C. Ian Schipper

Exceptional mobility of an advancing rhyolitic obsidian flow at Cordón Caulle volcano in Chile

The emplacement mechanisms of rhyolitic lava flows are enigmatic and, despite high lava viscosities and low inferred effusion rates, can result in remarkably, laterally extensive (>30 km) flow fields. Here we present the first observations of an active, extensive rhyolitic lava flow field from the 2011-2012 eruption at Cordón Caulle, Chile. We combine high-resolution four-dimensional flow front models, created using automated photo reconstruction techniques, with sequential satellite imagery. Late-stage evolution greatly extended the compound lava flow field, with localized extrusion from stalled, ~35 m-thick flow margins creating >80 breakout lobes. In January 2013, flow front advance continued ~3.6 km from the vent, despite detectable lava supply ceasing 6-8 months earlier. This illustrates how efficient thermal insulation by the lava carapace promotes prolonged within-flow horizontal lava transport, boosting the extent of the flow. The unexpected similarities with compound basaltic lava flow fields point towards a unifying model of lava emplacement.

Mechanisms of bubble coalescence in silicic magmas

Bubble coalescence is an important process that strongly affects magmatic degassing. Without coalescence, bubbles remain isolated from one another in the melt, severely limiting gas release. Despite this fact, very little has been done to identify coalescence mechanisms from textures of magmatic rocks or to quantify the dynamics of bubble coalescence in melts. In this paper, we present a systematic study of bubble-coalescence mechanisms and dynamics in natural and experimentally produced bubbly rhyolite magma. We have used a combination of natural observations aided by high-resolution X-ray computed tomography, petrological experiments, and physical models to identify different types of bubble–bubble interaction that lead to coalescence on the timescales of magma ascent and eruption. Our observations and calculations suggest that bubbles most efficiently coalesce when inter-bubble melt walls thin by stretching rather than by melt drainage from between converging bubble walls. Orders of magnitude are more rapid than melt drainage, bubble wall stretching produces walls thin enough that inter-bubble pressure gradients may cause the melt wall to dimple, further enhancing coalescence. To put these results into volcanogical context, we have identified magma ascent conditions where each coalescence mechanism should act, and discuss the physical conditions for preserving coalescence structures in natural pumice. The timescales we propose could improve volcanic eruption models, which currently do not account for bubble coalescence. Although we do not address the effect of shear strain on bubble coalescence, the processes discussed here may operate in several different eruption regimes, including vesiculation of lava domes, post-fragmentation frothing of vulcanian bombs, and bubbling of pyroclasts in conduits.

“Poseidic” explosive eruptions at Loihi Seamount, Hawaii

Much remains unknown about submarine explosive eruptions. Their deposits are found to great depths in all the world’s oceans, but eruptions are typically described by analogy to a subaerial nomenclature that ignores the substantial and inevitable infl uences of hydrostatic pressure and magma-water interaction at submerged edifi ces. Here we explore magmatic volatile exsolution and magma-water interaction for a pyroclastic cone-forming eruption at ~1 km depth on Loihi Seamount, Hawaii. We examine vesicle textures in lapilli—the physical manifestation of degassing; dissolved volatiles in matrix glasses and olivine-hosted glass inclusions—the geochemical record of ascent and volatile exsolution; and fi ne ash morphology—the evidence for if and how external water assisted in fragmentation. This approach allows a submarine explosive eruption style to be defi ned: the magma achieved ~40% vesicularity through almost perfectly closed-system volatile exsolution from ~3 km below the vent, which accelerated and weakened the melt, allowing it to be fragmented by explosive magma-water interaction. We introduce the name “Poseidic” for this end-member style of submarine basalt explosivity. Poseidic eruptions are identifi able from measurable features in pyroclasts, and are possible at all subaqueous basaltic volcanoes.

Rapid laccolith intrusion driven by explosive volcanic eruption

Magmatic intrusions and volcanic eruptions are intimately related phenomena. Shallow magma intrusion builds subsurface reservoirs that are drained by volcanic eruptions. Thus, the long-held view is that intrusions must precede and feed eruptions. Here we show that explosive eruptions can also cause magma intrusion. We provide an account of a rapidly emplaced laccolith during the 2011 rhyolite eruption of Cordón Caulle, Chile. Remote sensing indicates that an intrusion began after eruption onset and caused severe (>200 m) uplift over 1 month. Digital terrain models resolve a laccolith-shaped body ∼0.8 km3. Deformation and conduit flow models indicate laccolith depths of only ∼20–200 m and overpressures (∼1–10 MPa) that likely stemmed from conduit blockage. Our results show that explosive eruptions may rapidly force significant quantities of magma in the crust to build laccoliths. These iconic intrusions can thus be interpreted as eruptive features that pose unique and previously unrecognized volcanic hazards.

