Description: Well water level changes associated with magmatic unrest can be interpreted as a result of pore pressure changes in the aquifer due to crustal deformation, and so could provide constraints on the subsurface processes causing this strain. We use finite element analysis to demonstrate the response of aquifers to volumetric strain induced by pressurized magma reservoirs. Two different aquifers are invoked – an unconsolidated pyroclastic deposit and a vesicular lava flow – and embedded in an impermeable crust, overlying a magma chamber. The time-dependent, fully coupled models simulate crustal deformation accompanying chamber pressurization and the resulting hydraulic head changes as well as flow through the porous aquifer, i.e. porous flow. The simulated strain leads to centimetres (pyroclastic aquifer) to metres (lava flow aquifer) of hydraulic head changes; both strain and hydraulic head change with time due to substantial porous flow in the hydrological system. Well level changes are particularly sensitive to chamber volume, shape and pressurization strength, followed by aquifer permeability and the phase of the pore fluid. The depths of chamber and aquifer, as well as the aquifer's Young's modulus also have significant influence on the hydraulic head signal. While source characteristics, the distance between chamber and aquifer and the elastic stratigraphy determine the strain field and its partitioning, flow and coupling parameters define how the aquifer responds to this strain and how signals change with time. We find that generic analytical models can fail to capture the complex pre-eruptive subsurface mechanics leading to strain-induced well level changes, due to aquifer pressure changes being sensitive to chamber shape and lithological heterogeneities. In addition, the presence of a pore fluid and its flow have a significant influence on the strain signal in the aquifer and are commonly neglected in analytical models. These findings highlight the need for numerical models for the interpretation of observed well level signals. However, simulated water table changes do indeed mirror volumetric strain, and wells are therefore a valuable addition to monitoring systems that could provide important insights into pre-eruptive dynamics.

Description: The Central American Volcanic Arc (CAVA) is, and has been, one of the most active volcanic regions and generated numerous Plinian eruptions along his 1200 km extension. The best preserved archive of this volcanism can be found as ash layers in the marine sediments downwind from the volcanic sources on the Pacific floor. Numerous ash layers up to 8 Mio old, which occur in ODP and DSDP cores of Legs 66, 67, and 202, originated in Central America and southern Mexico. The cores lie across the ash distribution areas expected from dominant wind directions as identified by mapped fallout deposits. We have chosen 145 ash layers of all three Legs for first detailed analysis of these sites to built up a data base for upcoming IODP cruise 334: Costa Rica Seismogenesis Project. The ash layers commonly have sharp contacts at the bottom and diffuse transitions to terrigenous and pelagic sediments at the top. Ash layer thickness ranges from 0.5 to 60 cm with typical grain sizes from medium silt to coarse sand. The mineral assemblages are typical for arc volcanism (plagioclase, pyroxene, hornblende, and olivine). The most evolved tephras also contain biotite. Electron microprobe analyses of 1300 glass shards yield compositions ranging from basaltic andesite to rhyolite and trachyte. Felsic ashes can be divided into seven compositional groups by means of silica and potassium contents. Correlations between marine ashes and on-land tephras are constrained by petrographical and stratigraphical criteria, major element geochemistry of glasses and minerals, and trace element data from LA-ICP-MS analyses. Due to limited exposure on land, such correlations with individual tephras are only possible for deposits of late Pleistocene to Holocene age. Older ash layers, however, can be correlated with regional arc segments making use of systematic along-arc variations of trace-element characteristics (Zr/Nb, Ba/La, Ce/Yb, La/Yb and Ba/Zr) of the arc rocks. Results show that source areas of the ash layers are distributed along the entire CAVA, as well as at the Southern Mexican Arc. The marine tephra record provides important data for ongoing studies of CAVA volcanism: (a) dating of undated land tephra by correlation with marine ashes and the ages derived by sedimentation rates; (b) stratigraphic correlations along the entire arc can be traced much more completely in the marine sediment cores than by limited onshore outcrops alone; (c) long-term changes in magmatic evolution of volcanic complexes can be reconstructed by using the marine archive of ash layers.

