3: Geologic Time
- Page ID
- 20336
Introduction
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-” should be familiar to you: 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, which is covered in a later section.
- 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.
- 3.1: Relative Dating
- Review of important geologic principles used in the determination of relative ages of geologic features.
- 3.2: Unconformities-Missing Time
- The concepts described in the previous section apply when rocks and fossils are present. But what about when they aren’t? 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, which are described here.
- 3.3: Dating Rocks Using Fossils
- Review of the significance of fossils in the determination of relative geologic time.
- 3.4: Absolute Dating of Geologic Materials
- Review of the tools used to determine an absolute or numerical age for geologic materials.
- 3.5: The Geologic Time Scale
- An expanded recap of the structure and components of the geologic time scale.
- 3.6: Chapter Summary and Key Terms
- A concise summary of the highlights of each section in this chapter on geologic time.
- 3.7: Detailed Figure Descriptions
- A listing of relevant vocabulary encountered in this chapter, along with definitions.