Description: Drilling at Integrated Ocean Drilling Program Site U1381 on the Cocos Ridge offshore Costa Rica recovered 67 primary Miocene (ca. 8 Ma to ca. 16.5 Ma) marine fallout ash layers. Geochemical, volcanological, and geological criteria link these ashes to Plinian eruptions that carried ash to at least 50–450 km from the Galápagos hotspot. These ash layers are the first documentation of highly explosive Miocene Galápagos hotspot volcanism. This volcanism is bimodal with two-thirds of the tephra layers generated by basaltic magmas (glass compositions 〈57 wt% SiO2) and one-third by rhyolitic magmas. The temporal distribution of the tephra layers, inferred from sediment accumulation rates calibrated by 40Ar/39Ar and biostratigraphic ages, reveals a distinct increase in eruption frequency and hence increased volcanic activity of the Galápagos hotspot after 14 Ma which we interpret in the context of dynamic interaction between the Galápagos plume and spreading ridge.

Description: A rigorous detection of Milankovitch periodicities in volcanic output across the Pleistocene-Holocene ice age has remained elusive. We report on a spectral analysis of a large number of well-preserved ash plume deposits recorded in marine sediments along the Pacific Ring of Fire. Our analysis yields a statistically significant detection of a spectral peak at the obliquity period. We propose that this variability in volcanic activity results from crustal stress changes associated with ice age mass redistribution. In particular, increased volcanism lags behind the highest rate of increasing eustatic sea level (decreasing global ice volume) by 4.0 ± 3.6 k.y. and correlates with numerical predictions of stress changes at volcanically active sites. These results support the presence of a causal link between variations in ice age climate, continental stress field, and volcanism.

Description: The upper Quaternary pyroclastic flow deposits of Laacher See volcano show compositional and structural facies variations on four different scales: (1) eruptive units of pyroclastic flows, composed of many flow units; (2) depositional cycles of as many as five flow units; flow units containing (3) regional intraflow-unit facies; and (4) local intraflow-unit subfacies. These facies can be explained by successively overlapping processes beginning in the magma column and ending with final deposition. The pyroclastic flow deposits thus reflect major aspects of the eruptive history of Laacher See volcano: (a) drastic changes in eruptive mechanism due to increasing access of water to the magma chamber and (b) change in chemical composition and crystal and gas content as evacuation of a compositionally zoned magma column progressed. The four scales of facies result from four successive sets of processes: (1) differentiation in the magma column and external factors governing the mechanism of eruption; (2) temporal variations of factors inducing eruption column collapse; (3) physical conditions in the eruption column and the way in which its collapse proceeds; and (4) interplay of flow-inherent and morphology-induced transport mechanics.

Description: Lahars, debris flows, and sediment-rich floods are frequent and deadly hazards at all mountain-forming volcanoes. Their hazard potential is traditionally assessed through mass-conserving closed-system models, where peak conversion rates of potential energy to mechanical energy and hence maximum destruction potential are predicted to occur on the steepest volcano flanks. This belies evidence of extremely high-energy and deadly catastrophes caused by such flows at large distances from volcanoes. Here we use the first high-resolution record of a moving lahar to develop a new model of the temporally and spatially variable mass-flow structure. We show that bulk flow energy can grow dramatically in such systems over tens to hundreds of kilometers via momentum transfers from the lahar into water and particles along its path. We also demonstrate that dynamic transformations of such flows and their ultimate runout are primarily controlled by the mass flow front.

Description: Large explosive volcanic eruptions inject gases, aerosols, and fine ashes into the stratosphere, potentially influencing climate. Emissions of chlorine (Cl) and bromine (Br) from such large eruptions play an important role for catalytic destruction of ozone in the stratosphere, but hitherto the global effects of simultaneous catastrophic release of volcanic Br and Cl into the stratosphere have not been investigated. The Br release from 14 large explosive eruptions throughout Nicaragua covering an entire subduction zone segment in the past 70 ka was determined with petrologic methods. Melt inclusions in volcanic phenocrysts were analyzed using a new optimized synchrotron–X-ray fluorescence microprobe set-up. Single eruptions produced Br outputs of 4–600 kt, giving an average Br emission of 27 kt per eruption. Using the assumption that 10% of the emitted halogens reach the stratosphere, the average Br and Cl loading to the stratosphere would be 3 ppt and 1500 ppt, respectively, which together would account for 185% of the preindustrial equivalent effective stratospheric Cl loading. We thus conclude that many large tropical volcanic eruptions had and have the potential to substantially deplete ozone on a global scale, eventually forming future ozone holes.

Description: At convergent margins, fluids rise through the forearc in response to consolidation of the upper plate and dewatering of the subducting plate, and produce various cold-seep–related features on the seafloor (mud diapirs, mud mounds). At the Central American forearc, authigenic carbonates precipitated from rising fluids within such structures during active venting while typical mixed-mud sediments were ejected onto the surrounding seafloor where they became intercalated with normal pelagic background sediments, indicating that mud mounds evolved unsteadily through alternating active and inactive phases. Intercalated regional ash layers from Plinian eruptions at the Central American volcanic arc provide time marks that constrain the ages of mud ejection activity. U/Th dating of drill core samples of authigenic carbonate caps of mud mounds yields ages agreeing well with those constrained by ash layers and showing that carbonate caps grow inward rather than outward during active venting. Both dating approaches show that offshore Nicaragua and Costa Rica (1) active and inactive phases can occur simultaneously at neighboring mounds, (2) mounds along the forearc have individual histories of activity, but there are distinct time intervals when nearly all mounds have been active or inactive, (3) lifetimes of mounds reach several hundred thousand years, and (4) highly active periods last 10–50 k.y. with intervening periods of 〉10 k.y. of relative quiescence.

Description: Volcanic islands export clastic material to their surrounding oceans by explosive eruptions, lava emissions, biogenic production on their shelves, and failure of their slopes, amongst other processes. This raises the question of whether geological events (in particular, eruptions and landslides) can be detected offshore and dated, and whether any relationships (for example, with climate changes) can be revealed using sediment cores. The volcanically active central Azorean islands (Faial, Pico, São Jorge, and Terceira), with their neighboring submarine basins, are potentially good candidates for such an analysis. Here, chronostratigraphies of four gravity cores collected amongst the islands are constructed based on twelve radiocarbon dates and two dates derived by geochemically correlating primary volcaniclastic turbidites with ignimbrites on Faial and Terceira Islands. Age-depth models are built from the hemipelagic intervals to estimate individual turbidite dates. Volumes of turbidites are modeled by multiplying basin areas with bed thickness, allowing for various turbidite thinning rates and directions. The volumes of landslide-generated turbidites are only comparable with the largest volumes of their adjacent upper-slope submarine landslide valleys; therefore, such turbidites in the cores likely derive from these largest landslides. Emplacement intervals between turbidites originating from both landslides and pyroclastic density currents are found to be mostly a few thousand years. Frequencies of landslide-generated turbidites and hemipelagic sedimentation rates were both highest in the past 8 k.y. compared to preceding periods up to 50 k.y. High hemipelagic sedimentation rates are interpreted to be related to sea-level rise, allowing more shelf bioproduction and release of particles by coastal erosion. The coincident increased frequencies of submarine landslides may also be associated with the increased sediment supply from the islands, resulting in a more rapid build-up of unstable sediments on submarine slopes. Notably, the emplacement frequencies of turbidites of pyroclastic density current origins do not suggest the decreased eruption frequency toward the Holocene that has been found elsewhere.