Note: Dr. Steffi Burchardt, a senior lecturer in Structural Geology at Uppsala University in Sweden, presented a talk, “Xeno-pumice erupted offshore El Hierro, Canary Islands: A tale of stoped blocks in magma chambers?” at the LASI V workshop in Port Elizabeth, South Africa in October 2012. The article below is based on this talk and also an interview with Dr. Burchardt. Over a few weeks, I am highlighting some of the research that was presented at the LASI V workshop. This is the second post in that series.
Following a period of intense seismic activity, on October 10th, 2011 a submarine eruption began approximately 1 kilometer off the coast of El Hierro, the youngest and westernmost island in the Canary Islands, which is a group of volcanic islands believed to have been formed through hotspot volcanism. The eruption was evident from the unusual conditions on the sea surface: the sea bubbled, like a Jacuzzi, and was stained green. The large green stain was easily observable from space. In the midst of these strange conditions, some highly unusual rocks were erupted. For several days after the sea started bubbling, strange floating rocks were observed and collected off the coast of La Restinga, the closest town to the undersea eruption. These floating stones were generally tens of centimeters in size and resembled lava bombs in shape. The outsides of these floating rocks consisted of basanite , a rock type commonly observed in the Canary Islands and other volcanic ocean islands. Basanites don’t generally float. However, these basanite shells floated because their insides were filled with a white to light grey, pumice-like material. Pumice is a highly vesicular rock, which means that it is a rock full of voids or bubbles, which make the rock light enough to float on water.
While the pumice-like centers explained why the rocks floated, they also raised a multitude of questions and triggered some heated debates amongst geologists. This is because pumice is not commonly produced in the Canary Islands* or in other oceanic island hotspot environments, such as Hawaii and Iceland. The lavas erupted at oceanic island hotspots are generally mafic, low viscosity lavas such as basalts (and basanites). Viscosity is, in essence, a measure of how resistant lava is to flowing. The less viscous a lava, the more likely that lava is to flow. Therefore, low viscosity lavas such as basalts tend to flow easily and also tend to regularly release volatiles such as water and carbon dioxide. Therefore, the pressures in these lavas remain relatively low, and violent eruptions are uncommon. Pumice is most commonly produced during eruptions of felsic, highly viscous, volatile-rich lavas, which are found in environments such as island arcs, not oceanic island hotspots. The voids or bubbles in pumice represent places where volatiles have been rapidly released due to a pressure change, often during a violent eruption.
So, what was pumice-like material doing in an oceanic island eruption? A number of theories were put forward to try to explain the floating rocks that were erupted off of La Restinga. Some scientists thought the pumice-like material represented juvenile, highly silicic, highly viscous magma (such as rhyolite), which is very explosive. Other scientists proposed that the pumice-like material represented re-melted magmatic material, altered volcanic rock, or reheated hyaloclastite or zeolite from the slopes of El Hierro. Mysterious in origin, the floating stones were called “restingolites” after the nearby town of La Restinga.
After extensive analysis, a group of scientists (Troll et al., 2012) proposed an alternative hypothesis to explain the pumice-like material found in the restingolites. Based on the material’s high silica content, lack of igneous trace element signatures, and high oxygen isotope values as well as the presence of remnant quartz, jasper, carbonate, and wollastonite, Troll et al. concluded that the pumice-like material in the restingolites in fact represented xenolithic material from pre-island sedimentary layers that were picked up and heated by ascending magma, which caused the layers to partially melt and bubble. Looking like pumice and originating as xenoliths, Troll et al. dubbed the restingolites “xeno-pumice”.
Dr. Burchardt elaborates, “Xeno-pumice is definitely not an established term. We have coined it for the first time in the case of El Hierro eruption. The name comes from adding the preface ‘xeno-‘, which means foreign, to ‘pumice’. We used this term because the floating rocks of El Hierro present the characteristics of pumice, but they are actually not pumice in origin; they are actually xenoliths. We found out, based on mineralogy and also the fact that they contain detrital sand grains and fossils, that they are actually not magmatic in origin but rather that they are xenoliths from the sedimentary layers that underlay the Canary Islands. So, they are older than the volcanism. When the magma was rising, it stagnated at this level and interacted with the sedimentary rocks, sandstone and minor carbonate, and the magmas transported the xenoliths up with them to the ocean floor, where they were erupted. But in the process of the ascent of these xenoliths, they were subject to heat from the magma, so they started to melt. Since they contain a lot of water, this water started to boil and formed bubbles. The end product was something that looked like a pumice: lots of bubbles surrounded by a glassy matrix.”
Even though xeno-pumice was not a known rock type before the 2011 El Hierro eruption, Dr. Burchardt and her colleagues think that xeno-pumice may actually be a common—if not commonly recognized—rock type in other parts of the world.
Dr. Burchardt explains, “The El Hierro eruption was a very fortuitous circumstance for our work because my colleagues and I had been working on similar rocks from volcanoes worldwide, but that they were not previously recognized as xeno-pumice. The El Hierro eruption was therefore some kind of a breakthrough for our research in this field, and there will be a whole series of papers dealing with xeno-pumice from different parts of the world.”
By November, the xeno-pumice rocks were no longer being erupted, and worries that a dangerous, explosive eruption could occur at El Hierro abated. The identification of the restingolites as xeno-pumice was also good news for the hazard risk at El Hierro.
Dr. Burchardt explains, “It was good news that these xenoliths are sedimentary in origin because it means that there is no rhyolitic magma beneath the island, which means that a big explosive eruption isn’t likely.”
While the xeno-pumice rocks do not carry the message that an explosive eruption is likely to occur at El Hierro, they do carry other important messages from the deep. The unusual xeno-pumice rocks observed erupting at El Hierro in 2011 can provide much direct information about the interaction of magma and oceanic sediments and also may indicate that recycling of oceanic sediments into magma is an important process at ocean islands. Further study of xeno-pumice from the Canary Islands—and also from other parts of the world—will go a long way towards helping geologists better understand how volcanic eruptions at ocean islands interact with oceanic crust and sediments as they make their way to the surface.
*Update: Commentor Siim Sepp points out that intermediate composition pumice is found on the Canary Islands, most notably on the island of Tenerife. This is a very good point. I have perhaps oversimplified the explanation– pumice can be found at volcanic ocean islands under certain conditions. Thanks for pointing this out, Siim!
Reference:
Troll, V.R., Klügel, A., Longpré, M.-A., Burchardt, S., Deegan, F. M., Carracedo, J. C., Wiesmaier, S., Kuepper, U., Dahren, B., Blythe, L. S., Hansteen, T., Freda, C., Budd, D., Jolis, E. M., Jonsson, E., Meade, F. C., Harris, C., Berg, S. E., Macini, L., Polacci, M., and Pedroza, K. 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, Vol. 3: 97-110.
Georneys 
