Amatitlan, An actively resurging cauldron 10 km south of Guatemala City

A 14×16 km diameter collapse caldera has been recognized 10 km south of Guatemala City, Guatemala. The caldera is north of the presently active volcano Pacaya and west of Agua, a large stratovolcano. The caldera was not previously recognized because its eastern and western margins coincide with faults that outline the Guatemala City graben and because the northern margin of the caldera is buried by pyroclastic rocks. The existence of the northern caldera margin is now established by gravity data and a variety of geological observations including circumferential faults, hot springs, well-log data, and lithological changes in sedimentary rocks. A sequence of nine silicic pyroclastic deposits, totaling a volume of more than 70 km 3 dense rock were erupted from the caldera. The ages of these eruptions are mainly between about 300,000 years B.P. to less than 23,000 years B.P. The rocks erupted at the caldera and its associated vents consist of domes and nonwelded pyroclastic flow, surge, and fall deposits, mainly of rhyolitic to dacitic composition. Successive pyroclastic eruptions have generally become smaller in volume and more silicic with time. Major and minor element chemistry distinguish Amatitlan pyroclastics from those of other nearby calderas. The caldera lies at the intersection of an offset of the volcanic chain (the Palin Shear) and the faults along the volcanic front (Jalpatagua fault zone). The caldera has a heavily faulted resurgent dome crosscut by an impressive longitudinal graben. The graben\u27s alignment with the Jalpatagua fault zone suggests a genetic relationship. The longitudinal graben and resurgent dome are morphologically youthful and are the sites of many young silicic vents. Available seismic data show a heavy concentration of epicenters over the northern part of the resurgent dome, near a young silicic intrusion. The caldera is active and will probably erupt again. Over 1 million people live within 20 km and would be threatened in the event of a moderate eruption. Suggestions for future research focus on hazard assessment and forecastin

A volcanologist\u27s review of atmospheric hazards of volcanic activity: Fuego and Mount St. Helens

The large amount of scientific data collected on the Mount St. Helens eruption has resulted in significant changes in thinking about the atmospheric hazards caused by explosive volcanic activity. The hazard posed by fine silicate ash with long residence time in the atmosphere is probably much less serious than previously thought. The Mount St. Helens eruption released much fine ash in the upper atmosphere. These silicates were removed very rapidly due to a process of particle aggregation (Sorem, 1982; Carey and Sigurdsson, 1982; Rose and Hoffman, 1982). There is some evidence to suggest that particle aggregation is particularly successful in removing glass shards with high surface areas/mass ratios. The primary atmospheric hazard of explosive eruptions is volcanic sulfur, which is converted to sulfuric acid and sulfate crystals. Although the Mount St. Helens dacite magma had a very low sulfur content before eruption, the eruptions did make a significant contribution to the stratospheric sulfate layer (Newell, 1982). Evidence based on measurements of S and Cl in erupted rocks, glass inclusions, gas samples, and atmospheric samples collected for both Mount St. Helens and Fuego volcanoes, suggests that both volcanoes released substantial contributions of S from intrusive (non-eruptive) magma. The amount of sulfur contributed to the atmosphere by an explosive eruption thus depends not only on the volume of magma erupted and its sulfur content, but also on the degree of near-surface non-eruptive magma. The data collected to assess atmospheric hazard and to evaluate the processes and mechanisms of explosive volcanic eruptions have helped illuminate our understanding of: (1) the dispersion and atmospheric fractionation of volcanic ash and (2) the determination of the size and degassing energetics of shallow magma bodies beneath volcanoes

Quaternary tephra of Northern Central America

Silicic Plinian tephra units representing more than 30 Quaternary eruptions blanket Guatemala and El Salvador. They were erupted mainly from 5 principal sources, all of them calderas. Several of the eruptions were accompanied by ash flows. These eruptions also have the most extensive tephra deposits. The total volume of material erupted is equivalent to 300–500 km3 of dense rock. A major uncertainty is the volume of tephra scattered very far from the source. The volume of silicic magma erupted in the Quaternary is similar to the volumes of mafic lava produced at the volcanic front. The basaltic and andesitic cones of the volcanic front parallel the offshore Middle America trench and the active underthrust zone. The five caldera sources form a trend parallel to the volcanic front, on the side opposite the trench, where the older continental crust abuts the volcanic zone. The ages of silicic volcanism precede and overlap with the age of mafic volcanic front, which is largely younger than 50,000 years. All of the calderas have multiple eruptions which span at least many tens of thousands of years. Between the calderas the interfingering of ashes has allowed a network of relative ages to be established. We used a variety of techniques to characterize these units. They can be readily distinguished from units from many other provinces, but considerable effort is required to distiguish among the local units. Standard field and petrographic observations (stratigraphic data, thicknesses, grain size, lithic content, mineralogy) establish the critical framework which disallows most erroneous correlations. Geochemical analysis, particularly trace elements, provide a rapid means of ruling out many more possible corrections. Qualitative mineralogical analysis by electron microprobe of hornblende and Fe-Ti oxides was a very effective last resort for correlation

Gas emissions and the eruptions of mount St. Helens through 1982

The monitoring of gas emissions from Mount St. Helens includes daily airborne measurements of sulfur dioxide in the volcanic plume and monthly sampling of gases from crater fumaroles. The composition of the fumarolic gases has changed slightly since 1980: the water content increased from 90 to 98 percent, and the carbon dioxide concentrations decreased from about 10 to 1 percent. The emission rates of sulfur dioxide and carbon dioxide were at their peak during July and August 1980, decreased rapidly in late 1980, and have remained low and decreased slightly through 1981 and 1982. These patterns suggest steady outgassing of a single batch of magma (with a volume of not less than 0.3 cubic kilometer) to which no significant new magma has been added since mid-1980. The gas data were useful in predicting eruptions in August 1980 and June 1981

The Magmatic Gas Signature of Pacaya Volcano, With Implications for the Volcanic CO2Flux From Guatemala

Pacaya volcano in Guatemala is one of the most active volcanoes of the Central American Volcanic Arc (CAVA). However, its magmatic gas signature and volatile output have received little attention to date. Here, we present novel volcanic gas information from in-situ (Multi-GAS) and remote (UV camera) plume observations in January 2016. We find in-plume H2O/SO2and CO2/SO2ratios of 2-20 and 0.6-10.5, and an end-member magmatic gas signature of 80.5 mol. % H2O, 10.4 mol. % CO2, and 9.0 mol. % SO2. The SO2flux is evaluated at 885 ± 550 tons/d. This, combined with co-acquired volcanic plume composition, leads to H2O and CO2fluxes of 2,230 ± 1,390 and 700 ± 440, and a total volatile flux of ∼3,800 tons/d. We use these results in tandem with previous SO2flux budgets for Fuego and Santiaguito to estimate the total volcanic CO2flux from Guatemala at ∼1,160 ± 600 tons/day. This calculation is based upon CO2/total S (St) ratios for Fuego (1.5 ± 0.75) and Santiaguito (1.4 ± 0.75) inferred from a gas (CO2/Stratio) versus trace-element (Ba/La ratio) CAVA relationship. The H2O-poor and low CO2/Stratio (∼1.0-1.5) signature of Pacaya gas suggests dominant mantle-wedge derivation of the emitted volatiles. This is consistent with3He/4He ratios in olivine hosted fluid inclusions (FIs), which range between 8.4 and 9.0 Ra (being Ra the atmospheric3He/4He ratio) at the upper limit of MORB range (8 ± 1 Ra). These values are the highest ever measured in CAVA and among the highest ever recorded in arc volcanoes worldwide, indicating negligible4He contributions from the crust/slab