Selected Crater and Small Caldera Lakes in Alaska: Characteristics and Hazards

This study addresses the characteristics, potential hazards, and both eruptive and non-eruptive role of water at selected volcanic crater lakes in Alaska. Crater lakes are an important feature of some stratovolcanoes in Alaska. Of the volcanoes in the state with known Holocene eruptive activity, about one third have summit crater lakes. Also included are two volcanoes with small caldera lakes (Katmai, Kaguyak). The lakes play an important but not well studied role in influencing eruptive behavior and pose some significant hydrologic hazards. Floods from crater lakes in Alaska are evaluated by estimating maximum potential crater lake water volumes and peak outflow discharge with a dam-break model. Some recent eruptions and hydrologic events that involved crater lakes also are reviewed. The large volumes of water potentially hosted by crater lakes in Alaska indicate that significant flowage hazards resulting from catastrophic breaching of crater rims are possible. Estimates of maximum peak flood discharge associated with breaching of lake-filled craters derived from dam-break modeling indicate that flood magnitudes could be as large as 103–106 m3/s if summit crater lakes drain rapidly when at maximum volume. Many of the Alaska crater lakes discussed are situated in hydrothermally altered craters characterized by complex assemblages of stratified unconsolidated volcaniclastic deposits, in a region known for large magnitude (>M7) earthquakes. Although there are only a few historical examples of eruptions involving crater lakes in Alaska, these provide noteworthy examples of the role of external water in cooling pyroclastic deposits, acidic crater-lake drainage, and water-related hazards such as lahars and base surge.

The significance of the 1971 flank eruption of Etna from volcanological and historic viewpoints

The 1971 eruption represents a benchmark in the recent history of Etna volcano. From a volcanological point of view, this eruption was characterised by complex intrusive dynamics associated with significant ground deformation that induced the activation of the Moscarello seismogenic fault and the formation of a new summit crater: the Southeast Crater. At the same time, the 1971 event marks an important change in the eruptive style and composition of the magma towards products richer in K. It is no coincidence that, over the next fifty years, there would be an increase in the frequency of summit and flank eruptions and associated output rate. From an historical viewpoint, the eruptive event of 1971 was the first important flank eruption studied by the International Institute of Volcanology: the analysis of the scientific articles on this activity reveals a greater multidisciplinary content in the descriptions and explanations of volcanic activity. Particularly important were the collaborations of British and French research groups that, together with their Italian colleagues, succeeded in giving a complete picture of the eruption and describing the state of knowledge on the Sicilian volcano. The multidisciplinary methodology used to study this eruption is still valid today.

Landslide hypothesis for the origin of Haleakala volcano's crater and great valleys, Hawaii

Active Haleakala volcano on the island of Maui is the second largest volcano in the Hawaiian Island chain. Prominently incised in Haleakala's slopes are four large (great) valleys. Haleakala Crater, a prominent summit depression, formed by coalescence of two of the great valleys. The great valleys and summit crater have long been attributed solely to fluvial erosion, but two significant enigmas exist in the theory. First, the great valleys of upper Keanae/Koolau Gap, Haleakala Crater, and Kaupo Gap are located in areas of relatively low annual rainfall. Second, the axes of some valley segments are oblique for long distances across the volcanic slopes. This study tested the prevailing erosional theory by reconstructing the volcano's topography just prior to valley incision. The reconstruction produces a belt along the volcano's east rift zone with a morphology that is inconsistent with volcanic aggradation alone, but it is readily explained if it is assumed the surface was displaced along scarps formed by a giant landslide on Haleakala's northeastern flank. Although the landslide head location is well defined, topographic evidence is lacking for the toe and lateral margins. Consequently, the slope failure is interpreted as a sackung-style landslide with a zone of deep-seated distributed shear and broad surface warping downslope of the failure head. Maximum downslope displacement was likely in the range of 400–800 m. Capture of runoff at the headscarps formed atypically large streams that carved Haleakala's great valleys and explains their existence in low-rainfall areas and their slope-oblique orientations. Sackung-style landslides may be more prevalent on Hawaiian volcanoes than previously recognized.

