Examine the geologic time scale shown here. Which is the longest-lasting period of geologic time? Which is the shortest period? Why should these periods differ so much in duration?
What will be an ideal response?
Answer: The answer should not confuse eons or eras with periods. The period with the longest duration was the Cretaceous at 80 million years. The Silurian was the shortest period of the Phanerozoic, at 28 million years. (These numbers come from subtracting the "start" and "end" dates for each period from the geologic time scale image. The length of the period is based on changes in the fossil record, so the Cretaceous represents an extended time of relative stability in Earth's ecosystems and therefore its fossil record, while the Silurian was relatively a "flash in the pan," a biosphere that was stable for only a (relatively) short time.
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