If you've ever seen an explosive volcanic eruption - either live or on video - you know that there is an awful lot of ash produced. All that magma (well, most of it) that is erupting from the volcano is being fragmented into tiny glass shards that we call "ash" and all that ash is being shot into the air at astounding rates - for very large eruptions, it could be as high as 9,500 kg/s for a VEI 7 eruption. 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. So, how does the ash get spread so far from the site of the eruption? The simplistic view of ash behavior in the atmosphere would suggest that very small (< 30 μm) ash should stay aloft for days to weeks - the settling rate is between 10^-1 to 10^-3 m/s if you apply Stokes Law to the settling of the ash. However, Rose and others (2011) in Geology point out that in even large eruptions, this fine ash can settle in less than a day. This suggests that fine ash might stick together as it drifts in the plume, thus make larger particles that fall out faster than the initial size might suggest. Now, how these ash particle stick together is an open question that requires cooperation between the volcanological and meteorological communities. Some of the recent large, ashy eruptions worldwide (such as Chaitén and Puyehue-Cordón Caulle) have allowed volcanologists and atmospheric scientists to examine how ash is distributed during an eruption. This allows for the comparison of models of how ash will spread in the atmosphere with observations of the ash by observatories and satellite monitoring (such as the VAACs). The eruption of Eyjafjallajökull in Iceland spread ash over Europe quite rapidly thanks to its very small particle size, in part caused by the interaction with water during the April 2010 phase of the activity (and likely justified the closure of airspace over Europe). However, the ash varied during the course of the eruption and varied depending on the location in Europe.
Oddly, it is places like the ocean where volcanic ash is best preserved as layers in oceanic sediment, where is can collect as a layer and get covered by new sediment without the fears of terrestrial weathering, erosion and transport. In a new study by Salisbury and others (2012) in the Journal of Volcanology and Geothermal Research (you might recognized a few of the authors of the study from guest posts and Q&As on the blog), multiple ash layers were identified in cores off the coast of Sumatra in Indonesia. Some of these ashes could be potentially correlated with large eruptions such as the Younger Toba Tuff, but others, likely deposited in the last few hundred years, are from hitherto unidentified eruptions that ranked in the VEI 3-5 range. Now, that is no colossal event like Pinatubo or Novarupta, but it is close to the range of the 1980 St. Helens eruption (whose 32nd anniversary is today). So, somewhere on Sumatra over the last 500 years, multiple volcanoes produced significant eruptions, something that isn't fully realized. Yet, all the evidence we have right now are layers of ash deposited in the deep ocean potentially hundreds of kilometers from the source - similar to the sulfate record in the polar ice caps that capture large eruptions like the missing 1258 A.D. eruption that still eludes volcanologists. Today, we're only in the beginning stages of understanding how ash gets distributed globally, how it is preserved and the nature of its behavior once it is erupted into the atmosphere. However, as the 2010 eruption of Eyjafjallajökull and the 2011 eruption of Puyehue-Cordón Caulle (see above) showed us, ash cause major disruption to people's lives thousands of kilometers from the volcano. Combining volcanology, meteorology and remote sensing can begin to clear the path (so to speak) to predicting the behavior of volcanic ash and where it might ultimately end up.