What’s up in Bolivia?

James Hickey is a PhD student in the School of Earth Sciences at the University of Bristol. A geophysicist and volcanologist by trade, his PhD project is focussed on attempting to place constraints on volcanic unrest using integrated geodetic modelling.

To many, Bolivia is just an unassuming landlocked country in South America, perhaps most famous for its coca tea obsession and ‘gap yah’ alpaca wool sweaters. But to a number of enthused volcanologists it is a near-perfect playground. In the southwest of the country, sitting at 6008 m above sea level (ASL), Uturuncu volcano is inflating, and inflating over an unprecedented scale.

Uturuncu Volcano in the background with a gravimeter and campaign GPS setup in front. Image credit: James Hickey.

Interest first sparked in 2002 when it was initially found to be uplifting over a 70 km wide area following a satellite geodesy (InSAR) study. Volcanic inflation can often be a cause for concern – indicating a change in the underlying magmatic system is causing a build up of stress which is partly accommodated by the strain that manifests itself as deformation at the surface. This by no means implies an eruption is inevitable; however, numerous volcanoes inflate without erupting, and there are those that erupt without inflating. What was so special about this volcano was the size of the area that was uplifting!

A cartoon of how volcanic inflation affects the surrounding slopes, and how this is picked up by monitoring equipment. Image credit: USGS.

This lead to the initiation of a large, multi-disciplinary study (PLUTONS) to probe the volcano further, incorporating some of the world’s leading volcanologists. 10 years on we now have a much more informed picture of the dynamics driving the volcanic unrest at Uturuncu. The investigation was based on lots of fieldwork, incorporating the following:

• Geophysics (trying to work out what’s deep underground with fancy equipment)
• Geodesy (measuring the deformation in space and time)
• Geochemistry (bashing up rocks and working out what they’re made of)
• Geomorphology (inferring how the landscape has evolved through time)

More detailed exploration of the deformation uncovered a rather unique pattern, over an even wider 150 km scale. Mimicking the shape of a sombrero, the inner 70 km are uplifting while an outer ring of subsidence exists as well. Such a distinctive deformation pattern could allude to a very specific driving force.

It is thought that magma about 20 km below the surface is buoyantly rising up, much like the coloured blobs inside a lava lamp. This buoyant driving force is driven by the density difference between the magma and its surrounding host rock. The rising magma then causes uplift at the surface directly above it, while the surrounding subsidence is thought to be caused by the back flow of hot, ductile, crustal rocks that are pushed aside and down by the magma as it forces its way upward.

This specific process of magma migration is also backed up by geophysical observations. Investigations into minute changes of the gravity field around the volcano indicate areas of anomalous density in the subsurface. This has revealed vertically-elongated 15 km wide zones of low-density that resemble ‘fingers’, and have been inferred to represent regions of partially molten magma extending upward toward the volcano.

Similar patterns have also been observed in magnetotelluric studies. These investigate the subsurface for differences in resistivity; the amount a material opposes the flow of electric current. A low resistivity bulge is apparent directly beneath Uturuncu, and there are comparable low resistivity ‘finger-like’ zones too. All these low resistivity areas are believed to represent accumulations of magma.

The combination of observations from deformation, gravimetry and magnetotellurics allows a more robust conclusion to be drawn. All seem to indicate the rise of magma in a manner similar to that of a lava lamp. In a real world setting, this is known as diapirism, and represents a very specific method of magma migration. The fact that this is being monitored and studied in real-time is a very exciting research opportunity, affording volcanologists a unique insight into the process. With studies continuing, new and improved conclusions will surely become apparent. This helps build our understanding of how magma moves and accumulates which can be fed into better eruption forecasting and hazard monitoring.