Geologic Time and Dating
When looking at the history of the earth and the record of time preserved in the rock or what is
called the ROCK RECORD, two types of dating are distinguished. ABSOLUTE DATING puts a date on an
event in years. For example, something may be dated at 11,500 years BP (before present) plus or minus
500 years. Absolute dating is determined in a number of ways, that will not concern us in this lab. The
second type of dating is called RELATIVE DATING. Relative dating dates one event in relationship to
another event. Something happened before or after something else. Absolute dating is exemplified by when
you give your age, but relative dating is exemplified when you look at two adults and recognize that one is
older than another but you do not know when they were born. The law of superposition and the law of cross
cutting relations are both concerned with relative dating.
The calendar of earth history is called the GEOLOGIC TIME SCALE. On the geologic time scale,
the oldest events are found at the bottom and the youngest events are found at the top. Can you
figure out why geologists follow this convention? It follows the Law of Superposition! For the
laboratory, you will NOT need to know the geologic time scale; however, it is included here so
that you can see it.
The Geologic Time Scale
The geologic time scale is the result of a compilation of both relative dating and absolute dating.
The illustration above is a geologic cross section. List the three oldest rocks from oldest to youngest.
Answers: B, C, D.
How does the age of A compare to D?
Answer: A is younger than D.
How does the age of A compare to E? (Be careful!)
Answer: The relative ages of A and E can not be determined. A can be younger than E or older than E, or even the same age as E.
Remember that the law of superposition applies only to sedimentary rocks. No age relationship is evident in this depiction.
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