Question

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how are the divisions of the geologic time scale determined?
by the appearance of different types of metals.
by the formation of the worlds oceans.
by major events, such as mass extinctions or asteroid impacts.
by the start and end of each ice age.

Explanation:

The geologic time scale is divided based on significant, identifiable changes in Earth's history. Major events like mass extinctions, asteroid impacts, or major shifts in life forms create clear markers that separate distinct time periods, as these events leave measurable traces in the geological and fossil records. The other options are too narrow: metal use is a human-era development, ocean formation is a single early event, and ice ages are limited to more recent, shorter intervals.

Answer:

By major events, such as mass extinctions or asteroid impacts.