What’s brewing deep underground

On 18 May 1980, the world held its breath: Mount St. Helens in the US¡¯ Pacific Northwest was the site of one of the biggest natural disasters of modern times. Two months after the first warning signs that the dormant volcano was awakening, an earthquake of magnitude 5.1 shook the mountain. The unstable north flank of the mountain collapsed in a massive slide, uncorking it as a giant champagne bottle. A stupendous lateral blast blew away the mountain¡¯s summit and a surging mix of molten rock and gas devastated nearly 400 km² of forest. Within a short time, snow and ice that covered the summit thawed. Meltwater, debris and ash ¨C a mix known as lahar ¨C rushed down the valley floor, sweeping away everything in its path. The eruption led to 57 deaths and thousands of animals also perished.

Researching the magma system

In recent decades the mountain has calmed down, but it has not been completely dormant. It erupted again between 2004 and 2008, albeit less violently than in 1980. Mount St. Helens has remained a fixture or scientists¡¯ radar screens and they have followed the activity of the volcano to better understand it. But it hasn¡¯t been enough: Mount St. Helens remains unpredictable. Geologists are now hoping to change that. With the major external page iMush project, they want to create a high-resolution, three-dimensional image of the magma system that feeds the volcano down to a depth of 60 km to 70 km. ETH volcanologist Olivier Bachmann and his group are taking part in the project.

To date, only about the first 10 km under the volcano have been mapped; however, the resolution of the images gathered so far remain relatively low. And the scientific community knows even less about how the magma system is structured at greater depths and how the magma rises. The more scientists know about the structure of the magma system below Mount St. Helens, the better they will be able to read the warning signs that magma is rising and that an eruption might be imminent. The insights that geologists gain on this mountain should help scientists better predict eruptions not only of Mount St. Helens, but also other volcanoes in the Cascade Arc along the Pacific coast of North America and even worldwide.

Measuring earthquakes and magnetic flows

Using a combination of different methods, the researchers hope to unlock the secret of Mount St. Helens¡¯ destructive potential. Over the coming summer, they will install more than 2,500 seismometers to monitor the seismic activity in a 1,000 km² area surrounding the volcano. To take magnetotelluric measurements, they will position several thousand probes that register electrical and magnetic fields and they will detonate explosive charges positioned in 25 metre-deep drill holes at several locations around the mountains. This will generate seismic waves that are reflected or deflected on the rock; the different reflections can then be measured and will provide useful data on the structure of the sub-surface ¨C for instance, whether magma is present.

Management of the joint project is located at the University of Washington. In addition to Bachmann¡¯s group, scientists from other US universities and the US Geological Survey (USGS) are also taking part. Bachmann is to studying the rocks that have been spew out over the millennia and doctoral student Maren Wanke is collecting rock samples on Mount St. Helens and the surrounding area over two seasons. She then analyses the chemical and mineralogical properties of the rocks in the laboratories at the ETH.

Minerals reveal volcanic history

¡°The minerals contained in the rocks are data loggers,¡± says Bachmann. From the minerals it can be determined how they were formed, the depth from which they originate and the water, pressure and temperature conditions to which they were exposed. ¡°The trick is to extract this information,¡± says the ETH professor.

Rock samples collected by Maren Wanke in the summer of 2013 were also analysed by student Baptiste Lemirre as a part of his of his six-month master¡¯s thesis project. The initial data look promising. Some of the minerals contained in the rocks are between half a millimetre and two millimetres in size, some are even smaller than that, down to 0.2 millimetre. They weigh just a few dozen micrograms. These amounts are sufficient to enable the scientists to determine the age and chemical composition of the minerals and thus the origin of the rocks.

Results coming in 2016

¡°The rocks help us distinguish the periods of volcanic activity and dormant phases, because cycles of eruptions are reflected in the composition and the mineralogy of the rocks,¡± says Wanke. That, in turn, makes it possible to determine what phase of its cycle the mountain is currently in based on the most recent discharge. ¡°These rocks from the past are the key to understanding the present,¡± she emphasises. To gather even more evidence about the volatile life of Mount St. Helens, she will undertake her second collecting trip in late July for a period of four weeks. But whether the researcher has found the right key will take about two years to determine, with the first definitive results expected from 2016.