New research suggests forces pulling on Earth’s surface as the planet spins may trigger earthquakes and eruptions at volcanoes.
Seismic activity and bursts of magma near Italy’s Mount Etna increased when Earth’s rotational axis was furthest from its geographic axis, according to a new study comparing changes in Earth’s rotation to activity at the well-known Italian volcano.
Earth’s spin doesn’t always line up perfectly with its north and south poles. Instead, the geographic poles often twirl like a top around Earth’s rotational axis when viewed from space.
Every 6.4 years, the axes line up and the wobble fades for a short time – until the geographic poles move away from the spin axis and begin to spiral once again.
This phenomenon, called polar motion, is driven by changes in climate due to things like changing seasons, melting ice sheets or movement from tectonic plates.
As polar motion fluctuates, forces pulling the planet away from the sun tug at Earth’s crust, much like tides due to the gravitational pull from the sun and moon.
The tide from polar motion causes the crust to deform over the span of seasons or years. This distortion is strongest at 45 degrees latitude, where the crust moves by about 1 centimeter (0.4 inches) per year.
Now, a new study published in AGU’s journal Geophysical Research Letters suggests that polar motion and subsequent shifts in Earth’s crust may increase volcanic activity.
“I find it quite exciting to know that while climate drives Earth’s spin, its rotation can also drive volcanoes and seismicity,” said Sébastien Lambert, a geophysicist at Paris Observatory in France and lead author of the study.
The new findings, however, don’t allow scientists to forecast volcanic activity. Although the study suggests earthquakes might be more common or volcanic eruptions may eject more lava when the distance between Earth’s geographic and rotational axes is at its peak, the timescale is too large for meaningful short-term forecasts, according to the authors.
But the results point to an interesting concept. “It’s the first time we’ve found this relationship in this direction from Earth’s rotation to volcanoes,” Lambert said.
“It’s a small excitation process, but if you accumulate a small excitation over a long time it can lead to measurable consequences.”
Polar motion describes the motion of the Earth’s spin axis (shown in orange) with respect to the geographic north and south poles (shown in blue). Over time, the geographic poles appear to spin away from the spin axis when viewed from space and then back again.
Viewed from the perspective of someone on Earth, the spin axis instead appears to spiral away from the geographic poles and then spiral back.
The motion of the spin pole with respect to the geographic poles fixed to the Earth’s crust is called polar motion. Note: The size and speed of the spiral are greatly exaggerated for clarity. Credit: NASA/GSFC Science Visualization Studio
Shaking Earth
Previous work has shown the length of a day on Earth, which changes based on the speed of Earth’s spin, also deforms the crust and could affect volcanic behavior.
In the new study, Lambert and his colleague, Gianluca Sottili, a volcanologist from Sapienza University of Rome in Italy, wanted to study the relationship between polar motion and volcanic activity.
They focused on Mount Etna because the volcano is well-studied, meaning there’s plenty of data, and it sits just south of 45 degrees latitude.
There also weren’t any volcanic crises out of the ordinary at Mount Etna during the study period, which might otherwise mask the signal from polar motion.
Lambert and Sottili used seismic records from 11,263 earthquakes that happened within 43 kilometers (26.7 miles) of Mount Etna’s summit between 1999 and 2019.
The team also used records of how much magma erupted from the volcano since 1900. They included 62 eruptions in the analysis, based on the time span between events.
The pair then compared the distance between the geographic and rotational poles at the time each event occurred to determine whether volcanic activity was connected to Earth’s rotation.
Lambert and Sottili discovered there were more earthquakes when Earth’s rotational pole was furthest from the geographic axis—at the point in Earth’s top-like spin when it looks like it is about to fall over.
Between 1999 and 2019, those peaks were in 2002 and 2009. An expected peak in 2015 never materialized because one of the oscillations contributing to polar motion has been slowing down.
The team also uncovered a link between the amount of magma ejected during an eruption. Polar motion appears to drive the largest eruptions from Mount Etna, although to a lesser extent than its seismic activity, according to the researchers.
Examining volcanoes in the Ring of Fire to see if Earth’s spin impacts their activity would surely be interesting, Sottili said, who was senior author of the study. Even expanding to other planets might open scientists’ view of how external forces impact volcanoes on the surface, he added.