Response of a hydrothermal system to escalating phreatic unrest: the case of Turrialba and Irazú in Costa Rica (2007–2012)

This study presents the first hydrogeochemical model of the hydrothermal systems of Turrialba and Irazú volcanoes in central Costa Rica, manifested as thermal springs, summit crater lakes, and fumarolic degassing at both volcanoes. Our period of observations (2007–2012) coincides with the pre- and early syn-phreatic eruption stages of Turrialba volcano that resumed volcanic unrest since 2004, after almost 140 years of quiescence. Peculiarly, the generally stable Irazú crater lake dropped its level during this reawakening of Turrialba. The isotopic composition of all the discharged fluids reveals their Caribbean meteoric origin. Four groups of thermal springs drain the northern flanks of Turrialba and Irazú volcanoes into two main rivers. Río Sucio (i.e. “dirty river”) is a major rock remover on the North flank of Irazú, mainly fed by the San Cayetano spring group. Instead, one group of thermal springs discharges towards the south of Irazú. All thermal spring waters are of SO4-type (i.e. steam-heated waters), none of the springs has, however, a common hydrothermal end-member. A water mass budget for thermal springs results in an estimated total output flux of 187 ± 37 L/s, with 100 ± 20 L/s accounted for by the San Cayetano springs. Thermal energy release is estimated at 110 ± 22 MW (83.9 ± 16.8 MW by San Cayetano), whereas the total rock mass removal rate by chemical leaching is ~ 3000 m3/year (~ 2400 m3/year by San Cayetano-Río Sucio). Despite Irazú being the currently less active volcano, it is a highly efficient rock remover, which, on the long term can have effects on the stability of the volcanic edifice with potentially hazardous consequences (e.g. flank collapse, landslides, phreatic eruptions). Moreover, the vapor output flux from the Turrialba fumaroles after the onset of phreatic eruptions on 5 January 2010 showed an increase of at least ~ 260 L/s above pre-eruptive background fumarolic vapor fluxes. This extra vapor loss implies that the drying of the summit hydrothermal system of Turrialba could tap deeper than previously thought, and could explain the coincidental disappearance of Irazú’s crater lake in April 2010.

Monitoring of Merapi volcano, Indonesia based on Sentinel-1 data

<p>Despite the well-established interest of Synthetic Aperture Radar data for volcanoes study and monitoring, their integration to operational monitoring activities in volcanoes observatories remains limited so far. We here describe the effort in progress to integrate in near real time the information derived from Sentinel-1 satellites into the monitoring devices at BBPTKG in charge of Merapi volcano survey as well as the use of Sentinel-1 data during the recent period of  unrest. Merapi (7°32.5’ S and 110°26.5’ E) located in the densely populated Province of Yogyakarta in Central Java is one of the most active volcanoes in Indonesia. The eruptive history of Merapi is characterized by two eruptive styles: 1) recurrent effusive growth of viscous lava domes, with gravitational collapses producing pyroclastic flows known as « Merapi-type nuées ardentes » (VEI 2); 2) more exceptional explosive eruptions of relatively large size (VEI 3-4), associated with column collapse pyroclastic flows reaching distances larger than 15 km from the summit. The eruptive periodicity is 4 to 5 years for the effusive events and 50 to 100 years for the explosive ones. The last explosive events (VEI 3-4) occurred in November 2010 and was followed by a period of limited activity. In August 2018, a new dome was observed inside the summit crater, thus marking the start of a new phase of effusive activity. A new period of unrest then started in mid-October 2020, characterized by an increase in seismic activity as well as large and localized displacements in the summit area. Magma finally reached the surface on 4<sup>  </sup>January 2021. Deformation is currently recorded by EDM and tiltmeters together with a network of 10 permanent GNSS stations. GNSS data are automatically processed and inverted for a pressure source at depth. Both displacement time series as well as spatial probability distribution are directly available through WebObs (Beauducel et al., Frontiers, 2020), an integrated web-based system for monitoring. Sentinel-1 data are acquired over the volcano every 12 days on descending track 76 and every 6 days on ascending track 127. Since mid 2017, Sentinel-1 data are automatically downloaded on a local server at BPPTKG. Interferograms and coherence images are then produced using the NSBAS processing chain (Doin et al, 2012) and automatically integrated to WebObs to enable detection of potential rapid and significant changes in signal. Mean velocity maps are also produced as well as time series of surface displacement at given location enabling direct comparison with GNSS measurements. The descending InSAR time series shows a strong displacement away from the satellite in a 1.5 km wide area located on the north-eastern part  of the crater. This signal became significant in September 2020. It is consistent with field measurements recorded and allows to map the affected area. In mid-November 2020, Sentinel-1 data thus provided the first information on the spatial extent of the ongoing surface displacements, which was useful for crisis management.</p>

