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Xeno-Pumice: A New Kind of Floating Rock

We learn another neat trick that volcanoes can do

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Xeno-Pumice: A New Kind of Floating Rock

Dark lava shell around a breadlike filling

From Troll et al. 2012

When an undersea eruption began off the volcanic island of El Hierro in 2011, research ships showed up in the Canary Islands right away to study it. One thing they found in the first days was floating rocks of a peculiar kind, like a geological frybread or fritter. Now a paper in the open-access journal Solid Earth has given them a more scientific-sounding name: xeno-pumice.

The Floating Stones of El Hierro

The El Hierro eruption began with a plume of stained seawater, lots of volcanic gas bubbling up, and the appearance of floating rocks. These came in two kinds, neither of them taking the form of pumice, which is the usual suspect in these cases. Pumice is a sponge of lava, so full of gas bubbles that it is lighter than water. It is made by viscous, sticky lavas (like rhyolite) that produce dangerously explosive eruptions. Therefore it was important for researchers to find out what these rocks meant.

The first type of floating rock was lava balloons—thin lava shells, some as large as bathtubs, with a big gas-filled hole inside. Lava balloons are very rare in the historical record, and they break up within minutes. Scientifically they're not interesting, although they're certainly cool.

The second type was found early in the eruption period. These floating rocks had an exterior shell of glassy black lava and an interior of white or cream-colored material charged with gas, resembling white bread or the filling of malted milk balls. This put the scientists on alert, because light color is a telltale sign of the dangerous rhyolite lava.

Signs of Sediment

The authors of the Solid Earth paper, a 21-member team representing eight nations, ran tests on the breadlike filling and rejected previous ideas on how the stones formed.

First, the stuff didn't have any minerals typical of lava—no olivine, no pyroxene, no amphibole, not even any feldspar. That ruled out all three previous explanations: that it was rhyolite, that it was material from the flanks of the existing volcano of El Hierro, and that it was some kind of reheated, altered lava from below the Earth's crust.

Second, the white filling's chemistry was totally off the charts for known lava types. Nor did it match the chemistry of existing volcano flanks.

Third, some of the floating rocks had weird things inside them, including quartz grains and bits of red jasper. There were even shreds of wollastonite, which only forms from high-temperature alteration of calcite.

A New Name, a New Volcanic Trick

The authors concluded that this had once been seafloor sediment, laid down long before the volcanoes of the Canary Islands formed. This material, mostly clay and carbonate and silica particles of microscopic size, must have quickly melted when the lava invaded from below, then puffed up like perlite, or like the dough in frybread, in the overwhelming heat.

Because these chunks of material came from outside the lava and were engulfed in it, they are classified as xenoliths. But unlike ordinary xenoliths, the material puffed up into something resembling pumice—more precisely, it gained a pumiceous texture. The authors thus gave it the name xeno-pumice.

They also compared the El Hierro material to crustal xenoliths from other Canary Island volcanoes (Lanzarote, Tenerife and La Palma). They found a close match in a sedimentary xenolith collected on Gran Canaria. It had been a sandstone, but x-ray tomography showed that it had the same frybread consistency inside. Who knew that volcanoes could do such a trick? Now that we have a concept with its own name, we may start seeing other examples around the world.

Source: V. R. Troll et al., 2012, Floating stones off El Hierro, Canary Islands: xenoliths of pre-island sedimentary origin in the early products of the October 2011 eruption, Solid Earth 3, 97-110. (PDF)

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