Chapter 8: Geologic Time

Fig. 8.13

OBJECTIVES

• Define, compare, and contrast relative and absolute age dating.

• Describe the development of ideas that led to the modern view of geologic time.

• List and explain the rules that geologists apply to determine the relative age of geologic events.

• Explain how correlation allows individual rock layers to be traced from one place to another.

• Describe how relative dating methods allowed the geologic time scale to be established.

OBJECTIVES• Summarize radioactivity and its use in determining the absolute

age of geologic events.

• Explain how absolute dating has established numerical ages for the geologic time scale.

• Explain how relative and absolute dating together form the basis of our modern understanding of Earth history.

• Discuss how the vast span of geologic time has allowed plate tectonics to profoundly influence Earth history.

• Most of the processes that shape Earth operate extremely slowly – in vast geologic time.

• Geologists use two main methods of measuring geologic time.• Relative age• Absolute age

• These methods have been used to establish a geologic time scale.

Geologic Time: An Overview

Fig. 8.9b

• Relative Age• Age based on order in which geologic events occurred

• Absolute Age• The amount of time elapsed since the rock formed• May be based on measurement of radioactive decay

• Final determination of the sequence of a particular set of geologic events is usually found by combining clues from the outcrop that reveal relative age, along with evidence from laboratory measurements of absolute age.

Measuring Geologic Time: Relative and Absolute Ages

• Major (Incorrect) Hypotheses from Previous Generations • Principle of Catastrophism

• All geographic changes on Earth’s surface were attributed to supernatural disasters.

• Earth formed 6000 years ago.• Neptunists and Plutonists

• Neptunists believed that• A globe-engulfing sea formed Earth’s rocky layers• Earth must be ancient (much older than 6000 years)

• Plutonists insisted that• Crystalline rocks had crystallized from a molten state• Earth is dynamic and changing

A Short History of Geologic Time

• The Principle of Gradualism• James Hutton: “father of modern

geology” • Hutton observed weathering and

erosion and became convinced that Earth’s surface features were shaped by gradual processes rather than catastrophic flooding.

• Theory of the Earth, 1795

A Short History of Geologic Time

• Earth is powered by its own internal heat.

• Earth is in continuous but gradual change.• Earth is constantly decaying, renewing, and repairing itself.• Earth is truly ancient.

Hutton reasoned that if Hadrian’s Wall were really one-third as old as Earth itself, it should be as eroded as the mountains.

Fig. 8.4

(left) A woodcut of the Temple of Jupiter Serapis near Naples, Italy. Gradualists pointed to the holes bored by marine shells on the temple’s columns as proof of the gradual nature of geologic processes. (right) The temple in modern day.

• The Principle of Uniformitarianism • Charles Lyell, 1830• Geological processes have acted with the same intensity

from the earliest times to the present.

A Short History of Geologic Time

Fig. 8.5

• Rules to determine relative age• Superposition• Inclusions• Cross-cutting

relationships• Unconformity

Relative Dating: Determining Chronological Order

Fig. 8.15

Relative Dating: Determining Chronological Order

• Steno’s Rules (Superposition)1. Younger layers are deposited on top of older ones.2. Layers are flat-lying when they are deposited.

• Original horizontality3. Layers are deposited over wide areas.

• Original lateral continuity

Fig 8.6

Relative Dating: Determining Chronological Order

• Rule of Inclusion: Any rock must be younger than the rock from which the fragments were derived.

• Rule of Cross-Cutting Relationships: Any feature that cuts across a rock must be younger than the rock it cuts.

Figs. 8.7, 8.8

Relative Dating: Determining Chronological Order

• Unconformities• Erosion surfaces separating two sets of rock layers• Upper layers originally deposited horizontally on the lower ones• Three types: angular unconformity, disconformity, nonconformity

Fig. 8.10

Correlating Rock Strata• Principle of Faunal Succession

• The fossils present in any set of sedimentary rock layers succeed each other in a specific order that can be identified over large areas.

• Correlation• Individual rock layers can be traced from one place to another on

the basis of the fossils they contain.

• Stratigraphic Column• Column is the sequence of younger-upon-older rock strata.• The collection of fossils in any one layer shows the organisms

that lived during the time that layer was deposited.

Correlation: Correlation of sedimentary rock sequences and fossil assemblages above and below these four coal fields in the US Midwest indicates that the coal was deposited at or about the same time at all four sites.

Fig. 8.12

Relative dating can be used to reconstruct the geologic history of an area.

Relative Dating and the Geologic Time Scale

Fig. 8.14

Relative Dating and the Geologic Time Scale

• Subdivisions of the Geologic Time Scale• Many based on the emergence and extinction of fossil life forms• Eons: fundamental stages in the planet’s evolution

• Archean, Proterozoic, Phanerozoic, Precambrian• Eras: long intervals separated by major extinction events

• Paleozoic (“ancient life”), Mesozoic (“middle life”), Cenozoic(“recent life”)

• Periods• Shorter intervals of time subdividing eras• Examples: Devonian Period of the Paleozoic Era, Jurassic

Period of the Mesozoic Era• Epochs

• Index fossils subdivide periods

Geologic Time Scale: Numbers (millions of years) are determined by absolute dating and provide dates for the beginning of each time division.

Fig. 8.16

Absolute Dating: Finding a Geologic Clock• Radioactivity

• Unstable parent isotopes break down spontaneously to stable daughter isotopes

• Examples: U238 -Pb206; K40- Ar40; C14-N14

• Decay Rate: determined from the ratio of daughter isotope produced to remaining parent isotope

• Half-Life: length of time for a parent isotope to decrease to half its original quantity

Fig. 8.19

Different isotopes have different decay rates and are useful for rocks of different ages and mineralogies.

Commonly Used Isotope Systems

Resetting the Radiometric Clock

Fig. 8.20

• Radiometric ages of major boundaries of the stratigraphic column establish an absolute geologic time scale.

• Only certain types of rocks can be dated.

• Earth is currently thought to be 4,540,000,000 years old.• Paleozoic Era: began 542 million years ago• Mesozoic Era: began 252 million years ago• Cenozoic Era: began 66 years ago

Absolute Dating and the Geologic Time Scale

Dating Sedimentary Rocks: The age of sedimentary rocks that contain fossils is determined by dating associated volcanic lavas.

Fig. 8.21

• Knowing the absolute age of past geologic events can help determine the rates at which they occurred.

• Mass extinctions• Episodes of mountain building or volcanism• Patterns of climate change• Plate tectonics

• The geologic time scale is constantly modified with new and increasingly precise data.

• The vast scope of geologic time has allowed Earth’s slow-moving plates to bring about profound changes in geology, climate, and life.

Geologic Time and Plate Tectonics

SUMMARY• Relative dating involves determining the chronological order

of geologic events.

• Absolute dating involves determining the amount of time that has passed since events happened.

• The principle of catastrophism was replaced by the principles of gradualism and uniformitarianism in the 18th and 19th centuries.

• Relative dating involves examining rock formations in the field using the rules of superposition, inclusion, cross-cutting relationships, and unconformities.

SUMMARY• Rock strata can be correlated across large areas based on

fossil content.

• Absolute dating involves measuring radioactive isotopes in the laboratory.

• Geologic time is subdivided into eons, eras, periods, and epochs based on relative dating and the fossil record.

• The exact dates of events and boundaries in the geologic time scale are established with absolute dating.

• Radiometric dating confirms that Earth is ancient and explains how extremely slow processes can result in major changes to Earth’s surface.