Description: Highlights • A multidisciplinary approach to unravel the energetics of hydrothermal explosions. • Pressure failure caused by a lake drainage triggered the hydrothermal explosions. • Bedrock nature controlled the explosion dynamics and the way energy was released. • Approx. 30% of the available thermal energy is converted into mechanical energy. • Released seismic energy as proxy to detect past (and future?) hydrothermal explosions. Hydrothermal explosions frequently occur in geothermal areas showing various mechanisms and energies of explosivity. Their deposits, though generally hardly recognised or badly preserved, provide important insights to quantify the dynamics and energy of these poorly understood explosive events. Furthermore the host rock lithology of the geothermal system adds a control on the efficiency in the energy release during an explosion. We present results from a detailed study of recent hydrothermal explosion deposits within an active geothermal area at Kverkfjöll, a central volcano at the northern edge of Vatnajökull. On August 15th 2013, a small jökulhlaup occurred when the Gengissig ice-dammed lake drained at Kverkfjöll. The lake level dropped by approximately 30 m, decreasing pressure on the lake bed and triggering several hydrothermal explosions on the 16th. Here, a multidisciplinary approach combining detailed field work, laboratory studies, and models of the energetics of explosions with information on duration and amplitudes of seismic signals, has been used to analyse the mechanisms and characteristics of these hydrothermal explosions. Field and laboratory studies were also carried out to help constrain the sedimentary sequence involved in the event. The explosions lasted for 40–50 s and involved the surficial part of an unconsolidated and hydrothermally altered glacio-lacustrine deposit composed of pyroclasts, lavas, scoriaceous fragments, and fine-grained welded or loosely consolidated aggregates, interbedded with clay-rich levels. Several small fans of ejecta were formed, reaching a distance of 1 km north of the lake and covering an area of approximately 0.3 km2, with a maximum thickness of 40 cm at the crater walls. The material (volume of approximately 104 m3) has been ejected by the expanding boiling fluid, generated by a pressure failure affecting the surficial geothermal reservoir. The maximum thermal, craterisation and ejection energies, calculated for the explosion areas, are on the order of 1011, 1010 and 109 J, respectively. Comparison of these with those estimated by the volume of the ejecta and the crater sizes, yields good agreement. We estimate that approximately 30% of the available thermal energy was converted into mechanical energy during this event. The residual energy was largely dissipated as heat, while only a small portion was converted into seismic energy. Estimation of the amount of freshly-fragmented clasts in the ejected material obtained from SEM morphological analyses, reveals that a low but significant energy consumption by fragmentation occurred. Decompression experiments were performed in the laboratory mimicking the conditions due to the drainage of the lake. Experimental results confirm that only a minor amount of energy is consumed by the creation of new surfaces in fragmentation, whereas most of the fresh fragments derive from the disaggregation of aggregates. Furthermore, ejection velocities of the particles (40–50 m/s), measured via high-speed videos, are consistent with those estimated from the field. The multidisciplinary approach used here to investigate hydrothermal explosions has proven to be a valuable tool which can provide robust constraints on energy release and partitioning for such small-size yet hazardous, steam-explosion events.

