Around 74,000 years ago, a massive volcano on Indonesia’s Sumatra Island erupted, spewing thousands of cubic kilometers of material into the atmosphere and plunging the Earth into a "volcanic winter," a period of prolonged cooling. The Lake Toba event, which is thought to have been the largest eruption in the past 25 million years, extinguished a significant share of the planet's early inhabitants, including almost 60 percent of all humans, and thoroughly disrupted the climate system, reducing global temperatures by 3 to 5�C.
Though geologists have been studying calderas—or supervolcanoes as they are more commonly known—for years, their understanding of the basic geophysical processes underlying eruptions and magma dynamics has been limited. Considered among the most violent geologic events, supervolcanic eruptions have occurred every few hundred thousand years and are believed to have triggered mini-ice ages at several points in Earth's history.
The recent discovery of an area of crustal uplift in the Italian Alps' Sesia Valley has, for the first time, allowed researchers to peer deep below the surface at a fossil supervolcano's "plumbing"—the networks of tracks and trails that magma follows as it pushes upwards through the crust. The study, led by James Quick of Southern Methodist University, was detailed in the July issue of Geology.
The crustal uplift, which exposed rocks as deep as 25 km underground, was formed when the continents of Africa and Europe began colliding around 30 million years ago; the enormous friction this generated pushed Italy’s crust up. It granted the authors an unprecedented insight into magmatic underplating, the process by which erupted magma is driven by the intrusion of mantle-derived magma in the deep crust.
Quick and his colleagues collected five different depth samples and determined their ages using a sensitive high-resolution ion microprobe (SHRIMP), a mass spectrometer that blasts rocks with an ion beam to measure the isotopic and elemental abundances of minerals. Their analysis revealed that several magmatic events—the movement of mantle-derived melt into the deep crust—took place during the Permian period between roughly 289 and 284 million years ago.
The intrusion of mantle-derived magma at depths below 15 km induced anatexis, the partial melting of rocks, in the continental crust. This produced silica-rich melts that slowly made their way towards the surface, triggering intermittent eruptions between 289 and 280 million years ago. These repeated events were interspersed with periods of slow cooling and deformation in the crust. Crustal melting proceeded apace for several million more years until the rocks eventually crystallized during the early Permian.
These results agree with other studies that have shown that volcanic activity involving large silicic complexes elsewhere in the Alps was common at the time. Similar analyses of other supervolcanoes, including active ones in Long Valley and Yellowstone, could help geologists better interpret geophysical profiles and, more importantly, predict future eruptions.
Geology, 2009. DOI: 10.1130/G30003A.1
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