Page 2: The effects of volcanic ash on the atmosphere
“The rock sample to be analyzed is placed the lower chamber – an autoclave or pressurized furnace – in which it is heated under pressure,” Dingwell explains. The steel chamber on top essentially represents the atmosphere into which the fragmented rock is ejected by the explosion. “And the rupture disks form the bung in the system,” Dingwell adds. “The magma in volcanoes that erupt explosively is usually very viscous, and it can block the vent like a plug. As magma continues to rise in the chamber, the pressure exerted on the plug increases, and at some point the plug gives way – and the volcano erupts. The rupture disks simulate the effect of the magma plugging the vent. They ensure that the pressure is maintained as the rock sample is liquefied and begins to foam, and they allow the experiments to be controlled and calibrated.”
Don Dingwell can’t put a figure on the number of samples of igneous rock collected on the flanks of volcanoes all over the world that have been analyzed in the apparatus. The basic experimental protocol is always the same. Samples can be heated to temperatures of up to 900°C in the autoclave, while the pressure inside the vessel is gradually increased – to as much as 50 megapascals – by means of a gas line. Ultimately, the molten rock is explosively sheared into countless fragments which penetrate the fracture disks and are collected in the upper cylinder for subsequent analysis. The entire process, including pressure and temperature profiles, is controlled and documented by computer.
These profiles depict the overall course of each experiment, but Dingwell and his colleagues wanted to know more about each phase of an explosion. In particular they wished to understand what happens in the instant in which the particles of rock and ash are expelled from the vent. They therefore inserted a glass porthole at the point between autoclave and collector where the rock fragments emerge from the “vent”. The window is stable enough to withstand the shock wave – and allows them to photograph the ejection stream with a high-speed camera.
With the camera they can capture the detailed dynamics of the eruption plume, the clouds of ash and fragmented rock which, in the case of explosive eruptions, can reach heights of many kilometers. So the set-up now permits them to image “the volcanic ash at its place of birth and far beyond,” as Dingwell puts it. “For once the ash particles are in the system, we can allow them to react with the soil or ‘pedosphere’, the hydrosphere, the atmosphere and the biosphere.” Volcanic ash is very rich in minerals and can fertilize the soil, and the oceans. When the volcano Kasatochi on the Aleutian Islands erupted in 2008, the input of iron-rich ash triggered an algal bloom in the Gulf of Alaska. People and animals can also inhale very fine lava particles, which can cause chronic asthma and other respiratory diseases.
The effects of volcanic ash on the atmosphere are particularly serious, because it can have a far-reaching impact on the world’s climate. By reflecting solar radiation, and thus reducing the amount that reaches the Earth’s surface for periods of months or even years, volcanic ash causes a fall in global temperature. The outcome – as large-scale eruptions in the past have shown – is cold summers and poor harvests. As mentioned above, volcanic ash also presents a threat to air traffic. It acts like sand-paper on the exposed metal surfaces of aircraft and scratches windows, rendering them opaque. The fine particles can also clog airspeed and altitude sensors and affect their function. When drawn into the turbines, they melt and form a coating on the blades, which have a drastic impact on engine performance. Indeed, on three occasions between 1973 and 2000, volcanic ash led to a complete loss of engine power. In June 1982, a British Airways Boeing 747-200 got too close to an eruption plume emitted by the Galungung volcano on Java. When the engines cut out, the plane dropped from an altitude of 11 km to 4 km. In the denser air at that height, the pilot succeeded in restarting the engines, and landed the plane safely in Jakarta.
“Early on, when we used the high-speed camera, at 10,000 frames per second, we noticed peculiar white flecks between the flying ash particles,” says Bettina Scheu, one of Dingwell’s closest collaborators. Then the team members realized that they were looking at lightning flashes. “It was an ‘electrifying’ moment,” she says, “but was it a freak finding or something more interesting?” The team repeated the experiment, taking pictures at up to 60,000 frames per second – and found that very many of the flashes occurred immediately above the fracture disk – in other words, directly above the vent.
The surprising finding made waves among volcanologists, who have always been fascinated by the spectacular thunderstorms that often accompany the eruption of volcanic plumes. Up to now, the only way to study them was to fly close to the rising plume – a pretty risky undertaking. Now that volcanic lightning can be generated in the laboratory, one can elucidate the mechanism responsible for it.
“However, the flashes we observe directly above the vent have nothing to do with the lightning that is seen high up in the plume, which probably results from frictional interactions between ash particles and collisions with ice crystals,” says Dingwell. The reason for the flashes close to the outlet of the vent is charge separation. Due to turbulence at high pressure in an extremely dense cloud of particles, small particles end up negatively charged, while larger fragments acquire a net positive charge. “The two classes interact, rub against one another – and this combination leads to charge separation and to lightning,” Bettina Scheu explains.
With the help of two antennas installed in the vent of their laboratory volcano, Dingwell and his colleagues were able to document the electrical discharges that are the basis for the flashes – including those that occurred in the parts of the plume that were hidden from the eye of the camera. The researchers discovered a simple correlation between the average size of the particles and the frequency of the flashes: the smaller the particles, the greater the number of flashes. Conversely, the more flashes appear at the vent outlet, the higher is the proportion of very fine ash in the plume.
This result could allow one to estimate the level of risk to air traffic posed by a given eruption, as it is the class of extremely fine ash particles that are propelled to heights of up to 9000 meters, the altitude at which commercial airliners routinely fly. And these particles stay there until the next rainstorm washes back down to earth.
Corrado Cimarelli, a member of Dingwell’s team who has made a significant contribution to the laboratory experiments, is currently installing the high-speed camera on Sakurajima, a highly active volcano in Japan. He wants to find out whether the same phenomenon occurs in nature, with the help of colleagues from the US and Japan, who are observing the peak with aid of antennas.
The Munich volcanologists now plan to focus on what else happens to the ash within the plume. “At the moment, among other things, we are looking at how the particles of ash interact with the sulfur- and chlorine-containing gases in the plumes,” says Dingwell. Specialists in other disciplines, such as biologists and hydrologists are also involved in these studies. “These are among the important issues for experimental volcanology in the near future. At all events, volcanic ash will keep us busy for years to come,” he says. By Angelika Jung-Hüttl / Translation: Paul Hardy
Prof. Dr. Donald Bruce Dingwell
Chair of Mineralogy and Petrology and Director of the Department of Earth and Environmental Sciences at LMU. Dingwell (b. 1958) studied Geology and Geophysics at the Memorial University of Newfoundland, and obtained his PhD at the University of Alberta in Edmonton. He was Managing Scientific Officer of the Bavarian Geoinstitute at Bayreuth University, completing his Habilitation there in 1992, before moving to Munich in 2000. In 2009, he received an Advanced Investigator’s Grant from the European Research Council (ERC), and served as the ERC’s Secretary-General from 2011 until 2013.