Rapid Rates of Short Submarine Eruptions Measured at Monowai in the Kermadec Islands
- By Erik Klemetti
- May 14, 2012
3D bathymetric view of Monowai caldera in the Kermadec Islands. The red cone in the foreground is Monowai cone. You can see the scars of collapses on its slopes along with new domes near the summit. Image courtesy of Oregon State University by Susan Merle.
I saw (and was sent) a lot of articles about the findings from Anthony Watts and others in Nature Geosciences on the submarine volcanism in the Kermadec Islands north of New Zealand. A group of geologists were lucky enough to stumble across an eruption of Monowai in 2011 and in doing so, they set off a series of discoveries that seem to indicate that Monowai is a very active submarine volcano. Monowai is a fairly complex caldera volcano that has seen quite a bit of activity that has been captured either through subsurface acoustics or by finding the telltale signs of an eruption beneath the sea – discolored, bubbling water, pumice rafts – stuff like what we saw during last year’s activity at El Hierro. Watts and his collaborators were able to carefully map the volcano to find what the changes at Monowai have been during these eruptive periods and it boils down to something we find familiar for terrestrial volcanoes: collapse and healing, sometimes quite rapidly.
If you’re a close watcher of volcanic activity, you know that if a volcano erupts andesite, dacite to rhyolite, you can produce sticky domes of lava that can oversteepen and collapse. That is what happens on a regular basis at places like Soufriere Hills on Montserrat and Shiveluch in Kamchatka (amongst many others). These domes or spins can become incredibly impressive – the spine that formed at Pelee in Martinique in 1902-1903 was almost 300 meters tall and it eventually collapsed in 1903. However, these examples are all from terrestrial volcanoes.
Figures 6a and 6b from Watts et al. (2012) showing the changes in the shape of Monowai in the Kermadec Islands over the last 8 years. You can see the growth and destruction of domes and spines on the volcano across the years.This new study at Monowai may have captured a collapse like this, followed by new growth of a dome at a submarine volcano. Over the course of 14 days of mapping, the team found that the seafloor depth changed, first dropping almost 19 meters, then surging back almost 72 meters. That is quite a dramatic shift for only 2 weeks. This activity coincided with earthquake activity at Monowai was well, so even though the eruption was seen, the clues of changing seafloor depth – possibly a collapse followed by new lava – and seismicity all point to eruptive activity. If you look at their vertically-exaggerated views of the seamount (Figure 6a and b, above), it becomes quite clear that domes or spines of lava come and go from the summit area of Monowai, just like you might seen at its terrestrial brethren. Now, these cycles had been recognized at Monowai before, but this is the first time that the results of an eruption was caught in the act.
One of the implications that the authors push in the article is that these rates of activity are very rapid – and surely they are. The estimates of ~0.001-0.008 km3 of new material erupting within a few weeks – that works out to annual eruptive rates of ~0.11-0.63 km3*. I do worry a bit about their comparision of average growth rates at global volcanoes with these data, even looking at the last few years are Monowai. Volcanoes are notoriously inconsistent with their growth and have periods of heightened activity that punctuate long periods of low activity – they even point this out by comparing the average growth rate between 2007-2011 (0.08 km3) versus the rate they observed in the two weeks in 2011 (which would have produced, if constant over 4 years, ~2.8 km3). Whether or not “growth rates at Monowai are larger than all other oceanic volcanoes, including Montserrat, Azores, Hawaii, Iceland and the Canary Islands” is truly significant and not merely a product of small sample sizes we have for the volcanic history of many submarine volcanoes is unclear. However, it does show how dynamic some submarine arc volcanoes might be, which should come as little surprise considering the mischief their terrestrial counterparts can produce.
*Note: A few readers wondered why certain articles on this study insisted on reporting all volumes as “Olympic size swimming pools” of lava. Beats me, honestly. I know some writers love those comparators, but maybe we can give the reading audience some credit after you mention the comparison once.