Conduit Dynamics in Transitional Rhyolitic Activity Recorded by Tuffisite Vein Textures from the 2008–2009 Chaitén Eruption

The mechanisms of hazardous silicic eruptions are controlled by complex, poorly-understood conduit processes. Observations of recent Chilean rhyolite eruptions have revealed the importance of hybrid activity, involving simultaneous explosive and effusive emissions from a common vent. Such behaviour hinges upon the ability of gas to decouple from magma in the shallow conduit. Tuffisite veins are increasingly suspected to be a key facilitator of outgassing, as they repeatedly provide a transient permeable escape route for volcanic gases. Intersection of foam domains by tuffisite veins appears critical to efficient outgassing. However, knowledge is currently lacking into textural heterogeneities within shallow conduits, their relationship with tuffisite vein propagation, and the implications for fragmentation and degassing processes. Similarly, the magmatic vesiculation response to upper conduit pressure perturbations, such as those related to the slip of dense magma plugs, remains largely undefined. Here we provide a detailed characterization of an exceptionally large tuffisite vein within a rhyolitic obsidian bomb ejected during transitional explosive-effusive activity at Chaiten, Chile in May 2008. Vein textures and chemistry provide a time-integrated record of the invasion of a dense upper conduit plug by deeper fragmented magma. Quantitative textural analysis reveals diverse vesiculation histories of various juvenile clast types. Using vesicle size distributions, bubble number densities, zones of diffusive water depletion, and glass H2O concentrations, we propose a multi-step degassing/fragmentation history, spanning deep degassing to explosive bomb ejection. Rapid decompression events of ~3-4 MPa are associated with fragmentation of foam and dense magma at ~200-350 metres depth in the conduit, permitting vertical gas and pyroclast mobility over hundreds of metres. Permeable pathway occlusion in the dense conduit plug by pyroclast accumulation and sintering preceded ultimate bomb ejection, which then triggered a final bubble nucleation event. Our results highlight how the vesiculation response of magma to decompression events is highly sensitive to the local melt volatile concentration, which is strongly spatially heterogeneous. Repeated opening of pervasive tuffisite vein networks promotes this heterogeneity, allowing juxtaposition of variably volatile-rich magma fragments that are derived from a wide range of depths in the conduit. This process enables efficient but explosive removal of gas from rhyolitic

Magma-slurry interaction in Surtseyan eruptions

It is established that Surtseyan eruptions involve extensive magma-water interaction, but the specific volumes, geometries, and dynamic consequences of such interaction have not been precisely characterized. Textural studies seeking to understand phreatomagmatism have mainly focused on fine ash—an approach that is intuitive given the abundance of fine particles in Surtseyan deposits, but that neglects additional information preserved in coarser particles. Virtually without exception, scoria bombs from Surtsey (Iceland) and similar volcanoes show composite textures, with host material having entrained smaller clasts. Entrained clasts commonly show evidence of post-entrapment groundmass crystallization, and always are surrounded by void space indicating that they were wet at the time of entrapment. The composite textures—ubiquitous in bombs but also common in lapilli—support a classical model that describes Surtseyan volcanism as being driven by mingling of magma and water-saturated slurry in periodically flooded vents. We use textures, eruption observations, and basic thermodynamics to expand the magma-slurry model and relate it directly to the vapor dynamics that characterize Surtseyan jets and plumes.

