Chapter Overview

Learning Objectives

1. Uniformitarianism (actualism) implies that geologic processes operating today also operated in the past;"the present is the key to the past." Since rates of deposition and other activities are slow, the expanse of geologic time was necessarily broadened by application of uniformitarianism, but the term doesn't imply rates were uniform.

2. Absolute time provides a date in years or some other time unit to a rock, while relative time merely arranges events in a sequence.

3. Geologists think of the geology of an area in terms of the sequence of events that form its history. Four basic principles are applied to recognize the various steps in the geologic history of an area. Original horizontality implies that the rocks were horizontal when first formed, and any change from horizontal took place after deposition. Superposition implies that rock sequences get younger toward their tops. Lateral continuity states that sedimentary layers extend laterally until their edges pass into another environment reflected by different sediment types. Cross-cutting relationships imply that a disrupted rock unit is older than the cause of its disruption. Figure 8.1-8.11 and Table 8.1 apply these principles to understanding the sequence of events that developed Minor Canyon, a fictitious location similar to Grand Canyon.

4. Correlation establishes age relationships between rock units or events in separate areas. Physical continuity implies that a rock unit can be traced from one area to another. Similarity of rock types allows correlation, particularly if the character and sequence are distinctive. Correlation by fossils utilizes the observation of faunal succession: fossil species occur in a definite and recognizable succession through time. Index fossils are short-lived and widespread, but most correlations are accomplished by fossil assemblages.

5. The standard geologic time scale reflects relative time and is based on fossil assemblages. Eras are the largest divisions, followed by periods, and then epochs. Fossils become common with the beginning of the Paleozoic Era, and rocks that precede that era are called Precambrian. The Mesozoic Era succeeds the Paleozoic Era, followed by the Cenozoic Era. Those era boundaries are times of mass extinction. The Cenozoic Era includes the Holocene Epoch in which we are now living. Most geological investigations involve the use of relative time.

6. Unconformities are gaps in the geologic record developed as buried surfaces of erosion. A disconformity separates beds that parallel one another. Fossils may indicate a break in the record. Angular unconformities separate tilted older strata from horizontal younger strata. Fossils and cross-cutting relationships may be used to determine the relative time of the folding and tilting. Nonconformities separate older plutonic or metamorphic rocks from younger sedimentary rocks.

7. Absolute time provides ages, usually in years, to geological features and events. The earth is estimated to be 4.5-4.6 billion years old. The oldest rocks on earth, 4.03 billion years old, are found in northwestern Canada. The oldest dated mineral is a zircon from Australia, which is 4.4 billion years old.

8. Absolute dating is based on the decay of radioactive isotopes, particularly uranium. Decay is expressed in half-lifes, the time it takes for one-half of a given amount of radioactive isotope to be reduced to its stable daughter product. Radioactivity involves emission of alpha and beta particles and electron capture that may change the atomic number or atomic mass number of the atom. In contrast, radiocarbon dating involves the formation and breakdown of C-14. Dates are determined by comparing the amount of C-14 present to what would be expected in a living organism. The half-life is only about 5,730 years and the system only provides dates on organic materials for the last 40,000 years with accuracy.

9. Dating is based on a comparison of the amount of isotope originally present compared to the amount present at the time of the analysis and the half-life of the isotope involved. Usually the date provided by the analysis is the time that the rock or mineral became a closed system. In igneous rocks, that would date mineral crystallization, while in metamorphic rocks, it would be time of metamorphism. Absolute dates have been assigned to the geologic time scale by bracketing events whose relative time is known. It has been used to subdivide the Precambrian, which comprises the bulk of geologic time. (N. B. the Precambrian-Cambrian boundary has been assigned a date of 551 billion years on the geologic time scale illustrated in Figure 8.26, but it is stated to be 545 million years on p. 189 and p.194 in the 9th edition).

10. The age of the earth has been a controversial subject and several attempts were made to determine its antiquity prior to the discovery of radioactivity (biblical, rate of cooling). The age of the earth has been established as 4.5 - 4.6 billion years old based on isotopic dating of meteorites.

11. Geologic time is vast, mostly represented by the Precambrian, and human history represents an exceedingly small portion of that time.

Related Readings

Cowen, R. 1994. History of Life. 2nd ed. Cambridge, MA: Blackwell Scientific Publications.

Dalrymple. G. B. 1991. The Age of the Earth. Palo Alto, CA: Stanford University Press.

Faure, G. 1986. Principles of Isotope Geology. 2nd ed. New York: John Wiley and Sons.

Geyh, M. A., and H. Schleicher. 1990. Absolute Age Determination. New York: Springer-Verlag.

Gould, S. J. 1988. Time's Arrow/Time's Cycle. Boston: Harvard University Press.

Raup, D. M. 1991. Extinction: Bad Genes or Bad Luck? New York: W. W. Norton and Company.

Answers to EOC Questions

Following are answers to the End of Chapter Questions for Chapter 8:

Boxed Readings