We have long known of volcanic activity deep below the surface of our oceans, the processes that provide the new material for oceanic crust as it spreads away from the mid-ocean ridges and carries the continents along with it. The chains of volcanic mountains such as Hawaii, greater in their relief than Everest, and the volcanic seamounts scattered across the ocean floors. All this has been in the textbooks for decades, as has been the obvious fact that deepwater volcanic eruptions are rather benign and unspectacular events, their energy constrained by the overpowering pressure of the water. Obvious, that is, until recently.
The conventional wisdom that “these eruptions burbled gently rather than bursting in fiery explosions” has now been shown to be wrong, largely as a result of the work of David Clague, a geologist at the Monterey Bay Aquarium Research Institute (MBARI) – but it took an awful long time for his results to be accepted. The story is told in some detail in a recent MBARI press release, and it makes fascinating reading, for the research itself, but also for the all-too-common challenges of overturning conventional wisdom. And, as you might have guessed, sand has a role in the story.
Back in 1997, Clague was studying Loihi, Hawaii's youngest underwater volcano, diving in a deepwater submersible from the Hawaii Undersea Research Laboratory. As the press release describes:
“An explosive eruption basically produces a lot of little tiny particles,” says Clague. While the smaller dust-sized particles blow away on the currents, larger sand-sized pieces settle closer to home. When the Pisces submersible crested the lip of Loihi's newly formed crater, Clague saw black sand rubble everywhere. “That was indicative of fairly abundant, or common, explosive activity on Loihi,” Clague says. “What we learned was that it’s actually a pretty common event there, and that Loihi is a quite explosive volcano.”
The sand is shown in the photo at the left of the header image, settled between the outcrops of lava. The central photo shows the simple – but critical – approach that Clague used for sampling. Traditionally, such deepwater volcanic environments had been sampled using a dredge haul or similar techniques designed to collect large chunks of rock – nothing sampled the finer material. So, in 2000, Clague returned, using push cores and suction sampling to collect the sand and finer sediments. His analyses confirmed what he had observed on the original visit to the sands of Loihi – that they contain large amounts of limu o Pele, “Pele’s seaweed.” This delightful term describes the thin sheets and flakes of volcanic glass that are produced by bursting gas bubbles within an erupting lava; Pele is, of course, the fire goddess of Hawaiian volcanoes, whose wrath is incurred should you ever sample any sand from the islands; there is also “Pele’s hair,” threads of volcanic glass, and her tears. Limu o Pele and Pele’s hair are shown in this image from the MBARI article (Jenny Paduan © 2011 MBARI):
Now Pele’s seaweed and hair are commonly found where lavas flowing into the sea from an emergent volcano such as Kilauea mix with sea water that is instantly turned to steam and explosions on all scales. But they had never before been observed associated with deep water volcanoes – could these eruptions in fact be far more violent than had been assumed? The answer, as Clague was eventually to prove, is yes. But his findings, when they were first published in 2003, were essentially ignored:
It wasn’t until 2009, when a research team led by Robert Sohn of the Woods Hole Oceanographic Institute published similar findings in the high-profile journal Nature, that geologists finally began to accept that deep-sea volcanoes could erupt energetically… Sohn's paper was the clincher that solidified this new view of underwater eruptions.
It also helped that expeditions finally began to encounter, and capture images of, live eruptions of deepwater volcanoes. I wrote in my last Sunday sand post of the tectonic chaos of the southwest Pacific, and the Islands of Samoa. A clear demonstration of this chaos is the active volcanism going on continuously in the region, and it was from the West Mata volcano, a couple of hundred kilometres southwest of Samoa and whose peak lies more than 1000 meters below the ocean surface, that some of this dramatic evidence came in 2009. Explosive volcanic eruptions at this great depth – see the photo at right in the header image (Joe Resing/NOAA/NSF) and absolutely go to http://www.youtube.com/watch?v=hmMlspNoZMs&feature=fvwrel for some stunning video (narrated by Resing).
So, simply by David Clague’s sampling the previously un-sampled, and questioning conventional wisdom, we now know something new about how our Planet actually works – better late than never.
And, speaking of sampling and Samoan sand, among the grains of the latter were foraminifera: I’ve written often about these critters before, including the ones that build their shells out of other grains, the so-called agglutinating forams. The photo below shows one such foram, about 2mm across, that specialises in building its shell from limu o pele, and, as David Clague writes:
Benthic foraminifera often glue particles to their tests, perhaps for protection from predators. These particles may be sponge spicules, sand grains, or other detritus, depending on the materials available and the "specialty" of the foram. In sediment cores from the Gorda Ridge [off the coast of Oregon], we found forams that "specialized" in volcanic glass grains and others that "specialized" in limu o Pele. They effectively concentrated the glass samples for us!
[All images not credited in the text © MBARI]
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