The vast majority of volcanoes (those active on the earth today and those discovered in the geologic record) are “normal” volcanoes. These are volcanoes with relatively well understood origins, generally related to the geology of tectonic plate margins. “Normal” volcanoes are erupting as we speak along mid-ocean spreading ridges and above convergent margins where one tectonic plate is being subducted beneath another. Just about all the volcanoes within the Pacific Ring of Fire are “normal” volcanoes.
“Super” volcanoes are not just immense volcanoes, though their size is one of the criteria distinguishing them from “normal” volcanoes. To compare two well-known examples: the caldera of Mount St Helens, a “normal” large volcano, contains an estimated volume on the scale of one km3; the volume of the Yellowstone Super Volcano is estimated at 1000km3 – that’s three orders of magnitude larger than your everyday ordinary source of total destruction! Super volcanoes that have been active in the past and are still threats today include Yellowstone, Long Valley (Mono-Inyo valleys in California), the Valles Caldera of New Mexico, Lake Toba in Indonesia, Taupo Volcano in New Zealand, and the Aira Caldera in Japan. A super volcano that hasn’t yet erupted is proposed beneath Europe (deadset under Germany), and some consider Campi Flegrei in Italy to be a potential super volcano hazard.
The origins of super volcanoes are not well understood: it has been suggested that they are intimately related to mantle plumes (uprising columns of hot rock through the earth) that penetrate intra-plate localities as hot spots. However, the Hawaiian volcanoes, perhaps the best studied example of hot spot volcanism, though large, are not as huge and destructive as would be an eruption of a super volcano. Hawaii is well-behaved and tourist friendly. Yellowstone could be cataclysmic.
Michael Thorne at the University of Utah has been investigating deep earth structures located at the core–mantle boundary, huge piles of rock at a depth of ~2900 km beneath the Pacific and another beneath Africa. Using seismic waves as a sort of terrestrial-scale sonogram, these large rock piles appear to be in motion (very slow motion), and at some areas where they converge, seismic waves are transmitted with the characteristic signals of partially molten rock, ie, blobs of magma. These “blobs” are on the size scale of, say, Greece (including the Aegean). Seismologists give these blobs the friendly name of “mega ultra low velocity zones.”
Are mega ultra low velocity zones the source of super volcanoes? Thorne believes these blobs could be the roots of mantle plumes that give rise to hot spot volcanism, and when of large enough scale (super blobs!) could cause super volcanoes or flood basalt eruptions (as the Deccan Traps or Siberian flood basalts). It is estimated that the time scale from which a blob ascends from the core-mantle boundary as a plume to intersect the lithosphere is on the order of 100 million years. Thus, the mega-blobs beneath Pacific and Africa could be the root cause of an apocalyptic eruption and extinction in the distant future.
Whew! We’re safe for a while – at least until Yellowstone decides to blow up.
Graphic: BBC Earth the Biography
More on this story, hot spots and super volcanoes: