Could Glacial Retreat-Related Landslides Trigger Volcanic Eruptions? Insights from Mount Meager, British Columbia Gioachino Roberti, Brent Ward, Benjamin van Wyk de Vries, Nicolas Le Corvec, Swetha Venugopal, Glyn Williams-Jones, John J. Clague, Pierre Friele, Giacomo Falorni, Geidy Baldeon, Luigi Perotti, Marco Giardino, and Brian Menounos Abstract chamber at 3–16 km depth. Based on numerical model simulations carried out to constrain the stress change, the Mount Meager, a glacier-clad volcanic complex in British failure would affect the stress field to depths of up to Columbia, Canada, is known for its large landslides, as *6 km, with changes in effective stress of up to well as a major eruption about 2360 years ago. In 2010, *4 MPa. The change in effective stress following such after decades of glacier retreat, the south flank of Mount a landslide might destabilize the magmatic chamber and Meager collapsed, generating a huge (53 Mm3) landslide. trigger an eruption. This result also suggests that a In 2016, fumaroles formed ice caves in one of the glaciers previously documented major flank collapse may have on the complex. This glacier is bordered by a large had a role in the 2360 cal yr BP eruption. unstable slope presently moving about 3.5 cm per month. If this slope were to fail, a long-runout debris avalanche Keywords would reach the floor of the Lillooet River valley, with possible destructive effects on downstream infrastructure. Volcanic landslide Á Stress changes Á Eruption trigger Á The unloading of the volcanic edifice from an abrupt FEM Á InSAR failure would also have unknown effects on the magmatic plumbing system. From geochemical, geophysical, and petrological data, we infer the presence of a magmatic Introduction G. Roberti (&) Minerva Intelligence, 850 W Hastings St #301, Vancouver, BC, Mount Meager is a glaciated volcanic complex in the Canada e-mail: [email protected] southern Coast Mountains of British Columbia, with activity spanning the past *2 Ma (Read 1979) (Fig. 1). The most B. Ward Á S. Venugopal Á G. Williams-Jones Á J. J. Clague Earth Sciences Department, Simon Fraser University, Burnaby, recent eruption occurred 2360 years ago (Clague et al. 1995) BC, Canada and produced a tephra plume that tracked eastward into B. van Wyk de Vries Á S. Venugopal western Alberta (Hickson et al. 1999). Today, Mount Mea- CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, Université ger hosts an active hydrothermal system and is the site of Clermont Auvergne, Aubiere Cedex, France frequent shallow earthquakes (Friele et al. 2008; Venugopal N. Le Corvec et al. 2017). It is known for its very large pre-historic and 111 rue Jules Ferry, 33200 Bordeaux, France historic landslides (Friele et al. 2008), of which a 53 Mm3 P. Friele debris avalanche in 2010 is the largest landslide in Canadian Cordilleran Geoscience, Squamish, BC, Canada history (Guthrie et al. 2012). Following the 2010 landslide, G. Falorni Á G. Baldeon recent studies have focused on the ongoing instability on TRE-ALTAMIRA Inc., Vancouver, BC, Canada Mount Meager and its link to recent deglaciation and climate L. Perotti Á M. Giardino warming (Roberti et al. 2018a, b). These studies have Earth Sciences Department, GeoSitLab - GIS and Geomatics identified many large unstable slopes, one of which, on the Laboratory, University of Torino, Torino, Italy west flank of Plinth Peak (Fig. 1), is of particular concern as B. Menounos it moved 3 cm during a 24-day period during the summer of Geography Program and Natural Resources and Environmental 2016 (determined by InSAR interferometry; see Roberti Studies Institute, University of Northern British Columbia, Prince George, BC, Canada © Springer Nature Switzerland AG 2021 147 V. Vilímek et al. (eds.), Understanding and Reducing Landslide Disaster Risk, ICL Contribution to Landslide Disaster Risk Reduction, https://doi.org/10.1007/978-3-030-60319-9_15 148 G. Roberti et al. et al. 2018b). This slope is near an amphitheatre (A in collapse in the 2360 cal yr BP eruption. That collapse might Fig. 1) that hosts a retreating glacier and a fumarole field. have decompressed the volcanic plumbing system and In this study, we present a numerical model to investigate contributed to the eruption. the stress field evolution prior to and after a hypothetical collapse of the west flank of Plinth Peak. We discuss how this stress change could affect the hydrothermal and volcanic Methods plumbing systems and lead to an eruption. We also docu- ment another large edifice collapse from Job Valley, which We describe the geology and structure of the west flank of happened a few centuries before the 2360 cal yr BP erup- Plinth Peak and Job Valley from historical aerial pho- tion. This landslide originated