Temporal Variations in the Depth Estimate of Mass Density Changes at Mt Etna Volcano

<p>Continuous gravity measurements at Mt. Etna, Sicily demonstrate spatio-temporal variations that can be related to volcanic processes. Two iGrav superconducting gravimeters (SGs) were installed in 2014 and 2016 at Serra La Nave Astrophysical Observatory (SLN; 1,730 m elevation; ~6.5 km from the summit craters) and La Montagnola hut (MNT; 2,600 m asl; ~3.5 km SE of the summit crater). Since their installation both stations have been continuously recording, resulting in high-resolution (1 Hz sampling rate) time series. The persistent activity of Etna is maintained by a regular supply of magma to the shallower levels of the plumbing system. The bulk mass redistributions induced by the newly injected material result in minor variations in the local gravity field that are recorded by the two stations. By assuming that the observed gravity changes are due exclusively to mass changes in an almost spherical-shaped source, located beneath the craters, the amplitude ratio between the two signals can be used to estimate the depth to potential mass changes beneath the surface.</p><p>This study reports on the time-dependent nature of mass changes located beneath the craters of the volcano during 2019. Results highlight distinct periods of stability at different depths, as well as potential periods of transitory activity, where the predominant mass source switches between storage zones at different depth. These events are correlated to phases of strombolian and effusive activity, highlighting the potential of continuous gravimetry for the detection of eruption precursors.</p>

Long-term variations of diffuse CO2, He and H2 at the summit crater of Teide volcano, Tenerife, Canary Islands

<p>Tenerife (2,034 km<sup>2</sup>) is the largest of the Canary Islands. Its structure is controlled by a volcano-tectonic rift-system with NW, NE and NS directions, with the volcanic system Teide-Pico Viejo located in the intersection. Teide is 3,718 m.a.s.l. high and its last eruption occurred in 1798 through an adventive cone of Teide-Pico Viejo volcanic complex. Persistent degassing activity, both visible and diffuse, takes place at the summit cone of the volcano, being the diffuse degassing the principle mechanism.</p><p>During the period 1999-2020, more than 200 diffuse CO<sub>2</sub> efflux surveys have been performed in the summit crater of Teide Volcano. For each survey, 38 sampling sites homogeneously distributed inside the crater covering an area of 6,972 m<sup>2</sup> were selected. Diffuse CO<sub>2</sub> emission was estimated in each point by means of a portable non dispersive infrared (NDIR) CO<sub>2</sub> fluxmeter using the accumulation chamber method. Additionally, soil gas samples were taken at 40 cm depth and analyzed later in the lab for the He and H<sub>2</sub> content by means of quadrupole mass spectrometry and micro-gas chromatography, respectively. To estimate the He and H<sub>2</sub> emission rates at each sampling point, the diffusive component was estimated following the Fick’s law and the convective emission component model was estimated following the Darcy’s law. In all cases, spatial distribution maps were constructed averaging the results of 100 simulations following the sequential Gaussian simulation (sGs) algorithm, in order to determine CO<sub>2</sub>, He and H<sub>2</sub> emission rates.</p><p>During the study period, CO<sub>2</sub> emissions ranged from 2.2 to 176.1 t/d, He emissions between 0.013 and 4.1 kg/d and H<sub>2</sub> between 1.3 and 35.6 kg/d. On October 2, 2016, a seismic swarm of long-period events was recorded on Tenerife followed by a general increase of the seismic activity in and around the island (D’Auria et al., 2019). Since then, relatively high values have been obtained in the diffuse CO<sub>2</sub>, He and H<sub>2</sub> emission rate the crater of Teide. This increase reflects a process of pressurization of the volcanic-hydrothermal system.</p><p>The variations in CO<sub>2</sub>, He and H<sub>2</sub> emissions indicate changes in the activity of the system and can be useful to understand the behaviour of the volcanic system and to forecast future volcanic activity. Monitoring the diffuse degassing rates at Teide volcano has demonstrated to be an essential tool for predicting future seismic–volcanic unrest, and has become important to reduce volcanic risk in Tenerife (Melián et al., 2012; Pérez et al., 2013).</p><p>D'Auria .L, Barrancos J., Padilla G.D., Pérez N.M., Hernández P.A., Melián G., Padron E., Asensio-Ramos M., García‐Hernández R. (2019). J. Geophys. Res. 124, 8739-8752</p><p>Pérez N. M., Hernández P. A., Padrón E., Melián G., Nolasco D., Barrancos J., Padilla G., Calvo D., Rodríguez F., Dionis S. and Chiodini G. (2013). J. Geol. Soc., 170(4), 585-592.</p><p>Melián G., Tassi F., Pérez N. M., Hernández P., Sortino F., Vaselli O., Padrón E., Nolasco D., Barrancos J., Padilla G., Rodriguez F., Dionis S., Calvo D., Notsu K., Sumino H. (2012).  Bull. Volcanol, 74(6), 1465-1483.</p><p> </p>

