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Geosciences LibreTexts

7: Geologic Time

  • Page ID
    6887
  • Learning Objectives

    • Explain the difference between relative time and numeric time
    • Describe the five principles of stratigraphy
    • Apply relative dating principles to a block diagram and interpret the sequence of geologic events
    • Define an isotope, and explain alpha decay, beta decay, and electron capture as mechanisms of radioactive decay
    • Describe how radioisotopic dating is accomplished and list the four key isotopes used
    • Explain how carbon-14 forms in the atmosphere and how it is used in dating recent events
    • Explain how scientists know the numeric age of the Earth and other events in Earth history
    • Explain how sedimentary sequences can be dated using radioisotopes and other techniques
    • Define a fossil and describe types of fossils preservation
    • Outline how natural selection takes place as a mechanism of evolution
    • Describe stratigraphic correlation
    • List the eons, eras, and periods of the geologic time scale and explain the purpose behind the divisions
    • Explain the relationship between time units and corresponding rock units—chronostratigraphy versus lithostratigraphy

    The geologic time scale and basic outline of Earth’s history were worked out long before we had any scientific means of assigning numerical age units, like years, to events of Earth history. Working out Earth’s history depended on realizing some key principles of relative time. Nicolaus Steno (1638-1686) introduced basic principles of stratigraphy, the study of layered rocks, in 1669 [1]. William Smith (1769-1839), working with the strata of English coal mines, noticed that strata and their sequence were consistent throughout the region. Eventually, he produced the first national geologic map of Britain [2], becoming known as “the Father of English Geology.” Nineteenth-century scientists developed a relative time scale using Steno’s principles, with names derived from the characteristics of the rocks in those areas. The figure of this geologic time scale shows the names of the units and subunits. Using this time scale, geologists can place all events of Earth history in order without ever knowing their numerical ages. The specific events within Earth history are discussed in Chapter 8.

    It shows a man
    Figure \(\PageIndex{1}\): Nicolas Steno, c. 1670

    • 7.1: Relative Dating
      Relative dating is the process of determining if one rock or geologic event is older or younger than another, without knowing their specific ages—i.e., how many years ago the object was formed. The principles of relative time are simple, even obvious now, but were not generally accepted by scholars until the scientific revolution of the 17th and 18th centuries.
    • 7.2: Absolute Dating
      Relative time allows scientists to tell the story of Earth events, but does not provide specific numeric ages, and thus, the rate at which geologic processes operate. Based on Hutton’s principle of uniformitarianism, early geologists surmised geological processes work slowly and the Earth is very old. Relative dating principles was how scientists interpreted Earth history until the end of the 19th Century.
    • 7.3: Fossils and Evolution
      Fossils are any evidence of past life preserved in rocks. They may be actual remains of body parts (rare), impressions of soft body parts, casts and molds of body parts (more common), body parts replaced by mineral (common) or evidence of animal behavior such as footprints and burrows. The body parts of living organisms range from the hard bones and shells of animals, soft cellulose of plants, soft bodies of jellyfish, down to single cells of bacteria and algae.
    • 7.4: Correlation
      Correlation is the process of establishing which sedimentary strata are of the same age but geographically separated. Correlation can be determined by using magnetic polarity reversals (Chapter 2), rock types, unique rock sequences, or index fossils. There are four main types of correlation: stratigraphic, lithostratigraphic, chronostratigraphic, and biostratigraphic.

    Thumbnail: Perhaps no place on Earth better exemplifies the principles geologists use to determine the ages of rocks than Arizona’s Grand Canyon National Park.

    Summary

    Events in Earth history can be placed in sequence using the five principles of relative dating. The geologic time scale was completely worked out in the 19th Century using these principles without knowing any actual numeric ages for the events. The discovery of radioactivity in the late 1800s enabled absolute dating, the assignment of numerical ages to events in the Earth’s history, using decay of unstable radioactive isotopes. Accurately interpreting radioisotopic dating data depends on the type of rock tested and accurate assumptions about isotope baseline values. With a combination of relative and absolute dating, the history of geological events, age of Earth, and a geologic time scale have been determined with considerable accuracy. Stratigraphic correlation is an additional tool used for understanding how depositional environments change geographically. Geologic time is vast, providing plenty of time for the evolution of various lifeforms, and some of these have become preserved as fossils that can be used for biostratigraphic correlation. The geologic time scale is continuous, although the rock record may be broken because rocks representing certain time periods may be missing.