"Largest Volcano on Earth"? It Is All About Timing

Rocky Planet iconRocky Planet
By Erik Klemetti
Sep 6, 2013 8:26 PMNov 19, 2019 8:45 PM


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Bathmetry of the Shatsky Rise and the Tamu Massif in the Pacific Ocean. The lines represent paths of seismic profiles taken in order to interpret the structure of the oceanic crust in the area of Tamu Massif. The red dots are drilling locations on the volcanic center. Image: Figure 1 in Sager and others (2013), Nature Geosciences. Sometimes it can be difficult to separate the publicity-driven hooks for a new study in science from that actual findings of the study itself. A research paper itself is the distillation of years of works and reams upon reams of data and interpretation. Depending on the journal in which the study is published, that could mean trying to present the findings in as little as a few pages with a smattering of figures (the bigger the journal's reputation, the shorter the paper it seems to be these days). Then, that paper is taken by the press officers at an institution and distilled even more into a press release that might be a page where maybe a single figure and finding are touted with some quotes from the author that emphases BIG FINDING #1, with possibly a few brief quotes from other scientists who have had a day or two to look at some (possibly not all) of the paper. Once that press release is freed to the wild, then many in the media (I emphasize, not all in the media) descends, latches onto that BIG FINDING, makes it seem like that single paper unequivocally proves BIG FINDING and then takes chunks of the press release and maybe a secondary quote and we're left with a tiny vestige of what the original study had set out to do. This is what people hear about scientific research: "Dr. Someone makes BIG FINDING that is the biggest/fastest/deadliest/noisiest/hotter/furthest (circle one) discovery ever!" Now, I bring this up not to say that coverage of science in the media is a bad thing -- in fact, we likely need more of it. However, the way it is tackled, as trying to get across so much information in so little space, leaves us without the information needed to make any educated assessment of the study. The assumption that people can't understand complicated science (or don't want to understand) leaves us taking the "science" out of science and leaves us with an overdistilled undrinkable product. With that in mind, many of you have noticed a bevy of new articles proclaiming that the "largest single volcano on Earth" had been discovered. You look at the headlines and wow, it seems impressive: Scientists Confirm Existence of Largest Single Volcano On Earth

Largest volcano on Earth found under Pacific Ocean

Scientists just discovered Earth's largest volcano

Ocean volcano may be largest on Earth, biggest in solar system

With those sorts of claims, we need some great supporting evidence to back up the idea that this volcano is not only potentially the largest on Earth but also possibly the biggest is the solar system. What we need to do is go back to the source of these claims and try to make sense out of the idea that somehow, we're been missing a volcano larger than Olympus Mons (currently the biggest in the solar system) here on Earth. The study by William Sager and others (appearing in Nature Geosciences) looks at an area of the Pacific Basin called the Shatsky Rise. The rise itself is a large plateau about the size of California that contains 2,500,000 cubic kilometers of most basalt and gabbro (the stuff of oceanic crust). Sager and others (2013) looked at one part of the Shatsky Rise, now called the Tamu Massif (named after Texas A&M University) on the southern end of the Shatsky Rise. The Shatsky Rise is thought to be an oceanic plateau formed by voluminous basalt eruptions -- like a flood basalt, but under the ocean. Flood basalts on land are constructed from a multitude of lava flows that issue from fissures over the course of hundreds of thousands to millions of years, so their submarine cousins are thought to form the same way. Where Sager and others (2013) diverge from this idea is they propose that Tamu Massif is, in fact, a singular volcano where almost all the lava flows came from the summit vent area. So, instead of a series of fissures across thousands of square kilometers each erupting lava flows that coalesce into a large plateau of basalt, they say that all this basalt was formed from thick flows of basalt all coming from the same place. Now, that is quite a statement. No other single volcano on Earth covers the same area as the Tamu Massif, so if it is one volcano, then it not only sets the bar, it takes the bar and bends it into a pretzel. As I said, any statement like this needs to be backed with strong evidence, so what evidence does Sager and others (2013) present? Well, much of their argument is balanced on seismic profiles of the oceanic crust around Tamu Massif. Research vessels mapped the ocean floor and ran seismic profiles by shooting an airgun in the ocean to produce artificial seismic waves that would reflect off materials within the crust -- something that oil companies do all the time to find new prospects. The above map shows the locations that these profiles we made -- as as you can see, they only sample a very small portion of the area in question. Now, the seismic profiles that are constructed are tricky because the data you get is relatively low resolution and doesn't usually present an unique solution (see below). They need to be interpreted and by no means is that a straightfoward job. One thing that helps is to have some drill core of the crust in the area to compare to determine what materials might be reflecting your seismic waves. Once you decide what is reflecting (and what isn't), you start drawing lines on the profile to define the layers, in this case lava flows.

