Volcanic ash places a marker in time

Wooden stakes mark different ash layers. The fine-grained ash in the lower layer came from a distant volcano; the heavier, coarser tephra on top originated nearby.
Photo credit: Photo by Kristi Wallace, U.S. Geological Survey/Alaska Volcano Observatory

Volcanic eruptions spew ash over a short period of time, from a few hours to a few days. The ash accumulates quickly, and the layers create time posts which help scientists unlock the past.

Archaeologists use ash layers to determine the age of an ancient tool or campsite. Paleontologists use them to date fossil finds. And ash layers help geologists understand when certain features of the land were formed.

Combined, this information helps us understand the Earth and its history.

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