More information: S. Lambert et al. Is there an influence of the pole tide on volcanism? Insights from Mount Etna recent activity, Geophysical Research Letters (2019). DOI: 10.1029/2019GL085525
Journal information:Geophysical Research Letters
Basaltic volcanism is the most widespread volcanic activity on Earth and planetary bodies. On Earth, eruptions can impact global and regional climate, and threaten populations living in their shadow, through a combination of ash, gas and lava.
Ash emissions are a very typical manifestation of basaltic activity; however, despite their frequency of occurrence, a systematic investigation of basaltic ash sources is currently incomplete.
Here, we revise four cases of ash emissions at Mount Etna linked with the most common style of eruptive activity at this volcano: lava fountains (4–5 September 2007), continuous Strombolian activity transitioning to pulsing lava fountaining (24 November 2006), isolated Strombolian explosions (8 April 2010), and continuous to pulsing ash explosions (last phase of 2001 eruption).
By combining observations on the eruptive style, deposit features and ash characteristics, we propose three mechanisms of ash generation based on variations in the magma mass flow rate.
We then present an analysis of magma residence time within the conduit for both cylindrical and dike geometry, and find that the proportion of tachylite magma residing in the conduit is very small compared to sideromelane, in agreement with observations of ash componentry for lava fountain episodes at Mount Etna.
The results of this study are relevant to classify ash emission sources and improve hazard mitigation strategies at basaltic volcanoes where the explosive activity is similar to Mount Etna.
Introduction
Explosive volcanism is characterised by magma fragmenting into particles of different size varying from micrometre to metre. In the presence of an eruption column, ash particles are volcanic fragments up to 2 mm in size that are dispersed to large distances from the eruptive centre in comparison to coarser fragments (e.g., bombs and lapilli) that fall in more proximal areas.
Abundant ash has characterised most of the explosive activity at Mount Etna, Italy, since 1995 (Figure 1) (e.g., La Delfa et al., 2001; Alparone et al., 2003; Andronico et al., 2009a, 2015 and references therein), often deeply affecting people’s everyday life and the overall economy of Eastern Sicily (e.g., Barsotti et al., 2010; Andronico et al., 2014a; Andronico and Del Carlo, 2016; Horwell et al., 2017).
At Etna ash emissions accompany different eruptive styles, from mild to moderate Strombolian explosions to high energy lava fountain activity (e.g., Andronico et al., 2008a, 2015), from short-lasting ash explosions (Andronico et al., 2013) to long-lasting explosive eruptions like those occurred in 2001 (20 days; e.g., Taddeucci et al., 2002; Scollo et al., 2007) and in 2002–2003 (∼2 months; e.g., Andronico et al., 2005). All the 1995–2019 explosive activity producing significant ash emissions in the atmosphere and involving summit craters or flank areas has been summarised in Table 1.FIGURE 1
Figure 1. Satellite images of ash dispersal during the 2001 and 2002–2003 Mount Etna eruptions. (a) July 22, 2001; Image courtesy Jacques Descloitres, MODIS Land Rapid Response Team at https://visibleearth.nasa.gov/ view.php?id=56431. (b) October 27, 2002; image courtesy Jacques Descloitres, MODIS Rapid Response Team at NASA GSFC, at https://visibleearth.nasa.gov/view.php?id=10376. (c) November 12, 2002; image courtesy Jeff Schmaltz, MODIS Rapid Response Team, NASA GSFC at https://visibleearth.nasa.gov/view.php?id=10398. Mount Etna coordinates 37°45′18′N, 14°59′42′E.TABLE 1
Table 1. Summary of the explosive activity (summit eruptions, i.e., lava fountains, strong Strombolian episodes, Strombolian explosions and sustained ash emissions, and flank eruptions) producing significant ash emission up to tens–hundreds of kilometres of distance away from Etna from 1995 to 2017.
Based on visual observations of the eruptive activity and textural and compositional features of ash samples, it was found that the characteristics of ash particles at Etna usually vary with the eruptive style. For example, ash emitted during Strombolian explosions and at the peak of lava fountain activity is more vesicular, less crystallised and with a less compositionally evolved groundmass than that erupted during less explosive events or at the end of a long-lasting explosive eruption, and it contains less lithic material (Taddeucci et al., 2004; Andronico et al., 2008b).
