This paper examines phreatic eruptions which are driven by inputs of magma and magmatic gas. We synthesize data from several significant phreatic systems, including two in Costa Rica (Turrialba and Poás) which are currently highly active and hazardous. We define two endmember types of phreatic eruptions, the first (type 1) in which a deeper hydrothermal system fed by magmatic gases is sealed and produces overpressure sufficient to drive explosive eruptions, and the second (type 2) where magmatic gases are supplied via open-vent degassing to a near-surface hydrothermal system, vaporizing liquid water which drives the phreatic eruptions. The surficial source of type 2 eruptions is characteristic, while the source depth of type 1 eruptions is commonly greater. Hence, type 1 eruptions tend to be more energetic than type 2 eruptions. The first type of eruption we term “phreato-vulcanian”, and the second we term “phreato-surtseyan”. Some systems (e.g., Ruapehu, Poás) can produce both type 1 and type 2 eruptions, and all systems can undergo sealing at various timescales. We examine a number of precursory signals which appear to be important in understanding and forecasting phreatic eruptions; these include very long period events, banded tremor, and gas ratios, in particular H2S/SO2 and CO2/SO2. We propose that if these datasets are carefully integrated during a monitoring program, it may be possible to accurately forecast phreatic eruptions.

Identification of unambiguous signals of volcanic unrest is crucial in hazard assessment. Processes leading to phreatic and phreatomagmatic eruptions remain poorly understood, inhibiting effective eruption forecasting. Our 5‐year gas record from Poás volcano, combined with geophysical data, reveals systematic behavior associated with hydrothermal‐magmatic eruptions. Three eruptive episodes are covered, each with distinct geochemical and geophysical characteristics. Periods with larger eruptions tend to be associated with stronger excursions in monitoring data, particularly in SO₂/CO₂ and SO₂ flux. The explosive 2017 phreatomagmatic eruption was the largest eruption at Poás since 1953 and was preceded by dramatic changes in gas and geophysical parameters. The use of drones played a crucial role in gas monitoring during this eruptive period. Hydrothermal sealing and volatile accumulation, followed by top‐down reactivation of a shallow previously emplaced magma body upon seal failure, are proposed as important processes leading to and contributing to the explosivity of the 2017 eruption.

Gas measurements using unmanned aerial vehicles, or drones, were undertaken at Turrialba volcano, Costa Rica, and Masaya volcano, Nicaragua, in 2016 and 2017. These two volcanoes are the largest time‐integrated sources of gas in the Central American Volcanic Arc, and both systems are currently extremely active with potential for sudden destabilization. We employed a series of miniaturized drone‐mounted instrumentation including a mini‐DOAS, two MultiGAS instruments, and an optical particle counter, supplemented by ground‐based measurements. Payloads were typically 1–1.5 kg and flight times were 10–15 min. The measurements were both accurate and precise due to the inherent sensitivity of the instrumentation and the high gas concentrations, which the drones were able to sample. The quality of data obtained by our drones was comparable to that obtained by our ground‐based measurements. At Turrialba in April 2017, we measured an average SO₂ flux of 1,380 ± 280 T/day, CO₂/SO₂ of 6.5, and H₂O/SO₂ of 27.8. Using these values, we calculated a CO₂ flux of 6,170 T/day and an H₂O flux of 10,790 T/day. At Masaya in May 2017, the average SO₂ flux was 1,560 ± 180 T/day, with CO₂/SO₂ of 3.9 and H₂O/SO₂ of 62.3, giving a mean CO₂ flux of 4,150 T/day and mean H₂O flux of 27,330 T/day. The elevated carbon and water fluxes and ratios are indicative of underlying magmas that are enriched in these components, resulting in the high levels of activity observed.

The Valles Caldera complex in north–central New Mexico, USA, represents the type example of a resurgent caldera system, characterized by eruption of two voluminous high-silica rhyolite ignimbrites, the Otowi and Tshirege Members of the Bandelier Tuff. Central resurgence of ∼1000 m occurred within 27 ± 27 kyr following eruption of the Tshirege, or Upper Bandelier Tuff (UBT). This process was accompanied by small-volume eruptions of the Deer Canyon Rhyolite, followed closely by the Redondo Creek Rhyodacite. The Cerro del Medio Rhyolite lava dome complex is a product of ring fracture volcanism following resurgence. We have combined cathodoluminescence (CL) imaging and titanium-in-quartz geothermometry techniques for single quartz crystals from (1) different stratigraphic horizons of the UBT ignimbrite, (2) samples of the Deer Canyon Rhyolite and (3) the Cerro del Medio Rhyolite lavas to build an evolutionary model of the Valles Caldera lifecycle. CL imaging reveals that ∼70% by volume of the UBT ignimbrite contains unzoned quartz crystals (average concentration 28 ± 2 ppm Ti), recording relatively stable conditions. The abrupt appearance of compositionally zoned quartz crystals within the mid-to-late erupted UBT ignimbrite units 3–5 reveals evidence for interaction with hotter magma. Corresponding titanium-in-quartz measurements of outer, high-intensity CL rims (average of 71 ± 9 ppm Ti) reveal temperature increases of between ∼35 and 140°C relative to the initial, volumetrically large Upper Member eruptions (flow units 1 and 2). These temperature increases mimic those originally reported in the literature. Compared with the temperature increases, changes in pressure appear to play a secondary role. We have also discovered an interesting heterogeneity within the Deer Canyon Rhyolite lavas, with strong spatial control on the eruption of porphyritic lavas containing complexly zoned quartz crystals on the western parts of the resurgent dome. Conversely, crystal-poor to aphyric lavas containing small, unzoned quartz crystals are confined to eastern parts of the resurgent dome. The Cerro del Medio Rhyolite lavas are sparsely porphyritic to aphyric, and contain unzoned quartz with titanium concentrations more than 40 ppm higher than the cores of UBT quartz. The quartz-free Redondo Creek Rhyodacite is the most primitive silicic material erupted during the Valles Caldera cycle. Intrusion of this hotter magma into a residual UBT crystal mush zone may have facilitated eruption and geochemical–thermal heterogeneity within the Deer Canyon Rhyolite, resurgence of the caldera, and expulsion of hot, crystal-poor rhyolite batches from the mush zone, which were subsequently erupted as the Cerro del Medio complex.