GEOLOGIC TIME - a summary of the lecture.
Additional resources,
Geologists are interested in
-duration of geologic processes
-timing of geologic events
-age of the Earth
There are two ways to get an age:
1. relative age dating: gives ordering of things
2. absolute age dating: provides a number
I. Relative Age Dating
To establish a chronology, need material evidence for a sequence of events for example, layered rocks
Two fundamental principles - From these principles, get beginnings of a stratigraphic clock
1. principle of horizontality
2. principle of superposition
In addition one can detect an
-upward sequence of lithological changes
-upward sequence of faunal changes
Leads to idea of time units
-time interval for each rock layer
-length of time for all layers within a rock body
Once time intervals are established, we are concerned with correlating rock formations from one locality to another. How?
1. by fossils
2. by surfaces
unconformity
angular unconformity
Mapping of the surface of the Earth led to the construction of the geologic time scale (p. 39 text). This consists of eons that are divided into
Eras
Periods
Epochs
In an attempt to relate actual time span to a rock sequence, we must establish rates. Some that have been tried are:
rate of sediment accumulation
rate of salt influx to oceans
rate of evolution
- there are many problems with establishing precise rates
II. Absolute Age dating - The discovery of radioactivity gave scientists a means for measuring time spans.
Parent (radioactive) ----->Daughter. Unstable parent decreases in abundance with time and a more stable daughter increases correspondingly.
There are three types of decay (p. 185 text)
1. alpha (a) particles: emission of particles containing 2 protons and 2 neutrons
2. beta (b) particles: electron emission
3. electron capture: a proton picks up an orbital electron to make a neutron
The rate at which radioactive atoms decay is reported in terms of half-life, the time required for one half of the original number of radioactive atoms to decay. Another measure is the decay constant (l
) = ratio of the number of parent atoms decaying per second to the total number of atoms remaining. These are
-characteristic of particular radioactive atom under consideration
-represent probability that an atom will decay within a stated unit of time
radioactive decay can be expressed by formula (p. 44 text)
N = Noe-l
t
where N = number of parent atoms present today, No= original number of atoms (parent + daughter present in
the sample), e = 2.718... (this is a constant), l
= decay constant, t = time
One calculates time by modifying above equation to:
t = 1/l
ln (1+D/N)
where D = number of daughter atoms present today and N is the number of parents atoms present today.
Assumptions:
Assumptions
-radioactive decay is not disturbed by physical processes
-half-life measured accurately
-half-life constant
The age equation is valid only when the following four points are satisfied:
1. the rock or mineral containing the parent/daughter pair must be a closed system
2. value is realistic; synthesize all geologic information available
3. l
must be accurately known
4. D and N must be accurately measured
Geologists do not actually date a rock; rather what is dated are chemical compounds (minerals) found within the rock.
oldest date on Earth -- 3.8 b.y.
meteorites -- 4.6 b.y.
Additional resources
A history of the development of the concepts of geological time and the methods used
- Introduction to how to tell time and the geological time scale.
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