In the end, your average eruption is releasing millions to trillions cubic meters of ash into the atmosphere. Most of it falls near the volcano (within tens of km), but a significant portion can travel far away, drifting in the atmosphere for hundreds, thousands, tens of thousands of kilometers around the globe. That ash becomes the telltale signs of an eruption that may have much of its record erased by future eruptions or by the relentless powers of weathering, erosion and transport.
Volcanic ash is really just a mix of shattered rock, minerals and glass. The shattered rocks are from the physical breaking of the pre-existing material like solidified lava in the conduit (accidental material), while glass is quickly quenched magma from the eruption (juvenile material). The minerals could come from either the accidental or juvenile material of the eruption. When you’re trying to identify a layer of volcanic ash, you can look at the shape of the glass shards, the mineralogy of the ash or the composition of the glass. However, trying to definitely match an ash layer with a specific eruption of a volcano can be very tricky as not all ashes are clearly distinctive in their shard shape (see right), mineralogy or glass composition. Unfortunately, that is all we have to go on in many occasions when looking at layers of volcanic ash that are deposited far afield from the volcano of origin.