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.

Date Interpretation

sample cross section for 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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