One of the seismic profiles of Tamu Massif in Sager and others (2013). Look how they interpret the seismic profile from the top to the bottom images -- these are based on their model for what the lithology of the rocks might be (lava flows in this case) and a single drill core along their transects (see map above). Image: Figure 4 from Sager and others (2013). Sager and others (2013) set about defining all their reflectors (see above) as lava flows and what they concluded is that (a) all the reflectors, if lava flows, come from the summit vent area and (b) the reflectors can be traced for hundreds of kilometers, meaning their lava flows are very, very long. Some of the lava flows are upwards of 23 meters thick on their own, putting them in the same league as the lava flows of the Columbia River flood basalts. After looking at their profiles, they interpreted them a stack of lava flows all from a single vent. However, they do allow for some volcanism after the main building of the shield volcano, including some impressive parasitic cones and other "eruptive centers" that are not at the summit of Tamu Massif. Now, this is really the crux of their argument: seismic profiles can be interpreted as large lava flows all coming from the same vent. Interesting idea, but in my mind, a bit of a stretch with the data on hand. Sager and others (2013) go on to argue that this whole feature (which they never offer a total volume of erupted material) needed to form geologically quickly because the Pacific plate is moving, so if it spans a long period, you get a chain of islands like Hawaii rather than a single giant volcano. Right there is the Achilles' Heel of the study -- time. They provide a single age for the Tamu Massif taken from a single drill core on the volcano, putting the volcano being active at ~144 Ma. I like to remind my students that in order to understand a single, normal sized arc volcano like Lassen Peak in California, you need to disassemble the volcano and date as many of the volcanic materials as you can to understand how it evolved over time. With a massive volcanic package like Tamu Massif, a single age is not nearly enough to conclusively say that Tamu Massif is a singular volcano that formed very rapidly, no matter what the seismic profiles tell you. You need to date flows all over the volcano to see if they match in age if you are saying that the flows are single, long lava flows. Sager and others (2013) admit that the lack of samples from such a difficult to access location (the bottom of the Pacific) hinders the interpretation of the original of Tamu Massif, but in my mind, calling it a single volcano isn't valid until we know more about the age of those lava flows across the area. UPDATE: A reader pointed out that they also have magnetic lineation data (see red lines on the top map) for the region that help constrain some of the timescales. However, that data paints with a very broad brush -- in the millions of years. We need more precise ages of material erupted from the Tamu Massif to truly construct its history. Sager and others (2013) get into some more arm-waving material when it comes to figuring out how this "single volcano" might have formed. Previous work on lavas sampled from Tamu Massif suggest that the basalt was forming at ~6 km depth, or the boundary between the oceanic crust and mantle beneath. However, no mechanism currently exists to explain how you could store such massive amounts of eruptible material at those depths -- so if Tamu Massif is a single volcano that formed rapidly, we need to come up with a way to produce all the basalt to the right place at the right time. We know that terrestrial flood basalt can be incredibly productive for brief bursts, but those are flowing from fissures -- so why can't Tamu Massif just be a large fissure in a fissure field that the evidence of those fissures is obscured in the seismic data? Fissures lack a lot of topographic relief, so picking them out in such profiles could be difficult. Priming that much basalt to all erupt from a single location in a very brief (geologically) period -- which they don't ever suggest how long that might be -- would be a remarkable geologic event. Now, this is not to say that Sager and others (2013) haven't found something interesting. These large lava flows seem to be a hallmark of Tamu Massif and have been observed at other oceanic plateaus as well. How do such large lava flows get emplaced on the ocean floor? I find it hard to envision these thick lava flows traveling that far at the bottom of the oceanic without cooling rapidly, so the rates of eruption would need to be very high to keep things hot. Maybe these are really long and wide lava tube systems instead, to keep that lava hot. What about the ocean itself -- we know basaltic eruptions release lots of carbon dioxide and sulfur dioxide, so how would that effect the chemistry of the deep ocean if 1000s of cubic kilometers of basalt are erupted, potentially over a short period of time. Oceanic plateaus are not uncommon, so they might play a very interesting role in ocean chemistry. So, Sager and others (2013) present some interesting evidence that Tamu Massif is big. Going as far as to say, on this evidence along, that it is a single volcano might be stretching this data to its absolute limits. Without thorough dating of the lava flows, we can't be sure how long it took to form and if it was over the course of millions of years, how is that a single volcano (much like a flood basalt being miscategorized as a "single event"). It is easy to jump onto that startling and sexy BIG FINDING, especially something like "largest volcano on Earth" but as with almost all BIG FINDINGS in science, more work needs to be done before we can hand the medal over to Tamu Massif.

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