Description: Volcanic arcs are the surface expression of magmatic systems that result from subduction of mostly oceanic lithosphere at convergent plate boundaries. Arcs with a submarine component include intraoceanic arcs and island arcs that span almost 22,000 km on Earth’s surface, and the vast majority of them are located in the Pacific region. Hydrothermal systems hosted by submarine arc volcanoes commonly contain a large component of magmatic fluid. This magmatic-hydrothermal signature, coupled with the shallow water depths of arc volcanoes and their high volatile contents, strongly influences the chemistry of the fluids and resulting mineralization and likely has important consequences for the biota associated with these systems. The high metal content and very acidic fluids in these hydrothermal systems are thought to be important analogs to numerous porphyry copper and epithermal gold deposits mined today on land. During International Ocean Discovery Program (IODP) Expedition 376 (5 May–5 July 2018), a series of five sites was drilled on Brothers volcano in the Kermadec arc. The expedition was designed to provide the missing link (i.e., the third dimension) in our understanding of hydrothermal activity and mineral deposit formation at submarine arc volcanoes and the relationship between the discharge of magmatic fluids and the deep biosphere. Brothers volcano hosts two active and distinct hydrothermal systems: one is seawater influenced and the other is affected by magmatic fluids (largely gases). In total, 222.4 m of volcaniclastics and lavas were recovered from the five sites drilled, which include Sites U1527 and U1530 in the Northwest (NW) Caldera seawater-influenced hydrothermal field; Sites U1528 and U1531 in the magmatic fluid-influenced hydrothermal fields of the Upper and Lower Cones, respectively; and Site U1529, located within an area of low crustal magnetization that marks the West (W) Caldera upflow zone on the caldera floor. Downhole logging and borehole fluid sampling were completed at two sites, and two tests of a prototype turbine-driven coring system (designed by the Center for Deep Earth Exploration [CDEX] at Japan Agency for Marine-Earth Science and Technology [JAMSTEC]) for drilling and coring hard rocks were conducted. Core recovered from all five sites consists of dacitic volcaniclastics and lava flows with only limited chemical variability relative to the overall range in composition of dacites in the Kermadec arc. Pervasive alteration with complex and variable mineral assemblages attest to a highly dynamic hydrothermal system. The upper parts of several drill holes at the NW Caldera hydrothermal field are characterized by secondary mineral assemblages of goethite + opal + zeolites that result from low-temperature (〈150°C) reaction of rock with seawater. At depth, NW Caldera Site U1527 exhibits a higher temperature (~250°C) secondary mineral assemblage dominated by chlorite + quartz + illite + pyrite. An older mineral assemblage dominated by diaspore + quartz + pyrophyllite + rutile at the bottom of Hole U1530A is indicative of acidic fluids with temperatures of ~230°–320°C. In contrast, the alteration assemblage at Site U1528 on the Upper Cone is dominated by illite + natroalunite + pyrophyllite + quartz + opal + pyrite, which attests to high-temperature reaction of rocks with acid-sulfate fluids derived from degassed magmatic volatiles and the disproportionation of magmatic SO2. These intensely altered rocks exhibit extreme depletion of major cation oxides, such as MgO, K2O, CaO, MnO, and Na2O. Furthermore, very acidic (as low as pH 1.8), relatively hot (≤236°C) fluids collected at 160, 279, and 313 meters below seafloor in Hole U1528D have chemical compositions indicative of magmatic gas input. In addition, preliminary fluid inclusion data provide evidence for involvement of two distinct fluids: phase-separated (modified) seawater and a ~360°C hypersaline brine, which alters the volcanic rock and potentially transports metals in the system. The material and data recovered during Expedition 376 provide new stratigraphic, lithologic, and geochemical constraints on the development and evolution of Brothers volcano and its hydrothermal systems. Insights into the consequences of the different types of fluid–rock reactions for the microbiological ecosystem elucidated by drilling at Brothers volcano await shore-based studies.

Description: Phreatic eruptions, although posing a serious threat to people in crater proximity, are often underestimated and have been comparatively understudied. The detailed eruption catalogue for Ruapehu Volcano (New Zealand) provides an exceptional opportunity to study the statistics of recurring phreatic explosions at a crater lake volcano. We performed a statistical analysis on this phreatic eruption database, which suggests that phreatic events at Ruapehu do not follow a Poisson process. Instead they tend to cluster, which is possibly linked to an increased heat flow during periods of a more shallow-seated magma column. Larger explosions are more likely to follow shortly after smaller events, as opposed to longer periods of quiescence. The absolute probability for a phreatic explosion to occur at Ruapehu within the next month is about 10%, when averaging over the last 70 years of recording. However, the frequency of phreatic explosions is significantly higher than the background level in years prior to magmatic episodes. Combining clast ejection simulations with a Bayesian event tree tool (PyBetVH) we perform a probabilistic assessment of the hazard due to ballistic ejecta in the summit area of Ruapehu, which is frequently visited by hikers. Resulting hazard maps show that the absolute probability for the summit to be affected by ballistics within the next month is up to 6%. The hazard is especially high on the northern lakeshore, where there is a mountain refuge. Our results contribute to the local hazard assessment as well as the general perception of hazards due to steam-driven explosions.