Taking the Pulse of Andean Volcanoes

Yves Moussallam summarizes the Trail by Fire team’s findings to date.

It has been nearly two years since the Trail by Fire crew started preparing for their expedition along the South American Andes, which took place from October 2015 to February 2016 and for an additional month in February to March 2017. The team, Yves Moussallam (University of Cambridge, UK), Ian Schipper (Victoria University of Wellington, New Zealand), Nial Peters (University College London, UK), Philipson Bani (Laboratoire Magmas et Volcans, France), Aaron Curtis (NASA Jet Propulsion Laboratory, USA), Talfan Barnie (Nordic Volcanological Center, Iceland), and João Pedro Lages (Università deli Studie di Palermo, Italy), have published an initial suite of four papers. In this article, Yves Moussallam (recipient of the 2016 DCO Emerging Leader Award) summarizes the team’s findings to date.

Figure 1: Outlined map of South America showing the location of active volcanoes in relation to the Nazca subduction zone. Volcanoes targeted by the expedition are shown in red.
 

Quantifying degassing

The primary objective of the project was to quantify volcanic degassing at the scale of an entire subduction zone. Crucial in this endeavor was performing gas measurements at never-before measured volcanoes. Of those, satellite observations had indicated that the Peruvian volcanoes Ubinas and Sabancaya were particularly important gas emitters (Fig. 2). In November 2015, the team ascended these nearly 6000 m high volcanoes to measure their gas composition and fluxes. They found that these two volcanoes alone contribute two-thirds of all the gas released by volcanoes along the central section of the Nazca subduction zone. These two volcanoes also were the strongest gas sources the team encountered during their entire five-month expedition. A detailed investigation at Sabancaya revealed an intense and periodic degassing activity, suggesting magma convection at shallow depth (Moussallam et al., 2017b; view article here).

Figure 2: Monthly mean 0-4 km SO2 column amounts retrieved from IASI measurements in Peru and northern Chile for the months of April 2012 (left) and April 2014 (right). Ubinas and Sabancaya can be identified as the strongest SO2 sources in the region (Source: Moussallam et al., 2017b).
 

Surprises on the way

While volcanoes like Ubinas and Sabancaya were some of the main targets of the expedition, due to their known magmatic activity, others that were thought to be under quiet, hydrothermally dominated activity reserved some surprises. This was the case of El Misti volcano. This 5822 m high giant dominates the skyline of Arequipa, Peru’s second largest city, making it one of the world’s most dangerous volcanoes. From Arequipa, El Misti’s summit appears to hardly emit any gas. Its last major eruption was in the fifteenth century and the volcano has remained relatively quiet since. While climbing to the summit, the team wasn’t expecting much, assuming the gas coming out would be exclusively meteoric water being recycled through the volcano’s hydrothermal system. Within the crater was an old lava dome with a number of small vents releasing gases (Fig. 3). The first hint of something strange was the relatively high temperature of these vents, close to 300°C. Volcanic rocks can’t stay that hot for five hundred years…

Back in camp looking at the gas compositional data, the team realized that the gas coming out of the dome was clearly magmatic and not hydrothermal in origin. This means that the volcanic conduit at El Misti is permeable. Gases coming out of solution from the magma at depth are rising through a fracture network all the way to the surface. How a conduit made of such viscous rock has managed to stay permeable to gas for such a long time remains a mystery. The good news from a volcanic forecasting standpoint is that monitoring these gas emissions might prove useful for tracking any changes in magmatic activity at depth (Moussallam et al., 2017a; view article here).

Figure 3: Visible (A) and infrared (B) images of the lava dome of El Misti taken on 1 December 2015 from within the crater, looking south. The dome is about 150 m across. Note that the infrared image is saturated at 125°C (source: Moussallam et al., 2017a).   
 

Understanding magma motion and sustained degassing

The Trail by Fire team has a soft spot for lava lakes. Nearly all of the team members’ Ph.D. focused on volcanoes with lava lakes. While lava domes are the upper part of volcanic conduits in silicic volcanoes, lava lakes are the upper parts of the conduit of (usually) basaltic volcanoes. By studying lava lakes, volcanologists hope to understand how magma moves within volcanic conduits. The team hence made a long stop at Villarrica volcano, performing repeated ascents to acquire very precise time series observations of the gas flux, thermal emission, and gas composition emitted by the lava lake. At other lava lakes such as Erebus, Kilauea, or Ambrym, such time-series observations have revealed distinct periodicities operating at different time scales at each volcano. At Villarrica, however, the team found that the gas and thermal time series were non-periodic. This difference when compared to other lava lakes suggests a distinct conduit flow mechanism, whereby vigorous and turbulent mixing in the volcanic conduit results in steady gas and thermal emissions (Fig. 4). These observations suggest that a large variety of conduit flow mechanisms might operate at active volcanoes worldwide (Moussallam et al., 2016; view article here).

Figure 4: Schematic representation of the conduit dynamic and associated gas emission signature at Kīlauea, Villarrica, and Erebus lava lakes. Kīlauea’s Halema`uma`u’s lava lake exhibits gas-pistoning thought to be shallow driven and independent of conduit flow processes. The Erebus lava lake is sustained by a stable bi-directional flow, where new batch of magma regularly replace degassed sinking magma. The resulting gas signal oscillates periodically in term of both flux and composition with each episodic arrival of magma. The Villarrica lava lake appears to be sustained by continuous influx of magma turbulently and vigorously mixed within the volcanic conduit. The resulting gas emissions are stable or oscillate with no determined structure or periodicity. Right inset show the summit area of Villarrica at night (top) and the lava lake (bottom) (source: Moussallam et al., 2016).
 

The source of magmatic volatiles

There are two main reasons for attempting to measure volcanic gas emissions at the scale of a subduction zone. The first is quantifying the amount of gas emitted by volcanoes in the atmosphere and their impact on climate. The second is understanding what proportion of the volatiles contained in sediments and brought into the mantle are recycled at subduction zones. To answer this second question, measuring volcanic gas emissions is not enough: one needs to determine what proportion of these gases originates from the subducting plate. To obtain this percentage, the team carried a Delta Ray Isotope Ratio Infrared Analyser (IRIS) during the entire expedition. This instrument allowed them to measure the isotopic ratio of carbon and oxygen in the CO2 emitted by volcanic plumes. In turn, this information is used to determine the origin of the CO2 emitted. Did it come from the crust, the mantle or the subducting slab? The measurements the team made at several volcanoes of the central volcanic zone indicate that most of the CO2 originates from the subducting slab and has been efficiently recycled (Schipper et al., 2017; view article here).

Figure 5: δ13C versus δ18O of volcanic plumes, sources, and reservoirs. Diamonds show data measured by IRIS during the expedition, triangles show data from the literature (see Schipper et al., 2017 for references).  Our data shows that magmatic gas emissions from passively degassing volcanoes of the central volcanic zone are isotopically heavy, strongly suggesting a carbonate origin of the volcanic CO2 (source: Schipper et al., 2017)

The beauty of expedition-based science is that one never really knows what is going to be found. The only certainty is that the more we observe the natural world the more we learn. Stay tuned for more findings from the Trail by Fire that will be reported over the course of the next year.

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