Response of a Hydrothermal System to Escalating Phreatic Unrest the Case of Turrialba and Irazú in Costa Rica (2007-2012)

This study presents the first hydrogeochemical model of the hydrothermal systems of Turrialba and Irazú volcanoes in central Costa Rica, manifested as thermal springs, summit crater lakes, and fumarolic degassing at both volcanoes. Our period of observations (2007-2012) coincides with the pre- and early syn-phreatic eruption stages of Turrialba volcano that resumed volcanic unrest since 2004, after almost 140 years of quiescence. Peculiarly, the generally stable Irazú crater lake dropped its level during this reawakening of Turrialba. The isotopic composition of discharged fluids reveals the Caribbean meteoric origin; a contribution of “andesitic water” for Turrialba fumaroles up to ~50% is suggested. Four groups of thermal springs drain the northern flanks of Turrialba and Irazú volcanoes into two main rivers. Río Sucio (i.e. “dirty river”) is a major rock remover on the North flank of Irazú, mainly fed by the San Cayetano spring group. Instead, one group of thermal springs discharges towards the south of Irazú. All thermal spring waters are of SO4-type (i.e. steam heated waters), although none of the springs has a common hydrothermal end-member. A water mass budget for thermal springs results in an estimated total output flux of 187 ± 37 L/s, with 100 ± 20 L/s accounted for by the San Cayetano springs. Thermal energy release is estimated at 110 ± 22 MW (83.9 ± 16.8 MW by San Cayetano), whereas the total rock mass removal rate by chemical leaching is ~3,000 m3/y (~2,400 m3/y by San Cayetano-Río Sucio). Despite Irazú being the currently less active volcano, it is a highly efficient rock remover, which, on the long term can have effects on the stability of the volcanic edifice with potentially hazardous consequences (e.g. flank collapse, phreatic eruptions). Moreover, the vapor output flux from the Turrialba fumaroles after the onset of phreatic eruptions on 5 January 2010 showed an increase of at least ~260 L/s above pre-eruptive background fumarolic vapor fluxes. This extra vapor loss implies that the drying of the summit hydrothermal system of Turrialba could tap deeper than previously thought, and could explain the coincidental disappearance of Irazú’s crater lake in April 2010.

Harnessing Erebus volcano's thermal energy to power year-round monitoring

Year-round monitoring of Erebus volcano (Ross Island) has proved challenging due to the difficulties of maintaining continuous power for scientific instruments, especially through the Antarctic winter. We sought a potential solution involving the harvesting of thermal energy dissipated close to the summit crater of the volcano in a zone of diffuse hot gas emissions. We designed, constructed and tested a power generator based on the Seebeck effect, converting thermal energy to electrical power, which could, in principle, be used to run monitoring devices year round. We report here on the design of the generator and the results of an 11 day trial deployment on Erebus volcano in December 2014. The generator produced a mean output power of 270 mW, although we identified some technical issues that had impaired its efficiency. Nevertheless, this is already sufficient power for some monitoring equipment and, with design improvements, such a generator could provide a viable solution to powering a larger suite of instrumentation.