Module 3 Part 3 - The Geologic Time Scale and Earth History
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Types of dating:

• Relative dating: relative dating is used to tell that this group of rocks is older than that one but does not give a date for the age of the rocks. The various types of relative dating were used to construct the geologic time scale.
• Absolute dating: absolute dating gives an age for a rock or group of rocks.

Types of relative dating: Early geologists found ways to relatively date rocks. Some methods are useful to relatively date rocks at a particular locality while others may be used worldwide.

• Superposition is the principle that sedimentary beds deposited over lower are younger than the lower beds. In other words, beds at the bottom of a sequence of sedimentary rocks are older than those above them.
• Cross-cutting relationships occur when one feature cuts across another. When this happens the “cutter” has to be younger than the “cuttee”. This basic principle is used to date things or events in everything from microscopic studies to dating impact craters on the Moon. Things that can cut across other things include:
  • Igneous dikes or intrusions
  • Faults
  • Unconformities, which are erosional or non-depositional surfaces in sedimentary strata.
• Inclusions: good for both igneous and sedimentary rocks. An inclusion is a piece of a rock included in another rock. A piece of sandstone that was broken off country rock and fell into an igneous dike or sill must be older than that dike or sill. Similarly, a piece of underlying limestone that was broken off during erosion and included in an overlying sandstone must be older than the rock that contains it (the sandstone).
• Fossils: As geology developed in the 18th and 19th centuries, it became clear to early geologists such as William “Strata” Smith and Georges Cuvier that there are sequences of fossils that are unique to their rocks. It became clear, for instance, that a certain family of clams lasted from such a time (interval of rocks) to another and is never seen again. Today we know this is because once an organism becomes extinct, it never comes back. So, one can relatively date sedimentary rocks by the fossils they contain.
  
  This principle became known as fossil succession and was used by geologists on a world-wide scale to date rocks. Because fossil animals and plants, like organisms today, could occur in appropriate climatic zones world wide, geologists on one continent could relate their rocks to those on another. This was the great power of the Geological Time Scale, where the Earth’s rocks are divided into periods based on their fossil contents.

Absolute dating: Over the years, many ideas have been tried to put an exact age on rocks. However, it was not until the discovery of radioactivity in the late 19th century that a reliable method was found to date rocks. Recall that radioactivity is the spontaneous emission of particles and energy of different types from atoms due to unstable atomic configurations.

• The speed or rate of radioactive decay for an isotope is measured by its half-life, the amount of time necessary for one half of the radioactive atoms (parent isotope) to decay to their decay product atoms (daughter isotope).
• The rates of decay of radioactive isotopes are known from laboratory testing.
• The original radioactive parent atoms are included in a mineral’s lattice structure during the formation of the mineral crystal.
• Over time, a certain number of the atoms will decay to the daughter atoms.
• By measuring the ratio of daughter to parent atoms in the mineral crystal, and using the known rates of decay, an estimate can be made of the time elapsed since the crystal was formed.
• Where radioactive elements are not present in a rock sequence (as for many sedimentary rocks), a rock sequence may be dated by dating igneous rock bodies below and above the sedimentary series, obtaining a range of time for the formation of the sedimentary sequence.
• Thus, through radioisotope dating geologists have been able to put estimates of absolute ages for the different periods of the Geologic Time Scale that were originally subdivided on the basis of fossil assemblages. Figure 24: The Geologic Time Scale - BR.

Part 3 - The Geologic Time Scale and Earth History
Module 3 Home | Page 1 | Page 2 | Web Sites and References

Part 1 - Metamorphic Rocks | Part 2 - Structural Geology & Mtns  | Part 3 - Geologic Time Scale