Based on ground and satellite data, Andronico et al. (2009b) have attempted a first classification of ash-enriched Etnean volcanic plumes, subdividing ash emissions in the autumn of 2006 into five categories and demonstrating the utility of such classifications for volcanic hazard mitigation planners and civil protection purposes. It is worth specifying, however, that this classification is valid mostly for that period of activity in 2006 and it does not cover the whole range of explosive activity displayed by Etna.
Previous research on ash characteristics and the link between ash and eruption behaviour has improved our general knowledge on ash emissions at Etna (Taddeucci et al., 2002; Andronico et al., 2013). A better understanding of ash formation has the potential of further improving hazard assessment and forecasting at this volcano. However, while the sources and features of ash particles at Etna have been investigated in several cases, a systematic comparison is still lacking. The present paper represents a concrete step forward in the attempt of classifying the most common mechanisms through which ash originates at Etna.
In the following, first we briefly revise the characteristics of Etna ash, then we present four different case studies of ash emissions, each associated with a different eruptive style and marked by a different duration and intensity (i.e., mass eruption rate; MER) of the tephra emission. Ultimately, we link each ash emission case study to a different, peculiar mechanism of ash formation.
Although based on case studies from Etna, the proposed mechanisms can explain ash generated by other basaltic volcanic systems [e.g., Paricutin, Mexico (Pioli et al., 2008), and Villarrica, Chile (Romero et al., 2018)] which, during specific eruptive phases, resemble closely the Etna explosive activity in terms of duration, intensity and style of emission.
Characteristics of Ash at Etna
Textural and compositional features of ash particles erupted from explosive activity at Etna have been described in previous studies, and for details on the topic the reader is referred to those works (e.g., Taddeucci et al., 2002, 2004; Andronico et al., 2009a, 2013, 2014b). Here we summarise the common aspects that can help extending from specific to general cases. Juvenile ash particles at Etna consist of two end-members: sideromelane and tachylite (Figure 2). The former has fluidal to irregular morphology, is yellow to brown in colour, transparent, vesicular and generally glassy in the groundmass.
The latter is blocky, grey to black, generally opaque (sometimes it can be shiny), poorly vesicular and crystallised in the groundmass. There is however, a continuous, progressive transition of textural features between the two ash types, mainly generated by the different extent of groundmass crystallisation, far more pronounced in tachylite, and by the higher content of (sub) spherical vesicles, as opposed to vesicles with complex and/or irregular shapes, in sideromelane (Figure 2).
As a general rule, glass composition of Etna ash overlaps with the compositional field of pyroclastic material erupted from this volcano since 1995 (Corsaro et al., 2017; Pompilio et al., 2017). Within the same eruption, tachylite glass tends to be more compositionally differentiated in comparison to its sideromelane counterpart, with a higher silica, alkali and phosphorous content and lower magnesium and calcium (Taddeucci et al., 2002, 2004; Polacci et al., 2006). Compositional variations are mainly related to fractionation of a few tens of percent of crystal phases forming the groundmass. Lithic ash from Etna eruptions is not always easily distinguishable from tachylite juveniles, but is often marked by altered/oxidised surfaces and secondary minerals overgrowths.FIGURE 2
Figure 2. Secondary (top) and backscattered (bottom) scanning electron microscope images of typical sideromelane (a,c) and tachylite (b,d) ash particles at Mount Etna. Scale bar 20 μm in all images.
Visual observations of the eruptive activity integrated with investigations of ash componentry have revealed that the proportion of sideromelane ash increases with increasing eruption intensity (Taddeucci et al., 2004, and this work, section “Representative Episodes of Ash Emission at Etna”).
Therefore, Strombolian and lava fountain activity generally produce more sideromelane and fewer tachylite than low energy, impulsive ash explosions (Andronico et al., 2008b). This correlation, together with textural and compositional observations, suggested a longer co
nduit residence time for tachylite with respect to sideromelane ash. Hence, several authors hypothesised that tachylite ash particles at Etna are generated by the fragmentation of a cooler, viscous, crystallised and degassed magma at the conduit walls, while sideromelane represents the hotter, less viscous and vesiculating magma rising in the central portion of the conduit (Taddeucci et al., 2004; Polacci et al., 2006; Pompilio et al., 2017).
