The recent (19 September - 13 December 2021) eruption of the Cumbre Vieja massif on the island of La Palma was undoubtedly the most impactful volcanic event of 2021, and in the grand scheme of things, potentially the most costly eruption in the last decade. When weighed in terms of the number of evacuated and displaced people, infrastructural damage, and total economic loss of local industries, conservative impact estimates approach 1 billion Euros (European Commission), which eclipses the impact of the 2018 eruption of Kilauea on the big island of Hawaii. Natural events of this magnitude underscore the need to understand why and how volcanoes erupt, and in turn, how magma itself behaves as it migrates through feeder and eruptive portals to the Earth's surface and atmosphere. Much of the destruction on La Palma was dealt by rapidly advancing lavas. Our study on La Palma began with field observations and sampling of lava and ash from the complex system of vents that were erupting in mid- to late-November, 2021. We wanted to find out 'what' was erupting, both in terms of the lava's composition and physical properties. Our initial impressions were that the 2021 Cumbre Vieja lava was not like that of typical basaltic eruptions, ie., the emergent lavas appeared faster, thinner, and much more fluid than historical Hawaiian lavas that we had worked on in the past. Put another way, the lava exhibited strange "rheological" behavior.
Rheology is the study of the deformation and flow of matter. The term “rheology”, stemming from the Greek “rheo”, or flow, should elicit everyday experience with the flow of matter. Look no further than the rheostat on your wall, a device that moderates the electrical flow or current to a central heating system. However, in addition to being a scientific framework to characterize matter and how it behaves under stress, rheology encapsulates an important intensive property of magma that dictates nearly every volcanic process and thus the course of eruptions: Viscosity. In the most simple terms viscosity is a macroscopic physical property that governs a material's response to stress; low-viscosity materials flow easily while high-viscosity substances flow less readily.
It turns out that Cumbre Vieja's eruption produced a particularly low-viscosity form of mafic magma, a nepheline-normative basanite, which means that it is depleted in SiO2, and somewhat enriched in alkali-group components, like Na2O and K2O. Those compositional characteristics foster high-fluidity, as do other variables such as the high eruption temperature (~1150-1200˚C), which we divined by analyzing crystals in the ash.
Photo bottom: Scanning electron photomicrograph of a Cumbre Vieja basanite ash fragment. Present are glass (light grey) and crystalline phases (clinopyroxene, light grey striped; plagioclase, dark blocky; olivine, medium grey) that collectively record temperature (~1150-1200˚C) and pressure (~10 kbar) information of the erupted magma.
What we observed in the field on 18 November 2021, was however, even more impressive than chemical and thermal data might express. At about 8:30 p.m. local time, we witnessed two newly formed vents feeding rapidly advancing lava flows (~7-10 m/s), vigorous fire fountains, and spattering activity. These phenomena are best shown in the masthead image at this article's top, which is a montage of close-up frame grabs from video collected by public Radio Televisión Canaria (RTVC); one can also view a field image of this activity below. As shown in RTVC's live eruption feed (recording here: https://www.youtube.com/watch?v=19kgU-4uUOI), and in accordance with our own on-the-ground observations, the new erupting vents emitted an extraordinary glow, almost white and much brighter than the long-lived fire fountains erupting at the time. This radiance signaled higher temperature magma (upwards of 1200˚C according to geothermometry), and given what we know about temperature and magma viscosity, very fluid magma. Indeed, the twin lava streams seemed to effortlessly cascade down the steep crater wall, and for the better part of thirty minutes, the lavas produced repeated, tidal-bore-like structures, that to our knowledge, have never been documented before. These blunt and arcuate flow surges (see the white arrows in the top image), along with the speed at which they were propagating, indicated a magma low in viscosity and rheological behavior reminiscent of water drops wetting a window on a rainy day.