from the above-mentioned tographs, Lidar data, literature review and field work. Field amphitheatre near Plinth Peak that is now occupied by Job work at Plinth, Job Glacier, and the mouth of Job Valley was Glacier and the fumarole field. Discussion of decompression conducted in October 2017. We collected a core sample on mechanisms after the eventual collapse of Plinth Peak and the landslide deposit at the mouth of Job Valley to constrain documentation of the collapse in Job Valley enable an the temporal relation between the landslide and the examination of the possible role of the earlier edifice 2360 cal yr BP eruption. We used a 2D elastic finite element model (FEM) with COMSOL Multiphysics® to explore the effect of the collapse of Plinth Peak on the state of stress within the Mount Meager volcanic edifice. The FEM topography was built by superimposing E-W profiles of the west flank of Plinth (B-B’ in Fig. 1) and of Job Valley 500 m to the south, assuming that the failure would remove the bulge of the west flank of Plinth Peak to reach the profile of Job Valley. Two elastic domains were defined to represent both volcanic material (Plinth Peak) and the granitic basement rocks. We used Plinth Peak’s 2360 cal yr BP eruption dacite to model the volcanic edifice and the Fall Creek Stock monzogranite to model the substratum rock (Campbell et al. 2016; Kelly Russell, personal communication 2018). Fol- lowing previous work (Le Corvec et al. 2018), the FEM defined as a half-space, is loaded with gravitational forces. Except from its surface that is set free to move, the other boundaries are defined by a roller condition restricting dis- placements normal to the boundary. COMSOL allows to compute the differential stresses and the orientation of the stress field before and after the removal of the landslide material, subsequently allowing to compute the difference between the two cases. Results Plinth A prominent knob forms the west flank of Plinth Peak on the east side of Job Valley about 2 km from the Job Glacier headwall. The knob has an area of 2.7 Â 106 m2 and rises 1000 m from Job Glacier to the ridge crest (Fig. 2). DInSAR analysis of Sentinel-1 satellite images show displacement of Fig. 1 Job Valley, Plinth Peak, and the 2360 cal yr BP eruption crater. this part of the slope of 34 ± 10 mm over a 24-day period Scars, unstable slopes, landslides, eruption deposits, fumaroles, sample, fi fi from July to August 2016 (Roberti et al. 2018a) and and pro le locations are indicated. a Job Valley amphitheatre. b Pro le ± shown in Fig. 2. c 2010 landslide scar. d Job landslide deposit. Base 30 10 mm over a 12-day period in July 2017. The upper map is a hillshaded rendering of Lidar data from 2015–2016 slope extends 400 m above the lower slope (Fig. 2) and has Could Glacial Retreat-Related Landslides Trigger … 149 near-vertical cliffs and buttresses composed of horizontally the 2360 cal yr BP eruption. This debris flow unit is thus layered rhyodacitic lava flows of the Plinth assemblage interpreted as a close precursor to the eruption. (p9f, Read 1979). SqueeSAR analysis of the 1992–2000 ERS data indicate LOS (line of sight) displacements up to 17 mm yr−1. Analysis of the 2015–2018 Sentinel-1 data Discussion indicate LOS displacements of up to 10 mm yr−1. 2D stress field modeling in COMSOL shows that removal of the Plinth bulging rock mass would affect the effective stress to a depth *6kmbyupto *4 MPa (Fig. 2). Most of the decom- In view of displacement rates, geometry, geological setting, pression would occur at the toe of the slope, about 1 km and a low (1.084–1.077) factor of safety (FOS) calculated by downstream of the fumaroles. (Hetherington 2014), the west flank of Plinth Peak can be considered to be at failure and near catastrophic collapse threshold. The fumaroles on Job Glacier may accelerate Job glacier melting and bring the slope closer to catastrophic failure. The base of the slope, weakened by glacier erosion A 2-km long, >100 m thick, hummocky landslide deposit is and having been recently deglaciated, shows high defor- located at the Job Creek—Lillooet River confluence (D in mation rates. It is likely to fail first, followed quickly by Fig. 1). Considering the thickness and extent of the deposit, retrogressive failure involving the whole slope, similar to the we tentatively relate the landslide to the formation of the evolution of the 2010 collapse (Roberti et al. 2018b). The large amphitheatre at the head of Job Valley (*109 m3)(A failed rock mass would rapidly fragment, liquefy, and in Fig. 1). This amphitheatre is the only feature large enough transform into a debris flow, as observed at other large to be the source of the landslide deposit and furthermore has landslides at Mount Meager (e.g.