Ash dispersal at Etna is mostly controlled by eruption intensity and the ensuing plume height, as expected. For example, on 23 February and 23 November 2013, two lava fountain episodes characterised by relatively high eruption columns (at least for the most common explosive activity at Etna) of up to 9–10 km a.s.l. and high MERs, generated dispersal of ash particles up to 400 km from Etna, in Puglia (Italy) (Poret et al., 2018a, b). Lower intensity impulsive explosions and discontinuous explosive activity generally produce lower and intermittent eruption columns above the vent, producing minor ash dispersal.
However, if the intensity is low but the duration of the activity is prolonged for days to weeks, the continuous injection of relatively fine-grained ash in the atmosphere is able to form a sustained tephra column feeding an eruption cloud spreading hundreds of km away from the volcano. This occurred, for example, during the 2001 and 2002–2003 eruptions (e.g., Andronico et al., 2005; Villani et al., 2006; Scollo et al., 2007) (Figure 1).
Representative Episodes of Ash Emission at Etna
In the following we present and revise four case studies, in order of decreasing eruption intensity, during which ash was vented at Etna from either a single, short-lasting (minutes to tens of hours) explosive event (i.e., 24 November 2006, 4–5 September 2007, 8 April 2010) or a longer period of erupted activity that involved several successive explosive events over the course of a week (i.e., the last phase of the 2001 eruption).
We have chosen these specific case studies because they cover the most common eruptive styles and intensities that have characterised the explosive activity at Etna in the last decades: sustained lava fountain activity (the 2007 event), continuous Strombolian activity transitioning to quasi-steady or pulsing lava fountain (the 2006 event), isolated Strombolian explosions (the 2010 event) and nearly continuous to pulsing ash explosions (the 2001 eruption).
For each case study we provide a brief description of the event (or eruption), as well as a description of the associated deposit and ash particle features. This overall information represents a basic framework for classifying ash emissions at Etna and provides the constraints necessary to model mechanisms of ash formation.
Conclusions
Ash emissions are widespread during explosive basaltic activity, and often have a significant impact on people’s life and infrastructure. For example, between January and February 2019 Mount Etna North-East Crater produced several episodes of continuous to pulsing ash emissions, which, despite being characterised by a very low, uncommon sedimentation rate (a few g/m2 during several hours of activity), were able nonetheless to cause disruption at the Fontanarossa International airport of Catania (29 km from the vent).
To mitigate hazard at basaltic volcanoes, it is therefore imperative to improve knowledge on mechanisms of ash generation. In this study, we use Mount Etna as a general case study. Compositional and textural features of ash particles from Etna and other basaltic volcanoes have been well characterised and the explosive activity that produces them well studied. Yet, a systematic investigation of ash sources is still incomplete and mechanisms of ash generation poorly understood. With this study, we aim to fill this gap in knowledge.
By revising four ash emission episodes that are representative of the most common explosive activity at Etna in the last decades, we propose three mechanisms of ash generation based on variations in the magma mass flow rate that apply to other basaltic volcanoes erupting ash and whose explosive activity is similar to Etna.
Additionally, we provide an analysis of magma residence time in the volcanic conduit, which explains why different ash particles reside in the conduit for a shorter time than others. This analysis sheds light on the proportion of sideromelane and tachylite textures found in ash from lava fountaining and continuous Strombolian activity, in agreement with our first proposed ash generation mechanism.
The main finding of this study is that, integrating field observations with magma residence time calculations, we are able to provide improved information on both ash sources and mechanisms of ash formation from basaltic volcanoes erupting ash.
The broader implication of this investigation is that our results are significantly relevant to the wider volcanological community, particularly modellers of eruption dynamics and scientists involved with volcano monitoring and surveillance, and should be used to improve eruption forecasting and hazard assessment and to inform stakeholders on how to implement risk mitigation strategies in active volcanic areas.
