Question

Why are ancient volcanic ash deposits important to geologists?

Answer 1

They record the volcanic activity of a certain area. They allow geologists to deduce that there was vulcanism in that area 
The ash can also be radiometrically dated so scientists can tell when that area was volcanically active. It can also be used to relatively date the age of other geologic events in that particular area

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Answer 2

Volcanic ash tends to contain a mixture of tiny, sharp pieces of rock and glass, made from liquid droplets volcano that have cooled quickly. It's abrasive and mildy acidic and is known as tephra. Volcanic ash is expelled with great force from a volcano during an eruption. When the ash falls, it is hot and deadly, and can ignite fires, and it can also be spread over a very wide area by winds.

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