Once a year, a Javanese priest invokes Mount Merapi to please the people around him and not afflict them with angry eruptions. For volcanologists, however, spirituality is not enough for precautionary work - new rock samples from Mount St. Helens may now offer them more certainty.
It has become quiet around the Merapi volcano, although it was barely three months ago that the developments around the Javanese fire mountain caused fear and terror among the local population. Every day, authorities and researchers expected a potentially deadly eruption from the volcano, which was constantly grumbling and sending clouds of ash skyward. The mountain now only blows embers, dust and rock out of its crater every few days. No volcano researcher can predict with certainty whether and when the next strong eruption will follow.
Slightly less spectacular, but still obvious, another horror mountain reported back in May of this year: In the great caldera of Mount St. Helens - which exploded on March 20, 1980 with the force of 350 megatons of TNT and in 84 human lives in its surroundings – a huge stone dome arched up almost overnight, which remotely reminded observers of a diving back of a whale. At times, the hump grew two meters per day - pushed up by increased magmatic activity inside the mountain. An indication of a second disaster scenario like 1980?
Mount St. Helens has also slowed down somewhat, but its risky inner workings continue to spur researchers like Jon Blundy and Madeleine Humphreys of the University of Bristol and Kathy Cashman of the University of Oregon at Eugene to come up with new analyses. For example, not all processes involved in the rise of magma in and under the volcano are understood: how does the molten rock react to falling pressure and degassing? And what influence does this have on the timing and strength of the eruption? Thermodynamic models of magma, which loses water vapour, carbon dioxide and other gases – and does not crystallize in the process – do not rule out either cooling or further heating of the glowing liquid.
Real measurements on site are not yet technically possible, after all it is about detecting temperature fluctuations within glowing molten rock, which can reach temperatures of up to 1200 degrees Celsius and sometimes roll through the subsurface from the point of access for kilometers.
In order to be able to make statements about the changing pressures, temperatures and crystallization processes in the volcanic magma chambers, the scientists had to use small melt bodies in feldspar crystals, which were formed during different eruption phases on Mount St. Helens and the Russian Shiveluch were ejected on Kamchatka. The chemistry and structure of the glassy rhyolites suggest that even after they were attached to the feldspars, they were still in contact with the surrounding magma until they were completely isolated - so both evolved chemically equivalent for quite some time.
Like a snapshot, the rhyolites record the geochemical and physical conditions of the melt shortly before the eruption. And they surprised the researchers: Apparently, the magma crystallizes as it rises in the volcanic vent simply because of the falling pressure and not because of the falling temperatures, because heat is released into the surrounding rock.
During this crystallization, lattice energy is released, which in turn heats up the molten rock. This additional heat supply can be up to 100 Kelvin without impairing the precipitation of the rhyolite or feldspars, because at the same time the proportion of crystalline structures in the magma increases by up to forty percent by weight.
The additional heating could be an important trigger for the eruption, Blundy and his colleagues now suspect, because it also influences the flow properties of the magma - for example by changing the speed of crystal and bubble formation in the melt. It also counteracts the increased outgassing of water vapor and carbon dioxide, which originally reduced viscosity and would now be missing as a lubricant. A further surge of heat from magma injections from below would then not be necessary to bring the volcano to the boil.
Crystal formation through pressure relief also takes place much faster than through cooling: This process only takes years instead of centuries and can therefore also be observed within a researcher's lifetime. And it is possible that the glassy inclusions will make it possible to better estimate the active phases and the development of dangerous volcanoes such as Mount St. Helens or Mount Merapi in the future. However, this does not rule out further spiritual support from soothing high priests such as on Mount Merapi.