Super volcano: Self control revealed by synchrotron

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  • Published: Jan 15, 2014
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Super volcano: Self control revealed by synchrotron

Risky and eruptive

Supervolcano eruptions driven by melt buoyancy in large silicic magma chambers

The risk of a supervolcano eruption is, according to geochemical synchrotron X-ray studies carried out by scientists in Europe and Japan almost entirely associated with magma pressure and needs no external trigger. Details are reported in the journal Nature Geosciences this month.

A super volcano is defined as a geological feature that can produce more than 1000 cubic kilometers of ejected material in an eruption; this is three orders of magnitude greater than the ejecta volume of the volcanoes including the likes of Mount Etna on Sicily, Eyjafjallajökull in Iceland and Montserrat's Soufrière Hills volcano, which themselves erupt with devastating consequences. There only a few super volcanoes on Earth, but the lava and ash released in an eruption could cover a continent, perhaps trigger a mini-ice age by reducing how much sunlight reaches the earth and are predicted as being extinction-level geological events. For comparison the eruption of Mount Pinatubo in the Philippines in 1992 led to a decrease in average global temperature of 0.4 degrees Celsius. This effected last for several months. Geologists predict that a super volcano eruption would eject enough material to cut average temperatures by 10 degrees Celsius for a decade. These rare events rank alongside giant meteorite impacts as being among the biggest natural disasters that can occur on Earth.

There are thought to be two mechanisms by which a super volcano might erupt although details are unclear and seemingly the behaviour is very different from that taking place in a "conventional" volcanic eruption such as Mt Pinatube or Etna. It is possible that when the magma in the mantle rises into the crust from a hotspot and cannot break through the crust that pressure is able to build in a vast magma pool until it ultimately bursts through, as with the Yellowstone Caldera, USA. Alternatively, a super volcanic region might form where tectonic plate boundaries meet, for example, Toba on the Indonesian island of Sumatra.

Volcanic impact

Either way, given the potential impact on the environment and life on earth scientists are keen to understand the behaviour of super volcanoes in more detail so that they might a precise way to get an advance warning of an imminent eruption. The most recent super volcano eruption was in New Zealand more than 26,000 years ago - Lake Taupo on North Island. One might say that other known super volcanoes are worryingly long overdue for an outburst. Wyoming's Yellowstone last erupted 640,000 years ago but geologists are only too well aware that historically it seems to become active on a cycle lasting 600,000 years, although that is based on evidence from three past eruptions, so there is significant room for error in the timing estimates.

Of course, even the most imaginative science fiction writer has not devised a credible intervention that might avert a super eruption, so said a 2005 report by the Geological Society of London. But, the report did suggest more positively that, "We can, however, work to better understand the mechanisms involved in super-eruptions, with the goal of being able to predict them ahead of time and provide a warning for society. Preparedness is the key to mitigation of the disastrous effects of a super-eruption."

Wim Malfait and Carmen Sanchez-Valle of ETH Zurich and colleagues from the Paul Scherrer Institute in Villigen, Switzerland, Okayama University, Japan, the Laboratory of Geology of CNRS, Université Lyon and ENS Lyon and the European Synchrotron (ESRF) in Grenoble, France, have reproduced the conditions within the magma chamber of a super volcano to understand what it takes to trigger its eruption.

Critical to the behaviour of a super volcano is that vast magma chamber within is always located in an area where the heat flow from the interior of the Earth to the surface is very high. As a consequence, the magma chamber is very large and hot but also plastic: its shape changes as a function of the pressure when it gradually fills with hot magma. This plasticity allows the pressure to dissipate more efficiently than in a normal volcano whose magma chamber is more rigid. This is an advantage to life on earth because it means super volcanoes do not erupt very often. However Malfait suggests that something changes to lead to an eruption. "The driving force is an additional pressure which is caused by the different densities of solid rock and liquid magma. It is comparable to a football filled with air under water, which is forced upwards by the denser water around it," he explains. Whether this additional pressure alone could eventually become sufficiently high to crack the Earth's crust, leading to a violent eruption, or whether an external energy source like an Earthquake is required had not been ascertained before.

Extinction level

Whilst it is virtually impossible to drill a hole into the magma chamber of a super volcano given the depth at which these chambers are buried, one can simulate these extreme conditions in the laboratory. "The synchrotron X-rays at the ESRF can then be used to probe the state - liquid or solid - and the change in density when magma crystallises into rock," explains ESRF scientist Mohamed Mezouar. Temperatures of up to 1700 degrees Celsius and pressures of up to 3.6 gigapascals (36,000 times atmospheric pressure) can be achieved inside the so-called Paris-Edinburgh press. In this device a speck-sized rock sample is held between the tips of two tungsten carbide anvils and then heated with a resistive furnace. This allowed the researchers to accurately determine the density of the liquid "magma" over a wide range of pressures and temperatures. Magma often includes water, which as vapour adds additional pressure. The scientists also determined magma densities as a function of water content.

The measurements suggest that the pressure resulting from the differences in density between solid and liquid magma rock is all that would be need to crack many kilometres depth of the Earth's crust above the magma chamber, no external geological phenomenon, shifting plate or earthquake would be needed, just a sufficiently high build up of heat and pressure. The research has thus shown that the pressure is actually large enough for the Earth's crust to break. The magma penetrating into the cracks would eventually reach the Earth's surface, even in the absence of water or carbon dioxide bubbles in the magma. The team explains that as this material rushes towards the surface, the magma will expand violently as the pressure is released ejecting enormous amounts of materials kilometres into the air above.

"The next advances in understanding these systems will most likely come from improved numerical simulations that include more physics and material properties than have been included so far (our density data would be an input for those) and  ontinuing studies of the eruption products of past eruptions," Malfait told SpectroscopyNOW. "The ultimate goal would be to make predictions about what precursors we may expect as the magma chamber evolves and approaches an eruption," he adss, "How much ground deformation, volcanic tremor, degassing do we expect and how long in advance? This is quite a way off at the moment, but then again, there is probably no real urgency either."

Related Links

Nature Geosci, 2014, online: "Frequency and magnitude of volcanic eruptions controlled by magma injection and buoyancy"

Nature Geosci, 2014, online: "Supervolcano eruptions driven by melt buoyancy in large silicic magma chambers"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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