Geology Reference
In-Depth Information
the rock reached the surface of the earth, or became exposed. Answering this question may provide
geologists with a way to document the age and sequence of past dramatic geologic events such as
earthquakes, faulting, and uplift (see Chapter 9).
The answer is found in cutting-edge research into intergalactic cosmic rays and isotopes called
cos-
mogenicnuclides.
Similar to radioactive isotopes, cosmogenic nuclides (or cosmogenic isotopes) de-
cay only when they have been exposed to cosmic rays. These “rays” are streams of high energy
particles that pass through earth's atmosphere and into the first 3 meters (10 feet) or so of the earth's
surface. (You probably haven't noticed, but they are passing through you right now!) As these particles
collide with atoms in the rocks of earth's crust, they knock other particles out of the way — creating
cosmogenic isotopes.
Unlike radioactive isotopes, which have a fixed number of parent atoms, cosmogenic isotopes are con-
stantly being produced as the cosmic ray particles hit earth's surface. Therefore, in order to use these
isotopes as a method of dating, scientists must study and record their production rates as well as their
half-lives.
Because cosmic rays can't affect atoms in a rock until the rock is at the surface of the earth, measuring
the cosmogenic isotopes provides an age for when that rock became exposed. This tool is used by
some geologists to study rates of uplift and erosion.
When two isotope pairs are measured and they result in the same (or very
similar) age for a rock, the dates are described as
concordant
or in agreement. If
the two dates are not very similar, the dates are called
discordant,
and the scient-
ist must take new samples and new measurements until there is enough concord-
ant result to draw a trustworthy conclusion.
Other exacting methods of geological dating
Radiometric dating is by far the most common method of assigning numerical ages to
rocks and rock layers, but other methods provide absolute ages as well.
Counting tree rings
Another method of absolute dating that is useful for very recent history is a method called
dendro-
chronology.
Dendrochronology examines the growth rings in trees and uses them to count back in time
