methods just described are sufficient to trace a rock formation over relatively short distances, they are not adequate for matching up rocks separated by great distances. When correlation between widely separated areas or between continents is the objective, geologists must rely on fossils.
Fossils and Correlation
The existence of fossils had been known for centuries, yet it was not until the late 1700s and early 1800s that their significance as geologic tools was made evident.
During this period, English engineer and canal builder William Smith discovered that each rock formation in the canals he worked on contained fossils unlike those in the beds either above or below. Further, he noted that sedimentary strata in widely separated areas could be identified—and correlated—based on their distinctive fossil content.
Principle of Faunal Succession Based on Smith’s classic observations and the findings of many later geologists, one of the most important basic principles in historical geology was formulated: Fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content. This is known as the principle of fossil succession.
In other words, when fossils are arranged according to their age, they do not present a random or haphazard picture. To the contrary, fossils document the evolution of life through time.
For example, an Age of Trilobites is recognized quite early in the fossil record. Then, in succession, paleontologists recognize an Age of Fishes, an Age of Coal Swamps, an Age of Reptiles, and an Age of Mammals. These “ages” pertain to groups that were especially plentiful and characteristic during particular time periods. Within each of the “ages” are many subdivisions, based, for example, on certain species of trilobites and certain types of fish, reptiles, and so on. This same succession of dominant organisms, never out of order, is found on every continent.
Index Fossils and Fossil Assemblages When fossils were found to be time indicators, they became the most useful means of correlating rocks of similar age in different regions. Geologists pay particular attention to certain fossils called index fossils (Figure 2).
These fossils are widespread geographically but limited to a short span of geologic time, so their presence provides an important method of matching rocks of the same age. Rock formations, however, do not always contain a specific index fossil. In such situations, a group of fossils, called a fossil assemblage, is used to establish the age of the bed. Figure 3 illustrates how an assemblage of fossils may be used to date rocks more precisely than could be accomplished by the use of any single fossil.
In addition to being important, and often essential, tools for correlation, fossils are important environmental indicators. Although we can deduce much about past environments by studying the nature and characteristics of sedimentary rocks, a close examination of the fossils present can usually provide a great deal more information.
For example, when the remains of certain clam shells are found in limestone, a geologist quite reasonably assumes that the region was once covered by a shallow sea. Also, by using what we know of living organisms, we can conclude that fossil animals with thick shells, capable of withstanding pounding and surging waves, inhabited shorelines.
On the other hand, animals with thin, delicate shells probably indicate deep, calm offshore waters. Hence, by looking closely at the types of fossils, the approximate position of an ancient shoreline may be identified. Fossils also can be used to indicate the former temperature of the water. Certain kinds of present-day corals must live in warm and shallow tropical seas like those around Florida and The Bahamas. When similar types of coral are found in ancient limestones, they indicate the marine environment that must have existed when they were alive. These examples illustrate how fossils can help unravel the complex story of Earth history.
Determining Numerical Dates for Sedimentary Strata
Explain how reliable numerical dates are determined for layers of sedimentary rock.
Although reasonably accurate numerical dates have been worked out for the periods of the geologic time scale, this is not an easy task. The primary difficulty in assigning numerical dates to units of time is the fact that not all rocks can be dated by using radiometric methods. For a radiometric date to be useful, all the minerals in the rock must have formed at about the same time. For this reason, radioactive isotopes can be used to determine when minerals in an igneous rock crystallized and when pressure and heat created new minerals in a metamorphic rock.
However, samples of sedimentary rock can only rarely be dated directly by radiometric means. Although a detrital sedimentary rock may include particles that contain radioactive isotopes, the rock’s age cannot be accurately determined because the grains composing the rock are not the same age as the rock in which they occur.
Rather, the sediments have been weathered from rocks of diverse ages.
Radiometric dates obtained from metamorphic rocks may also be difficult to interpret because the age of a particular mineral in a metamorphic rock does not necessarily represent the time when the rock initially formed.
Instead, the date might indicate any one of a number of subsequent metamorphic phases.
If samples of sedimentary rocks rarely yield reliable radiometric ages, how can numerical dates be assigned to sedimentary layers? Usually a geologist must relate the strata to datable igneous masses, as in Figure 4.
In this example, radiometric dating has determined the ages of the volcanic ash bed in the Morrison Formation and the dike cutting the Mancos Shale and Mesaverde Formation. The sedimentary beds below the ash are obviously older than the ash, and all the layers above the ash are younger. The dike is younger than the Mancos Shale and the Mesaverde Formation but older than Wasatch Formation because the dike does not intrude these Paleogene age rocks.
From this kind of evidence, geologists estimate that the last part of the Morrison Formation was deposited about 160 million years ago, as indicated by the ash bed. Further, they conclude that the Paleogene period began after the intrusion of the dike, 66 million years ago. This is just one example of literally thousands that illustrate how datable materials are used to bracket the various episodes in Earth history within specific time periods. It shows the necessity of combining laboratory dating methods with relative dating principles applied to field observations of rocks.
The Geologic Time Scale
Distinguish among the four basic time units that make up the geologic time scale and explain why the time scale is considered to be a dynamic tool. Geologists have divided the whole of geologic history into units of varying length. Together, they compose the geologic time scale of Earth history (Figure 5). The major units of the time scale were delineated during the nineteenth century, principally by scientists in Western Europe and Great Britain. Because radiometric dating was unavailable at that time, the entire time scale was created using methods of relative dating. It was only in the twentieth century that radiometric methods permitted numerical dates to be added.
Structure of the Time Scale
The geologic time scale subdivides the 4.6-billionyear history of Earth into many different units and provides a meaningful time frame within which the events of the geologic past are arranged. As shown in Figure 5, eons represent the greatest expanses of time. The eon that began about 542 million years ago is the Phanerozoic, a term derived from Greek words meaning “visible life.” It is an appropriate description because the rocks and deposits of the Phanerozoic eon contain abundant fossils that document major evolutionary trends.
Another glance at the time scale reveals that eons are divided into eras. The Phanerozoic eon consists of the Paleozoic era (paleo = ancient, zoe = life), the Mesozoic era (meso = middle, zoe = life), and the Cenozoic era (ceno = recent, zoe = life). As the names imply, these eras are bounded by profound worldwide changes in life-forms.*
Each era of the Phanerozoic eon is further divided into time units known as periods. The Paleozoic has seven, and the Mesozoic and Cenozoic each have three. Each of these periods is characterized by a somewhat less profound change in life-forms as compared with the eras.
Each of the periods is divided into still smaller units called epochs. As you can see in Figure 5, seven epochs have been named for the periods of the Cenozoic. The epochs of other periods usually are simply termed early, middle, and late.