Chapter 19: Time and Geology
•Geoscientists (that work in fields or with maps and illustrations in a laboratory) are
usually concerned with relative time (geologic processes are long), and they try to figure
out the sequence in which geologic events have occurred in the past.
oA geoscientist may be able to characterize how old a rock is relative to another
set of rocks (i.e: rocks near the surface (rock A) is younger than the rocks
underlying it (rock B), which is younger than the rock underlying that (rock C)
oBut this doesn’t tell us the EXACT age of the rock, which can be determined
using radioactive isotope dating.
oIsotopic dating has shown the North American craton to be between 4 billion
years and 1 billion years old.
Main Focus of chapter 19
•How can we apply some basic principles to figure out the sequence in which geologic
events have taken place?
•It is not only important to know how old rocks are relative to one another, but to figure
out the actual age of the rock. This helps us compare rocks that are geologically
separated. By doing so, we can understand the history of a region, the continent and the
•The plate tectonics theory depends on intercontinental correlation of rock units and
•Fossil use has helped us identify certain strata that recurred in the same order. (Note:
Strata is defined as a thin layer of rock)
oThis helped geologists identify stratigraphic successions and eventually allows
the creation of the standard geologic time scale.
•Prior to the 1800’s the earth was thought to have created in merely 6,000 years. It was
thought that the features we observe in rocks and landscapes were created
supernaturally and the formation of the earth was placed in biblical chronology. Some
marine fossils that we find on mountains were believed to have gotten there through a
large flood that affected the entire world, and reached to the top of mountains (Noah’s
•However, James Hutton (father of modern geology) argued that the geological features
in the past can be explained using present-day processes. Thus, he concluded that the