Helge M. Gonnermann

Explosive volcanism may not be an inevitable consequence of magma fragmentation

The fragmentation of magma, containing abundant gas bubbles, is thought to be the defining characteristic of explosive eruptions. When viscous stresses associated with the growth of bubbles and the flow of the ascending magma exceed the strength of the melt, the magma breaks into disconnected fragments suspended within an expanding gas phase. Although repeated effusive and explosive eruptions for individual volcanoes are common, the dynamics governing the transition between explosive and effusive eruptions remain unclear. Magmas for both types of eruptions originate from sources with similar volatile content, yet effusive lavas erupt considerably more degassed than their explosive counterparts. One mechanism for degassing during magma ascent, consistent with observations, is the generation of intermittent permeable fracture networks generated by non-explosive fragmentation near the conduit walls. Here we show that such fragmentation can occur by viscous shear in both effusive and explosive eruptions. Moreover, we suggest that such fragmentation may be important for magma degassing and the inhibition of explosive behaviour. This implies that, contrary to conventional views, explosive volcanism is not an inevitable consequence of magma fragmentation.

Preserving noble gases in a convecting mantle

High 3He/4He ratios sampled at many ocean islands are usually attributed to an essentially undegassed lower-mantle reservoir with high 3He concentrations. A large and mostly undegassed mantle reservoir is also required to balance the Earth’s 40Ar budget, because only half of the 40Ar produced from the radioactive decay of 40K is accounted for by the atmosphere and upper mantle. However, geophysical and geochemical observations suggest slab subduction into the lower mantle, implying that most or all of Earth’s mantle should have been processed by partial melting beneath mid-ocean ridges and hotspot volcanoes. This should have left noble gases in both the upper and the lower mantle extensively outgassed, contrary to expectations from 3He/4He ratios and the Earth’s 40Ar budget. Here we suggest a simple solution: recycling and mixing of noble-gas-depleted slabs dilutes the concentrations of noble gases in the mantle, thereby decreasing the rate of mantle degassing and leaving significant amounts of noble gases in the processed mantle. As a result, even when the mass flux across the 660-km seismic discontinuity is equivalent to approximately one lower-mantle mass over the Earth’s history, high 3He contents, high 3He/4He ratios and 40Ar concentrations high enough to satisfy the 40Ar mass balance of the Earth can be preserved in the lower mantle. The differences in 3He/4He ratios between mid-ocean-ridge basalts and ocean island basalts, as well as high concentrations of 3He and 40Ar in the mantle source of ocean island basalts, can be explained within the framework of different processing rates for the upper and the lower mantle. Hence, to preserve primitive noble gas signatures, we find no need for hidden reservoirs or convective isolation of the lower mantle for any length of time.

Plume capture by divergent plate motions: implications for the distribution of hotspots, geochemistry of mid-ocean ridge basalts, and estimates of the heat flux at the core–mantle boundary

Abstract The coexistence of stationary mantle plumes with plate-scale flow is problematic in geodynamics. We present results from laboratory experiments aimed at understanding the effects of an imposed large-scale circulation on thermal convection at high Rayleigh number (106≤Ra≤109) in a fluid with a temperature-dependent viscosity. In a large tank, a layer of corn syrup is heated from below while being stirred by large-scale flow due to the opposing motions of a pair of conveyor belts immersed in the syrup at the top of the tank. Three regimes are observed, depending on the ratio V of the imposed horizontal flow velocity to the rise velocity of plumes ascending from the hot boundary, and on the ratio λ of the viscosity of the interior fluid to the viscosity of the hottest fluid in contact with the bottom boundary. When V≪1 and λ≥1, large-scale circulation has a negligible effect on convection and the heat flux is due to the formation and rise of randomly spaced plumes. When V>10 and λ>100, plume formation is suppressed entirely, and the heat flux is carried by a sheet-like upwelling located in the center of the tank. At intermediate V, and depending on λ, established plume conduits are advected along the bottom boundary and ascending plumes are focused towards the central upwelling. Heat transfer across the layer occurs through a combination of ascending plumes and large-scale flow. Scaling analyses show that the bottom boundary layer thickness and, in turn, the basal heat flux q depend on the Peclet number, Pe, and λ. When λ>10, q∝Pe1/2 and when λ→1, q∝(Peλ)1/3, consistent with classical scalings. When applied to the Earth, our results suggest that plate-driven mantle flow focuses ascending plumes towards upwellings in the central Pacific and Africa as well as into mid-ocean ridges. Furthermore, plumes may be captured by strong upwelling flow beneath fast-spreading ridges. This behavior may explain why hotspots are more abundant near slow-spreading ridges than fast-spreading ridges and may also explain some observed variations of mid-ocean ridge basalt (MORB) geochemistry with spreading rate. Moreover, our results suggest that a potentially significant fraction of the core heat flux is due to plumes that are drawn into upwelling flows beneath ridges and not observed as hotspots.

