Lava domes and their drumbeats
The emplacement of lava domes below volcanic summits is often accompanied by periodic swarms of micro-earthquakes. New work involving LMU researchers shows that this “drumbeat seismicity” reflects the stop-and-go nature of magma´s final ascent.
Mount St. Helen (Photo: USGS Cascades Volcano Observatory)
Burgeoning lava domes constitute a major volcanic hazard, but the dynamics of dome-building is highly unpredictable and poorly understood. Dome development is initiated when highly viscous lava within the mass of rising magma blocks its further ascent. As the pressure rises behind the plug, the roof of the dome may suddenly fracture, resulting in an explosive eruption. The formation of lava domes is often accompanied by recurrent sequences of small-scale earthquakes, a phenomenon which has been well documented at Mount St. Helens in the Pacific Northwest of the US. “The origin of this repeated seismic activity, which appears to arise in the same localized region of the conduit, has remained unexplained until now,” says Professor Donald Dingwell, Director of the Department of Earth and Environmental Sciences at LMU.
In collaboration with researchers at English, Italian and Japanese institutions, Dingwell has uncovered the mechanism that underlies such “drumbeat” seismicity. The first clues were provided by samples of so-called “pseudotachylites”, which were collected from the roof of a lava dome on Mount St. Helens. This glassy silicic rock is produced by frictional heating at fault surfaces, and its presence suggested the occurrence of episodes of frictional heating either between heterogeneous blocks within the rising magma or between the magma and the rock that forms the conduit wall.
Successful laboratory simulations
“With the aid of new techniques developed in our laboratories, we were able to characterize the mechanism responsible for this phenomenon,” Dingwell says. The study focused on lava samples obtained from domes at Mount St. Helens and the Soufriere volcano on the Caribbean island of Montserrat. By characterizing their response to frictional shearing, the researchers showed that, during the build-up of a lava dome, magma does not ascend continuously but spasmodically. In other words, the lava rises in fits and starts. This happens because, in localized regions, frictional heating at contact surfaces between solid magma and the conduit wall becomes so significant that the magma melts. The melt is highly viscous and acts as a brake to reduce the rate of frictional slippage. The melt then solidifies, causing the pressure to rise again. At some point, the plug fractures and sliding abruptly resumes.
“We were able to reproduce this cycle of sticking and slippage in the laboratory, as the magma repeatedly solidifies and abruptly fractures at the same point in the conduit,” Dingwell explains, “and this mechanism accounts for the hitherto enigmatic sequences of weak earthquakes that have been observed during the formation of volcanic domes.” The researchers hope that these insights will contribute to improvements in eruption forecasting and the assessment of volcanic hazards.
(Nature Geoscience 2014) göd