Thus, in our study, we aimed to "ground truth" our visual observations of highly fluid lava behavior by measuring magma viscosity in the lab. Our measurements were made on a high-temperature furnace and rheometer assembly designed to bring samples of basanite ash back to molten magmatic conditions. Our laboratory investigation involved filling a crucible with ash, melting it, and then introducing a platinum-rhodium spindle into the melt that, when rotated by the rheometer, generates torque that can be sensitively measured and converted to a viscosity value. We performed these experiments across a range of temperatures that span super-liquidus (no crystals) to sub-liquidus (crystals present) conditions. Our viscosity measurements show that for both dry and hydrous magma, the Cumbre Vieja's basanite viscosity would be at most a few tens of Pa sec for the permissible eruption temperatures (~1150-1200˚C). Minimum permissible viscosities according to our analysis are less than 10 Pa sec. Examples of how we run our rheological measurements can be found in the video links here: https://www.youtube.com/watch?v=UOUT4eqX9kA&t=4s and here: https://go.nature.com/3sW3yrz
Our experiments indeed confirmed that the Cumbre Vieja basanite erupted with a very low-viscosity, having values (<10-160 Pa sec) that are at least ten times lower than recently (2018) erupted basalt at Hawaii. These findings help explain why the lava erupted with relative ease, why it advanced so quickly and posed severe destructive potential, and why it exhibited odd flow behavior, including hydraulic jumps (standing waves) and cascading bores that have only rarely been observed.
An important outcome of knowing a magma's viscosity is that we can determine its flow regime(s) by calculating various dimensionless physical parameters. One such parameter is the Reynolds number (Re), which weighs the relative importance of inertial to viscous forces. The size of Re at a given set of conditions (e.g., viscosity, fluid density, length scale) will indicate a flowing magma's proximity to the transition from laminar to turbulent (eddy producing) flow. Most lava on Earth flows in the laminar regime. However, with our measurements of viscosities and field-based estimates of flow rates, we found that the Cumbre Vieja basanite may have been periodically flowing in a transitional, high-Re regime where turbulence was important.
Water is an high-Re system owing to its low viscosity. Water flows are often characterized by turbulence and hydraulic jumps, the latter of which are so-called for the step function change in flow thickness occurring when the flow transitions from sub-critical to supercritical flow conditions. Tidal bores and swash-zone waves in a beach environment are natural examples of moving hydraulic jumps. While it remains to be proven that ultralow-viscosity lavas may also exhibit such dramatic structures, many of our observations of Cumbre Vieja's lavas suggest some qualitative dynamic similarity between flow regimes in volcanic and hydrologic environments. Ornamental fountains offer an illustrative analogy to the lava cascades at Cumbre Vieja (Photo below); here one can view a low-viscosity, high-Reynolds number flow on a water fountain: https://youtu.be/WeOEfNL4UO0. Notice how the blunt flow fronts in the successive sheet flows remain defined despite advancement of the flow. Such features could have been forming in a similar manner within the lava cascades at Cumbre Vieja, where the requirements of both low viscosity and high rates of advance were met.
Photo below: A water feature with vigorous foamy water erupting at center, and producing sheets of fast-moving water in radial directions. The fluid dynamics here are very much like those occurring on a beach, in the swash zone where one sees the blunt fronts of successive "bores" maintain their height with out-flow distance. These hydraulic jumps might be a good analogy for the features observed in the lava cascades at Cumbre Vieja.
A final implication of our study is the realization that mafic magma of alkaline affinity rising through the crust and mantle will likely be even less viscous than what we have determined for Cumbre Vieja's newly emergent lava, given that in subterranean conditions, cooling during magma ascent will be negligible, the concentration of hydrous species in the melt--the most important influencer of melt viscosity--will undoubtedly be higher, and pressure driving flow will be more intense. This all means that we are beginning to catch a glimpse of just how mobile alkaline basanite magma is in the Earth's interior. Such insights will hopefully lead to better eruption forecasting and interpretations of precursory seismic activity and other signs of unrest at active alkaline volcanic systems.
More details on this work can be found in our manuscript "Eruption of ultralow-viscosity basanite magma at Cumbre Vieja, La Palma, Canary Islands". Link: https://www.nature.com/articles/s41467-022-30905-4
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