5: Geologic Time
- Page ID
- 20336
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Introduction to Geologic Time
At the heart of geology is the story of Earth’s past. Part of telling a good story is placing events in a sequential order. Geologists have almost always focused on reconstructing the order of geologic events based on relationships in the rocks and between rock units. This approach, when combined with a study of the relative relationships of units with fossil sequences, is the basis for the geologic time scale. This time scale can be considered like a calendar in which the largest subdivisions of time (Eons) are subdivided into Eras, which in turn are subdivided into Periods and Epochs. Epochs are then subdivided into early, late and middle sections and approximate ages in Ma are labeled. With the discovery of radioactivity around the turn of the 20th century, the relative order of events could be placed in a specific timeline, and rates of processes could be quantified. The determination of numerical (or absolute) ages for these minerals and the rocks in which they were found added an additional level of detail to our ability to reconstruct the geologic history of different terranes and the earth as a whole.
Before starting, consider the units of geologic time. Geologists measure events in Earth’s history in years before the present date. They use certain conventions for abbreviating intervals of time. One thousand years is represented by the abbreviation "ka," which means "kilo-annum." The prefix "kilo-" means "thousand". A kilogram equals one thousand grams and a kilometer equals one thousand meters. "Annum" means "year." Similarly, one million years is represented by "Ma," meaning "mega annum." One billion years is abbreviated as "Ga" for "giga annum." As an example: dinosaur species went extinct at approximately 66 Ma. The process of assigning actual ages – in years before the present date – is referred to as numerical (absolute) dating.
By the end of this chapter, you should be able to:
- Explain the difference between relative and absolute (numeric) dating.
- Describe major relative time Principles of Superposition, Original Horizontality, Lateral continuity, Cross-cutting Relations, Included Fragments, and Faunal Succession.
- Compare and contrast three major Unconformities (Angular Unconformities, Disconformities, and Nonconformities).
- Describe how radioisotopic dating is accomplished and the commonly used isotopic systems.
- List the Eons, Eras, and Periods of the geologic time scale and explain the basis behind the divisions.
- 5.1: Relative Dating
- Several important principles are used by geologists to place geologic features in a relative sequence that can be used to construct relative timing of events. This section discusses these principles.
- 5.2: Unconformities
- Geologists know that there are periods of erosion and non-deposition that also equate to events that take time and must be recognized. These episodes don’t leave rock units behind, but they do leave distinctive relationships called unconformities. This section discusses these concepts.
- 5.3: Dating Rocks Using Fossils
- Fossils are important indicators of periods of geologic time. When present in rocks, they provide an opportunity to correlate units that are not in direction connection with one another. This section presents a review of the significance of fossils in the determination of relative geologic time.
- 5.4: Absolute Dating of Geologic Materials
- The discovery of radioactivity opened the door for geologists to apply it's concepts to the determination of absolute or numerical ages of geologic materials. This section discusses this approach and some of the tools that are used.
- 5.5: The Geologic Time Scale
- The geologic time scale is a composite representation of all of Earth's history. It combines information gleaned from relative dating studies, fossil histories and absolute dating into one framework. This section presents the major elements of this time scale.
- 5.6: Chapter Summary
- This page presents a concise summary of the highlights of each section in this chapter on geologic time.
- 5.7: Detailed Figure Descriptions
- Descriptions of complex images within this chapter, for users who have difficulty seeing and interpreting images.