#supereruption

Supereruptions can be primed in decades.

In the first part (http://tinyurl.com/p88v7vq) we discussed how volcanoes can be filled with magma directly from the mantle over a short period of time. Here we discuss a similar process in supervolcanoes. Supervolcanoes produce devastating eruptions that have regional effects such as covering large areas in metres of burning ash, as well as global effects on climate. The last supereurption, that of Toba in Indonesia, occurred around 70,000 years ago is thought to have reduced the global human population to less than 5,000, as evidenced by a genetic bottleneck. Over 2,500 cubic kilometres of rock were violently ejected onto the Earth's surface, producing metres deep layers of ash as far away as India. For comparison, Krakatoa's explosion produced about 12 cubic Km.

Santorini erupted more recently in the Bronze age 3500 years ago, with powerful impacts on nascent Mediterranean civilisations (though only 60 cubic Km of rock flew out). Though this island is small by supervolcano standards, it remains the most recent large scale eruption of silica rich magma available for study, and shares similar processes with its larger cousins. As the magma chamber emptied its roof collapsed like a piston, taking most of the island down with it and generating a large tsunami. The small island in the centre of the caldera is an emergent lava dome from renewed activity deep below.

Modern geologists have never witnessed a catastrophic supereruption, since the rest period between explosions is long (18,000 years between Santorini's last two blowouts). Geophysical data from currently simmering examples such as Yellowstone and Campi Flegrei near Naples has proved hard to interpret. Researchers are therefore using geochemistry in order to understand how these enormous explosions developed in the past, and published a paper in Nature last year. They hope to use this knowledge in assessing volcanic hazards at active supervolcanoes worldwide and to develop understanding of the pre-eruptive phase of these powerful beasts.

The team analysed the diffusion of elements such as titanium through crystals of alkali feldspar in the lava from Santorini, hoping to learn about its journey from the mantle. Lavas tend to stop and stew in their own juices on the way up, especially felsic (silica rich) ones. They were shocked to discover that rather than the expected thousands of years, the magma only took about a century to rise from mantle to surface. They also showed that large changes in magma composition can happen very quickly. The chamber was recharged from below as expected, different magmas were stewing and mixing, but all in a much shorter time frame than expected.

This has obvious implications for monitoring and prediction at the world's dozen or so known supervolcanoes. Deep movements of magma are now clearly a priority target for monitoring, and will be betrayed by the earthquakes they cause. However, we need to be listening in order to make use of the warning they provide. Many volcanoes in the Andes for example are poorly monitored, and a large, sudden eruption in a remote area could impact the world's climate and affect us all

Loz

Santorini Caldera image credit: SOKOBAN/Wikimedia Commons

http://www.livescience.com/31111-super-volcano-eruption-prediction.html

http://whyfiles.org/2012/reading-magma-predicting-giant-eruptions/

These tools, excavated from a site in central India, may have been continuously produced between 80,000 and 25,000 years ago.

Chris Clarkson

When Mount Toba erupted on the Indonesian island of Sumatra some 74,000 years ago, ash fell like snow on the Indian subcontinent, including on human toolmakers who had shaped stone flakes into sharp cutting instruments. Debate over the identity of these craftspeople—and whether a cataclysmic “volcanic winter” wiped them out—has raged for decades, because it has implications for when our species first left Africa. A new study of these people’s tools suggests they not only survived the eruption, but thrived for another 50,000 years. Others, however, say there isn’t enough evidence that the tools were made by Homo sapiens at all.

The researchers may have found “an early wave of modern humans … or it might be another kind of early human altogether,” says Martin Richards, an archaeogeneticist at the University of Huddersfield, Queensgate, who was not involved with the work.

Some scientists have argued that the massive ash cloud from the Mount Toba eruption would have partially blotted out the Sun, sending global temperatures plummeting and threatening the survival of numerous species, including our own.

In 2007, though, anthropologists found evidence of stone tools in southern India that dated to before and after the eruption, suggesting the event may not have been as devastating as previously thought. But critics said it wasn’t clear whether the tools were made by our species or another archaic human such as Neanderthals or Denisovans.

In the new study, the team behind the 2007 study returned to the subcontinent, to a site known as Dhaba in central India on the banks of the Son River. The researchers uncovered thousands of stone flake tools used for cutting and scraping.

Most of the tools in the lower sediment layers are consistent with a specific production method known as the Levallois technique, which was used by both modern humans and Neanderthals during the Middle Stone Age, about 250,000 to 25,000 years ago. In the upper layers, however, the tools transition to smaller, more complex stone blades that are unquestionably made by our own species, the researchers argue.

Using a technique called optically stimulated luminescence, which measures electrons to infer when layers of sediment were last exposed to light, the researchers dated the site to a continuous occupation stretching from about 80,000 to 25,000 years ago. The dates, combined with the uninterrupted manufacture of increasingly complex tools, suggests modern humans were not only present in the region when Toba exploded, but that they survived for many thousands more years, the researchers report today in Nature Communications.

The findings bolster arguments by others, including geochronologist Kira Westaway at Macquarie University, that some members of H. sapiens made it as far as northern Australia some 65,000 years ago—though these early migrants apparently died out without leaving any traces in living people. “This paper finally connects the dots between India, southeast Asia, and Australia for modern human dispersal,” Westaway says.

Team leader Michael Petraglia, an anthropologist at the Max Planck Institute for the Science of Human History, suggests the eruption wasn’t so devastating. “The impact of the Toba eruption on climate has been overestimated,” agrees Chad Yost, a geoscientist at Arizona State University, Tempe, who was not involved with the work.

Stanley Ambrose, an anthropologist at the University of Illinois, Urbana-Champaign, and a fierce critic of the team’s 2007 study, remains unconvinced. He says it’s impossible to tell which species made the tools based on the evidence presented. What’s more, he questions the dates because the team found no well-defined layer of ash in the sediment, which would have more conclusively shown that the toolmakers’ occupation spanned the eruption.

Richards doesn’t doubt the team’s dates, but says identifying these toolmakers will require human fossils. “Without them, we can’t really be confident that we’re talking about modern humans [or] some other archaic hominin.”