5: Geologic Time
- Page ID
- 20336
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\( \newcommand{\dsum}{\displaystyle\sum\limits} \)
\( \newcommand{\dint}{\displaystyle\int\limits} \)
\( \newcommand{\dlim}{\displaystyle\lim\limits} \)
\( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)
( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\id}{\mathrm{id}}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\kernel}{\mathrm{null}\,}\)
\( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\)
\( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\)
\( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)
\( \newcommand{\vectorA}[1]{\vec{#1}} % arrow\)
\( \newcommand{\vectorAt}[1]{\vec{\text{#1}}} % arrow\)
\( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vectorC}[1]{\textbf{#1}} \)
\( \newcommand{\vectorD}[1]{\overrightarrow{#1}} \)
\( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}} \)
\( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)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
- This page covers the principles of relative dating in geology, detailing methods to determine the chronological order of rock layers without precise ages. Key concepts include superposition, original horizontality, and lateral continuity, among others, which aid in interpreting geological history. Additionally, the page lists educational resources on geology, covering topics like geological features and rock ages, to enhance foundational knowledge for students and enthusiasts.
- 5.2: Unconformities
- This page explains the three types of geological unconformities: nonconformity, angular unconformity, and disconformity, with examples from the Grand Canyon's stratigraphy. It describes how different rock layers relate, emphasizing that older rocks are overlain by younger sedimentary formations. The page notes the significance of these unconformities in revealing gaps in the geological record and highlights principles such as Superposition and Lateral Continuity in understanding Earth's history.
- 5.3: Dating Rocks Using Fossils
- This page explains the role of fossils in dating rocks and understanding Earth's historical conditions, emphasizing the Principle of Faunal Succession for establishing relative ages. It highlights the significance of index fossils, which aid in dating due to their brief existence. The discussion includes Paleozoic fossils, especially Smilodon, and notes how specific fossil features can help determine their ages within geological timelines.
- 5.4: Absolute Dating of Geologic Materials
- This page covers numerical dating in geology, contrasting it with relative dating and detailing radiometric methods, particularly geochronology, which uses radioactive decay and half-lives for age determination. Key decay types like alpha and beta are explained, alongside decay chains involving isotopes such as uranium-238 and potassium-40.
- 5.5: The Geologic Time Scale
- This page details the geological time scale, which categorizes Earth's 4.6 billion-year history into Eons, Eras, Periods, and Epochs. The International Commission on Stratigraphy maintains this classification, revealing significant evolutionary events, particularly at mass extinction boundaries.
- 5.6: Chapter Summary
- A summary of the Geologic Time chapter.