Non-equilibrium degassing and a primordial source for helium in ocean-island volcanism

Radioactive decay of uranium and thorium produces 4He, whereas 3He in the Earth’s mantle is not produced by radioactive decay and was only incorporated during accretion—that is, it is primordial. 3He/4He ratios in many ocean-island basalts (OIBs) that erupt at hotspot volcanoes, such as Hawaii and Iceland, can be up to sixfold higher than in mid-ocean ridge basalts (MORBs). This is inferred to be the result of outgassing by melt production at mid-ocean ridges in conjunction with radiogenic ingrowth of 4He, which has led to a volatile-depleted upper mantle (MORB source) with low 3He concentrations and low 3He /4He ratios. Consequently, high 3He/4He ratios in OIBs are conventionally viewed as evidence for an undegassed, primitive mantle source, which is sampled by hot, buoyantly upwelling deep-mantle plumes. However, this conventional model provides no viable explanation of why helium concentrations and elemental ratios of He/Ne and He/Ar in OIBs are an order of magnitude lower than in MORBs. This has been described as the ‘helium concentration paradox’ and has contributed to a long-standing controversy about the structure and dynamics of the Earth’s mantle. Here we show that the helium concentration paradox, as well as the full range of noble-gas concentrations observed in MORB and OIB glasses, can self-consistently be explained by disequilibrium open-system degassing of the erupting magma. We show that a higher CO2 content in OIBs than in MORBs leads to more extensive degassing of helium in OIB magmas and that noble gases in OIB lavas can be derived from a largely undegassed primitive mantle source.

Effects of crystal shape‐ and size‐modality on magma rheology

Erupting magma often contains crystals over a wide range of sizes and shapes, potentially affecting magma viscosity over many orders of magnitude. A robust relation between viscosity and the modality of crystal sizes and shapes remains lacking, principally because of the dimensional complexity and size of the governing parameter space. We have performed a suite of shear viscosity measurements on liquid-particle suspensions of dynamical similarity to crystal-bearing magma. Our experiments encompass five suspension types, each consisting of unique mixtures of two different particle sizes and shapes. The experiments span two orthogonal subspaces of particle concentration, as well as particle size and shape for each suspension type, thereby providing insight into the topology of parameter space. For each suspension type, we determined the dry maximum packing fraction and measured shear rates across a range of applied shear stresses. The results were fitted using a Herschel-Bulkley model and augment existing predictive capabilities. We demonstrate that our results are consistent with previous work, including friction-based constitutive laws for granular materials. We conclude that predictions for ascent rates of crystal-rich magmas must take the shear-rate dependence of viscosity into account. Shear-rate dependence depends first and foremost on the volume fraction of crystals, relative to the maximum packing fraction, which in turn depends on crystal size and shape distribution.

Modeling Volcanic Processes: Dynamics of magma ascent in the volcanic conduit

Overview This chapter presents the various mechanisms and processes that come into play within the volcanic conduit for a broad range of effusive and dry explosive volcanic eruptions. Decompression during magma ascent causes volatiles to exsolve and form bubbles containing a supercritical fluid phase. Viscous magmas, such as rhyolite or crystal-rich magmas, do not allow bubbles to ascend buoyantly and may also hinder bubble growth. This can lead to significant gas overpressure and brittle magma fragmentation. During fragmentation in vulcanian, subplinian, and plinian eruptions, gas is released explosively into the atmosphere, carrying with it magma fragments. Alternatively, high viscosity may slow ascent to where permeable outgassing through the vesicular and perhaps fractured magma results in lava effusion to produce domes and flows. In low-viscosity magmas, typically basalts, bubbles may ascend buoyantly, allowing efficient magma outgassing and relatively quiescent magma effusion. Alternatively, bubbles may coalesce and accumulate to form meter-size gas slugs that rupture at the surface during strombolian eruptions. At fast magma ascent rates, even in low-viscosity magmas, melt and exsolved gas remain coupled, allowing for rapid acceleration and hydrodynamic fragmentation in hawaiian eruptions. Introduction In the broadest sense, volcanic eruptions are either effusive or explosive. During explosive eruptions magma fragments and eruption intensity is ultimately related to the fragmentation mechanism and associated energy expenditure (Zimanowski et al ., 2003). If the cause of fragmentation is the interaction of hot magma with external water, the ensuing eruption is called phreatomagmatic. Eruptions that do not involve external water are called “dry,” in which case the abundance and fate of magmatic volatiles, predominantly H 2 O and CO 2 , as well as magma rheology and eruption rate, are the dominant controls on eruption style. Eruption styles are often correlated with magma composition and to some extent this relationship reflects differences in tectonic setting, which also influence magmatic volatile content and magma supply rate.

Relating vesicle shapes in pyroclasts to eruption styles

Vesicles in pyroclasts provide a direct record of conduit conditions during explosive volcanic eruptions. Although their numbers and sizes are used routinely to infer aspects of eruption dynamics, vesicle shape remains an underutilized parameter. We have quantified vesicle shapes in pyroclasts from fall deposits of seven explosive eruptions of different styles, using the dimensionless shape factor

Convection in a volcanic conduit recorded by bubbles

\Omega

Explosive to effusive transition during the largest volcanic eruption of the 20th century (Novarupta 1912, Alaska)

, a measure of the degree of complexity of the bounding surface of an object. For each of the seven eruptions, we have also estimated the capillary number, Ca, from the magma expansion velocity through coupled diffusive bubble growth and conduit flow modeling. We find that

Biochar particle size, shape, and porosity act together to influence soil water properties

\Omega