Pumice is rock that is ejected from a volcano, and has so much gas trapped in it that it can float. So when a pumice-ejecting volcano (not all volcanoes produce pumice) goes off near a body of water, you can get a raft of rock floating around for quite some time. By and by, water replaces the gas within the rock and it sinks. Like a rock. So, you can get layers of pumice on the bed of lakes, seas and oceans. A forthcoming paper in Deep Sea Research I describes two such pumice deposits of "Drift Pumice" in the Indian Ocean.
The fact that the two deposits are more or less on the surface of the seabed tells us that they are not too old. The central Indian Ocean basin collects sediment at a rate of just a couple of millimeters every thousand years, and given the way these samples were collected (using techniques that access near-surface material), it has to be the case that they date to within the last quarter of a million years. There are two distinct types of pumice samples showing very different degrees of "weathering" (the buildup of oxides on the surface) suggesting two distinct events. But the Indian Ocean is surrounded by regions of volcanic activity, in Africa and Indonesia, for example, so there are a lot of possible sources.
One way to tell where a particular volcanic rock comes from is by "fingerprinting." This is a technique developed in the late 1970s and early 1980s. Each volcanic eruption has a distinct chemical signature associated with any of the products it produces. There are two main factors influencing this. The main factor distinguishing effluence of specific volcanoes is the nature of the volcano itself. Geologists classify volcanoes into distinct categories along a couple of different dimensions. One dimension relates mainly to the geomorphology of the volcano (a cinder cone or stratified conical mountain shape being the typical volcano morphology for most people). Another is the specific chemical composition of the ejecta itself. These two characteristics are not independent, but rather, are closely interrelated. The second factor is more a matter of random chance. No two volcanoes come from the same exact magma, pass through the same rock on the way to the surface (some of that stuff is melted into the magma) or undergo the same exact processes of flow or ejection and cooling. Therefore, the exact list of elements represented in the ejecta, and the exact proportion of these various elements, will differ between volcanoes, and typically, between major eruptions of a given volcano.
So, in the study at hand, Pattan et al. were able to match the pumice from each of these two deposits on the floor of the Indian Ocean with specific eruptions by matching the chemical fingerprints of the rock with samples from actual known volcanoes.
One of the samples (given the poetic name "Group I") matches the explosion of Krakatau in 1883. The other sample, Group II, matches what has come to be known as YTT, for Younger Toba Tuff, dating to about 74,000 years ago.
The eruption of Krakatau in August of 1883 may be the most powerful volcanic eruption in recent history. More than 25 cubic kilometers of material may have been ejected. That's enough to put a one cm. layer of ash over all of Lake Superior, for instance. The sound of that blast was heard up to 5,000 km away, and more than 35 thousand people were killed. The tsunamis were bad. A similar eruption today would have a much, much higher death toll owing to a greater number of people in the immediate area and along tsunami affected coasts.
And, as far as "explosive volcanoes" go, that was a baby. The Younger Toba Tuff (YTT) eruption was much bigger. Lake Toba, in Indonesia, is sitting in the crater left behind by the YTT event. How big was the eruption? It was so big that it is classified as a "mega colossal" eruption. The amount of material ejected from this volcano was not a mere 25 cubic kilometers, but rather between 2500 and 3000 cubic kilometers, with a third of this in the form of ash and stuff (pumice) strewn into the air. The hole left behind by this eruption is one of the largest extant calderas on the planet, just bit smaller than the Yellowstone Caldera. Much of South Asia and the Indian Ocean was covered in the ash, ranging in depth from a centimeter or so to up to 6 meters. This was probably a significant mass extinction event for that region, and global consequences owing to the "nuclear winter" effect would have been devastating. It has been proposed (by Stanley Ambrose) that this was very nearly an extinction event for humans.
Whew, close call.
Pattan, J.N., A.V. Mudholkar, S. Jai Sankar, D. Ilangovan. (2008) Drift pumice in the Central Indian Ocean Basin: Geochemical evidence. Deep-Sea Research I. In press. doi:10.1016/j.dsr.2007.12.005.
So is that where the pumice stone that I use comes from?
I think this would be excellent information for teachers. We are often confronted with questions of, "How do you know how old (fill in the blank) is?" This post does a good job of explaining in understandable terms how geologists analyze volcanic ashes.