Question Geologists have divided geologic time into eras. What have they chosen as the demarcation of when one era ends and another begins? A. It is based on the fossil record, in particular when a major extinction has taken place. B. They have chosen the creation of major mountain ranges by plate tectonics as their indicators of the change in eras. C. Geologists looking at the rocks in the Earth’s history have decided that the advance and retreat of ice sheets is a world wide event and thus a fair marker for the boundaries of the eras. D. They arbitrarily have designated era boundaries ever 250 million years.
Question
Geologists have divided geologic time into eras. What have they chosen as the demarcation of when one era ends
and another begins?
A. It is based on the fossil record, in particular when a major extinction has taken place.
B. They have chosen the creation of major mountain ranges by plate tectonics as their indicators of the change in eras.
C. Geologists looking at the rocks in the Earth’s history have decided that the advance and retreat of ice sheets is a world wide event and thus a fair marker for the boundaries of the eras.
D. They arbitrarily have designated era boundaries ever 250 million years.
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