Learning Objectives

Each statement represents the primary learning objective for the corresponding major heading within the chapter. After you complete the chapter, you should be able to:

8.1 Explain the principle of uniformitarianism and discuss how it differs from catastrophism.

8.2 Distinguish between numerical and relative dating and apply relative dating principles to determine a time sequence of geologic events.

8.3 Define fossil and discuss the conditions that favor the preservation of organisms as fossils. List and describe various fossil types.

8.4 Explain how rocks of similar age that are in different places can be matched up.

8.5 Discuss three types of radioactive decay and explain how radioactive isotopes are used to determine numerical dates.

8.6 Explain how reliable numerical dates are determined for layers of sedimentary rock.

8.7 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.

A Few Fossil Resources

Amazing fossils found by ordinary people thrill scientists

Fossils in Big Bend National Park

La Brea Tar Pits

Rainbow Basin Natural Area

Concept Checks 

8.1 Contrast catastrophism and uniformitarianism. How did each view the age of Earth?

8.2 Distinguish between numerical dates and relative dates. Sketch and label four simple diagrams that illustrate each of the following: superposition, original horizontality, lateral continuity, and cross-cutting relationships. What is the significance of an unconformity? Distinguish between angular unconformity and disconformity.

8.3 Describe several ways that an animal or a plant can be preserved as a fossil. List three examples of trace fossils. What conditions favor the preservation of an organism as a fossil?

8.4 What is the goal of correlation? State the principle of fossil succession in your own words. In addition to being important time indicators, how else are fossils useful to geologists?

8.5 List three types of radioactive decay. For each type, describe how the atomic number and the atomic mass change. Sketch a simple diagram that explains the idea of half-life. For what time span does radiocarbon dating apply?

8.6 Briefly explain why it is often difficult to assign a reliable numerical date to a sample of sedimentary rock. How might a numerical date for a layer of sedimentary rock be determined?

8.7 What are the four basic units that make up the geologic time scale? List the specific ones that apply to the present day. Why is zoic part of period names on the geologic time scale?. What term applies to all geologic time prior to the Phanerozoic eon? Why is this span not divided into as many smaller units as the Phanerozoic eon?

Concepts in Review Geologic Time

8.1 A Brief History of Geology

Explain the principle of uniformitarianism and discuss how it differs from catastrophism.

Key Terms: catastrophism, uniformitarianism

*Early ideas about the nature of Earth were based on religious traditions and notions of great catastrophes.

*In the late 1700s, James Hutton emphasized that the same slow processes have acted over great spans of time and are responsible for Earth’s rocks, mountains, and landforms. This similarity of processes over vast spans of time led to this principle being called uniformitarianism.

8.2 Creating a Time Scale—Relative Dating Principles

Distinguish between numerical and relative dating and apply relative dating principles to determine a time sequence of geologic events.

Key Terms: numerical date, relative date, principle of superposition, principle of original horizontality, principle of lateral continuity, principle of cross-cutting relationships, inclusion, conformable, unconformity, angular unconformity, disconformity, nonconformity

*The two types of dates that geologists use to interpret Earth history are (1) relative dates, which put events in their proper sequence of formation, and (2) numerical dates, which pinpoint the time in years when an event took place.

*Relative dates can be established using the principles of superposition, original horizontality, lateral continuity, cross-cutting relationships, and inclusions. Unconformities, gaps in the geologic record, may be identified during the relative dating process.

QUESTION: The accompanying photo shows four features. Place the features in the proper sequence, from oldest to youngest. Explain your reasoning.

8.3 Fossils: Evidence of Past Life

Define fossil and discuss the conditions that favor the preservation of organisms as fossils. List and describe various fossil types.

Key Terms: fossil, paleontology

*Fossils are remains or traces of ancient life. Paleontology is the branch of science that studies fossils.

*Fossils can form through many processes. For an organism to be preserved as a fossil, it usually needs to be buried rapidly. Also, an organism’s hard parts are most likely to be preserved because soft tissue decomposes rapidly in most circumstances.

QUESTION: What term is used to describe the type of fossil that is shown here? Briefly describe how it formed.

8.4 Correlation of Rock Layers

Explain how rocks of similar age that are in different places can be matched up.

Key Terms: correlation, principle of fossil succession, index fossil, fossil assemblage

*Matching up exposures of rock that are the same age but are in different places is called correlation. By correlating rocks from around the world, geologists developed the geologic time scale and obtained a fuller perspective on Earth history.

*Fossils can be used to correlate sedimentary rocks in widely separated places by using the rocks’ distinctive fossil content and applying the principle of fossil succession. The principle states that fossil organisms succeed one another in a definite and determinable order, and, therefore, a time period can be recognized by examining its fossil content.

*Index fossils are particularly useful in correlation because they are widespread and associated with a relatively narrow time span. The overlapping ranges of fossils in an assemblage may be used to establish an age for a rock layer that contains multiple fossils.

*Fossils may be used to establish ancient environmental conditions that existed when sediment was deposited.

8.5 Numerical Dating with Nuclear Decay

Discuss three types of radioactive decay and explain how radioactive isotopes are used to determine numerical dates.

Key Terms: nuclear (radioactive) decay, radiometric dating, half-life, radiocarbon dating

*Radioactivity is the spontaneous decay of certain unstable atomic nuclei. Three common forms of nuclear decay are (1) emission of an alpha particle from the nucleus, (2) emission of a beta particle (electron) from the nucleus, and (3) capture of an electron by the nucleus.

*An unstable radioactive isotope, called a parent, will decay and form daughter products. The length of time for one-half of the nuclei of a radioactive isotope to decay is called the half-life of the isotope. If the half-life of an isotope is known and the parent/daughter ratio can be measured, the age of the sample can be calculated.

8.6 Determining Numerical Dates for Sedimentary Strata

Explain how reliable numerical dates are determined for layers of sedimentary rock.

*Sedimentary strata are usually not directly datable using radiometric techniques because they consist of the material produced by the weathering of other rocks. A particle in a sedimentary rock comes from some older source rock. If you were to date the particle using isotopes, you would get the age of the source rock, not the age of the sedimentary deposit.

*One way geologists assign numerical dates to sedimentary rocks is to use relative dating principles to relate them to datable igneous masses, such as dikes, lava flows, and volcanic ash beds. A layer may be older than one igneous feature and younger than another.

QUESTION: Determine the age of the sandstone layer in the illustration as accurately as possible.

8.7 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.

Key Terms: geologic time scale, eon, Phanerozoic eon, era, Paleozoic era, Mesozoic era, Cenozoic era, period, epoch

*PrecambrianEarth history is divided into units of time on the geologic time scale. Eons are divided into eras, which each contain multiple periods. Periods are divided into epochs.

*Precambrian time includes the Archean and Proterozoic eons. It is followed by the Phanerozoic eon, which is well documented by abundant fossil evidence, resulting in many subdivisions.

*The geologic time scale is a work in progress, continually being refined as new information becomes available.

QUESTION: Is the Mesozoic an example of an eon, an era, a period, or an epoch? What about the Jurassic?