The Surtsey Magma Series

The volcanic island of Surtsey (Vestmannaeyjar, Iceland) is the product of a 3.5-year-long eruption that began in November 1963. Observations of magma-water interaction during pyroclastic episodes made Surtsey the type example of shallow-to-emergent phreatomagmatic eruptions. Here, in part to mark the 50th anniversary of this canonical eruption, we present previously unpublished major-element whole-rock compositions, and new major and trace-element compositions of sideromelane glasses in tephra collected by observers and retrieved from the 1979 drill core. Compositions became progressively more primitive as the eruption progressed, with abrupt changes corresponding to shifts between the eruption’s four edifices. Trace-element ratios indicate that the chemical variation is best explained by mixing of different proportions of depleted ridge-like basalt, with ponded, enriched alkalic basalt similar to that of Iceland’s Eastern Volcanic Zone; however, the systematic offset of Surtsey compositions to lower Nb/Zr than other Vestmannaeyjar lavas indicates that these mixing end members are as-yet poorly contained by compositions in the literature. As the southwestern-most volcano in the Vestmannaeyjar, the geochemistry of the Surtsey Magma Series exemplifies processes occurring within ephemeral magma bodies on the extreme leading edge of a propagating off-axis rift in the vicinity of the Iceland plume.

Incipient melt segregation as preserved in subaqueous pyroclasts

Melt segregation is the extraction of residual melt from the rigid but permeable network of growing crystals with which it co-evolved. It is recognized as an effective mechanism of igneous differentiation that acts over many geologic time and length scales. Here we present evidence for rapid melt segregation in subaqueous basaltic pyroclasts. Segregation produced intravesicular extrusions, i.e., partly hollow balloons of glass penetrating the walls of earlier formed vesicles set in a microcrystalline groundmass. The segregation process can be described as a form of gas filter pressing, where microlite crystallization produced local gradients in volatile supersaturation and vapor pressure that drove melt extrusion into adjacent vesicles. Unlike in previously described segregation features, the structures presented here are shown to have formed after the extruded melt became enriched in fast-diffusing H2O, but before it became measurably enriched or depleted in slower diffusing major elements. We show that melt segregation of this embryonic type must occur in seconds, rather than in the days to centuries required for similar processes to occur in lava flows or magma chambers, and that it occurs within the short time scales that characterize explosive fragmentation of basaltic melt.

Submarine Explosive Eruptions

No large submarine explosive eruption has ever been witnessed, yet such eruptions are known to be fairly common from deposits of ancient seafloors, from tiny eruptions recently witnessed by remote vehicle, and from when submarine explosions breach the ocean surface. Most of the seafloor comprises basaltic lavas, but on seamounts and even some seafloor spreading centers explosive eruptions have produced primary volcaniclastic deposits up to hundreds of meters thick. Particles are characteristically glassy, dense to highly vesicular, and in some cases comprise tiny folded glass sheets, the remnants of burst bubbles. The eruptions producing all these deposits are seafloor-specific, modulated both by the ocean environment and the extent of pre-eruption volatile exsolution and escape from magma. Surtsey’s birth in 1963 highlighted features of submarine basalt eruptions as they breach the ocean surface, while in Japanese waters Myojinsho’s 1957 eruption had showcased explosive submarine eruption of silicic magmas. The modern seafloor south of Japan, and in many other places in the geological past have hosted large-scale, caldera-forming explosive submarine eruptions of silicic magmas, and in 2012 the sea-surface expression of such a large-scale explosive eruption was identified for the first time ever. Our understanding of submarine explosive volcanism grows as the seafloor becomes better known, but much remains to be learned.

Temporal redox variation in basaltic tephra from Surtsey volcano (Iceland)

The oxidation state of magma controls and/or tracks myriad petrologic phenomena, and new insights into oxidation are now made possible by high-resolution measurements of Fe3+/∑Fe in volcanic glasses. We present new μ-XANES measurements of Fe3+/∑Fe in a time series of basaltic tephra from the 1963–1967 eruption of Surtsey (Iceland), to examine if the magma mixing between alkalic and tholeiitic basalts that is apparent in the major and trace elements of these glasses is also represented in their oxidation states. Raw Fe3+/∑Fe data show a temporal trend from oxidized to reduced glasses, and this is accompanied by decreasing indices of mantle enrichment (e.g., La/Yb, Zr/Y). When expressed as composition- and temperature-corrected fO2, the trend has a similar magnitude (~0.3 log units) to the variation in fO2 due to ridge-plume interaction along the Reykjanes Ridge. These data indicate that the oxidation state of mixed magmas can be retained through fractionation and degassing processes, and that matrix glass Fe3+/∑Fe in tephras can be used to make inferences about the relative oxidation states of parental magmas during nuanced magma mixing.