Could rising sea levels mean more eruptions at Rainier? Possibly, but not for thousands of years (if at all). Image: J. Brew / Flickr. People love to associate climate and volcanoes -- and they want volcanoes to do all sorts of stuff to climate and vice versa. Just check out these recent headlines: Alaska Dispatch: How to Reverse Global Warming? Mimic Volcanic Eruptions. Atlantic Wire: Earth's Forecast Looks Warm and Full of Volcanic Eruptions. I mean, honestly, what can't volcanoes do? I find it amusing how easily the complex interaction between climate and volcanism is boiled down to something sensational when the DOOOM isn't even there. I have already tackled why manmade volcanic eruptions aren't our solution to a warming climate. However, I haven't touched the idea that as the climate warms, we should expect more volcanic eruptions. Could this be possible? I have to admit, when I first saw these headlines regarding a new paper in Geology by Steffen Kutterolf and others, I was highly skeptical. It has been suggested before that as you unload a volcanic region from a large ice cap, then we sometimes see increased volcanic activity. This is due to the lowering of pressure of the mantle rocks below the volcanic region, which enhances melting. Evidence of this has been seen in Iceland and in the Cascades of North America. However, that seems limited -- how many volcanoes are currently buried under thick (I'm talking kilometer thick) ice caps? Iceland and Antarctica are the most likely places, but in the grand scheme of global volcanism, that isn't too many volcanoes. How could the current warming climate of the planet produce much additional volcanism? Well, it actually has a surprisingly elegant answer. Kutterolf and others looked at ash deposits in ocean sediment that recorded upward of 1 million years of sedimentation in the oceans. These drillcores they examined were from offshore volcanically active regions like Central America, the Philippines and Japan. By dating the ash layers using a variety of methods, they could look to see if the number of VEI 5 or greater eruptions that would send ash over the ocean to be deposited varies with any type of cycle. Interestingly, they found that there does appear to be a correlation between amount of VEI 5+ eruptions and the 41 k.y. Milkankovich obliquity frequency for the most recent sediment. (The dating can become more problematic with older sediment.) This 41 k.y. obliquity is a cycle based on the tilt of the Earth's poles from 22.5 to 24.5 degrees, and as the obliquity changes, the climate also changes, so that ice volume decreases and sea level increases as climate warms. However, the peak volcanism trailed behind the highest rates of sea level change by around 4,000 years. Why might that be?
A sketch illustrating how rising sea level could enhance volcanic activity. As sea level rises (SL1 to SL2), more mass is added to one end of the plate, causing bending of the plate. This reduces pressure in the volcanic arc and underlying mantle (at 20 km depth), potentially producing more magma. Image: Erik Klemetti This comes down to geodynamics. Think about a tectonic plate like a plank of wood. The ocean overlaps one end of the plank and the other end is fixed to the ground (the interior of the tectonic plate). As sea level rises, the weight on one end increases, causing the plank to bend -- so pressure goes up at the end but down in the middle where the plank bends upward (see sketch above). Now, rocks don't equilibrate instantly, so imagine the mantle behind that bending plate is feeling that decrease in pressure. The result will be an increase in magma production, but not at exactly the same time as the sea level changes. This effect might be enhanced in plates without a very rigid continental plate, such as Central America as opposed to the Cascades. Why is this cool? Well, it means that you don't need an ice cap to have a warming climate produce more volcanism! Now, before you jump off the deep end of our plank, Kutteroff and others really don't tackle the big question: How much more volcanism occurs during these periods of increased sea level? It is apparently enough to be statistically significant in the sediment record, but likely not enough to cause MASSIVE GLOBAL CATASTROPHE. Even doubling the rates of VEI 5 eruptions (which would require a significant increase in magma production) isn't likely to have a profound, singular effect on global climate as the eruptions are distributed through timescales of thousands to hundreds of thousands of years. Even so, the change in sea level that warming climate today might cause isn't likely to be manifested in increased volcanism until, oh, around 6000 A.D. So, by the time those eruptions might start, we may be far beyond needing volcanism to cool the climate.
