Physical Geology - Geologic Time
Physical Geology
The Sill (dark band with baked upper and lower contacts) was injected into the limestone country rock. Therefore, the sill is younger than the limestone since the limestone had to be present before the sill was injected.
Preface
You may wish to print a copy of the lecture outline (minus the illustrations) and you have two options:
Chapter Objectives
After reading Chapter Eight, you should be able to:
- Explain the difference between absolute and relative dates.
- Define the principles that help determine the relative ages of rock units.
- List and explain the types of unconformities.
- Describe the methods used to correlate rock units in different regions.
- Explain radioactivity and how it is used to find the absolute ages of rocks.
- Describe the geologic time scale.
Introduction
History records numerous attempts to determine the age of the Earth. Archbishop Usher of Ireland in 1664 argued that the Earth was "born" on October 26, 4004 BC at 9:00 AM in the morning. Lord Kelvin (1866) assumed that the Earth was originally molten and that it had taken from 100 million to 20 million years for the Earth to cool to its present temperature distribution. Kelvin's arguments struck a blow at the prevalent geological theories of the time that argued for a much older Earth. Kelvin assumed that there was no source of "new" heat within the Earth. With the discovery of radioactive isotopes at the end of the previous century Kelvin's hypothesis was abandoned as it was shown that the decay of unstable radioactive isotopes as accompanied by the release of heat energy. Today, most geoscientists believe that the Earth formed about 4.6 billion years ago. It is important to note that the oldest continental crust is approximately 4.0 b.y. and, in general, the volume of rock varies inversely with the age of the rock. That is, with increasing age, the amount of rock of that age decreases. This is supported by the fact that the Earth is a dynamic planet and that the processes introduced in the section on Plate Tectonics have resulted in a recycling of crustal rocks.
Internet Resources
- Geologic Time Scale
"The history of the Earth is broken up into a hierarchical set of divisions for describing geologic
time. As increasingly smaller units of time, the generally accepted divisions are Eon, Era,
Period, Epoch, and Age. In the time scale shown below, two levels of this hierarchy are
represented."
- Spend some time viewing the various animations of plate movement as a function of geologic time
- Interpreting Geologic Sections
Geological sections and maps are pictures of what rocks do. Geologic maps show the pattern of rocks on the surface of the
earth. Geologic sections show the patterns of rocks as they might be exposed on the side of a road cut or on the wall of a
trench. One of the things that geologic sections show very well is the geological history of an area. Try these exercises!
A Virtual Field Trip to the Calvert Cliffs
The Calvert Cliffs "on the western shore of the
Chesapeake Bay in Calvert County are justly famous as a
fossil collecting area. The fossiliferous deposits belong to
the Chesapeake Group of Miocene age geological strata
in the Atlantic Coastal Plain region. These deposits are
exposed in cliffs up to 100 feet high between Chesapeake
Beach and Drum Point and constitute the most complete
section of Miocene deposits in the eastern United States."
Learning about geology from a computer screen is only half as fun as enjoying it in the field!
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Copyright by John C. Butler, July 29, 1995