The topics covered in this chapter can be summarized as follows:
19.1 The Geological Time Scale
The work of William Smith was critical to the establishment of the first geological timescale early in the 19th century, but it wasn’t until the 20th century that geologists were able to assign reliable dates to the various time periods. The geological timescale is now maintained by the International Commission on Stratigraphy. Geological time is divided into eons, eras, periods, and epochs.
19.2 Relative Dating Methods
We can determine the relative ages of different rocks by observing and interpreting relationships among them, such as superposition, cross-cutting, and inclusions. Gaps in the geological record are represented by various types of unconformities.
19.3 Dating Rocks Using Fossils
Fossils are useful for dating rocks back to ~600 Ma. If we know the age range of a fossil, we can date the rock in which it is found, but some organisms lived for many millions of years. Index fossils represent shorter geological time spans, and if a rock has several different fossils with known age ranges, we can narrow the time during which the rock formed.
19.4 Isotopic Dating Methods
Radioactive isotopes decay at constant known rates, and can be used to date igneous and metamorphic rocks. Some commonly used isotope systems are potassium-argon, rubidium-strontium, uranium-lead, and carbon-nitrogen. Radiocarbon dating can be applied to sediments and sedimentary rocks, but only if they are younger than 60 ka, and contain organic material, or minerals of calcium carbonate.
19.5 Other Dating Methods
There are many other methods for dating geological materials. Two that are widely used are dendrochronology and magnetic chronology. Dendrochronology, based on studies of tree rings, is widely applied to dating glacial events. Magnetic chronology is based on the known record of Earth’s magnetic field reversals.
19.6 Understanding Geological Time
While understanding geological time is relatively easy, actually comprehending the significance of the vast amounts of geological time is a great challenge. To be able to solve important geological problems and certain societal challenges, we need to really appreciate the vastness of geological time.
- A granitic rock contains inclusions (xenoliths) of basalt. What can you say about the relative ages of the granite and the basalt?
- Explain the differences between a) disconformity and paraconformity; and b) nonconformity and angular unconformity
- What are the features of a useful index fossil?
- Figure 19.29 shows a geological cross-section. The granitic rock F at the bottom is the rock that you estimated the age of in the exercise in 19.4, Radiometric Dating with Potassium-Argon. A piece of wood from layer D has been sent for radiocarbon dating and the result was 0.55 14C remaining. How old is layer D?
- Based on your answer to question 4, what can you say about the age of layer C in the figure above?
- What type of unconformity exists between layer C and rock F?
- What type of unconformity exists between layer C and rock B?
- We cannot use magnetic chronology to date anything older than ~780,000 years. Why?
- How did William Smith apply the principle of faunal succession to determine the relative ages of the sedimentary rocks of England and Wales?
- Access a copy of the geological time scale at http://www.stratigraphy.org/index.php/ics-chart-timescale. What are the names of the last age of the Cretaceous and the first age of the Paleogene? Print out the time scale and stick it on the wall above your desk!