Matthias Hort

Are the regional variations in Central American arc lavas due to differing basaltic versus peridotitic slab sources of fluids

Central American arc volcanism shows strong regional trends in lava chemistry that result from differing slab contributions to arc melting. However, the mechanism that transfers slab-derived trace elements into the mantle wedge remains largely unknown. By using a dynamic model for mantle flow and fluid release, we model the fate of three different slab-fluid sources: sediment, ocean crust, and serpentinized mantle. In the open subarc system, sediments lose almost all their highly fluid mobile elements by ∼50 km depth, so other fluid sources are necessary to explain the slab signal in arc-lava compositions. The well-documented transition from lavas with a strong geochemical slab signature (i.e., high Ba/La ratios) found in Nicaragua to lavas with a weaker slab signature (i.e., low Ba/La ratios) erupted in Costa Rica seems easiest to produce by a higher fraction of serpentine-hosted fluids released from the deeply faulted, highly serpentinized lithosphere subducting beneath Nicaragua than from the less deeply faulted, thicker, amphibolitic oceanic-crust and oceanic-plateau lithosphere subducting beneath Costa Rica.

MeMoVolc report on classification and dynamics of volcanic explosive eruptions

Classifications of volcanic eruptions were first introduced in the early twentieth century mostly based on qualitative observations of eruptive activity, and over time, they have gradually been developed to incorporate more quantitative descriptions of the eruptive products from both deposits and observations of active volcanoes. Progress in physical volcanology, and increased capability in monitoring, measuring and modelling of explosive eruptions, has highlighted shortcomings in the way we classify eruptions and triggered a debate around the need for eruption classification and the advantages and disadvantages of existing classification schemes. Here, we (i) review and assess existing classification schemes, focussing on subaerial eruptions; (ii) summarize the fundamental processes that drive and parameters that characterize explosive volcanism; (iii) identify and prioritize the main research that will improve the understanding, characterization and classification of volcanic eruptions and (iv) provide a roadmap for producing a rational and comprehensive classification scheme. In particular, classification schemes need to be objective-driven and simple enough to permit scientific exchange and promote transfer of knowledge beyond the scientific community. Schemes should be comprehensive and encompass a variety of products, eruptive styles and processes, including for example, lava flows, pyroclastic density currents, gas emissions and cinder cone or caldera formation. Open questions, processes and parameters that need to be addressed and better characterized in order to develop more comprehensive classification schemes and to advance our understanding of volcanic eruptions include conduit processes and dynamics, abrupt transitions in eruption regime, unsteadiness, eruption energy and energy balance.

Volcanic eruption velocities measured with a micro radar

A novel technique to quantitatively assess volcanic eruption dynamics is presented. We report on measurements of volcanic eruption velocities with a modified micro radar Doppler anemometer on Stromboli volcano, Italy. The mean vertical eruption velocity recorded during a four day period in late 1996 was around 10 m/s which is somewhat on the small side for strombolian eruptions. However, the activity of Stromboli volcano during this period was very weak explaining the relatively small velocities. The technique used is much better in resolution than older techniques based on static photos and allows a continuous monitoring of the volcanic activity. In addition to the velocity information recorded by the Doppler Spectrum it also contains information on the density of the ejected material and the size distribution of the particles in the volcanic jet.

Cooling and crystallization in sheet-like magma bodies revisited

Abstract In order to characterize the interaction of convection and crystallization during the solidification of magmatic sills, a one-dimensional model for the solidification of magmatic sills cooling from above has been used. The model is based on a Stefan-type solution for a mushy zone and takes into account the inward moving solidification front, whose speed is exclusively controlled by crystal growth. Below the solidification front, the magma cools by convection. It may eventually cool below its liquidus and start to partially crystallize. A kinetic model has been implemented to account for this crystallization. The model calculations lead to three main conclusions: (1) Crystal growth is not the only factor controlling the advance of magmatic solidification fronts. The advance appears to be mainly controlled by nucleation processes and only to a small extent by crystal growth. (2) The longevity and vigor of convection inside a magma chamber is strongly dependent on the kinetics of crystallization of magma inside the chamber. (3) Immediately after the initiation of cooling the melt becomes laced with nuclei.

Crystallization calculations for a binary melt cooling at constant rates of heat removal: implications for the crystallization of magma bodies

Abstract We report on thermodynamic non-equilibrium crystallization calculations for a unit volume of a binary melt subject to a constant, prescribed rate of heat loss. Crystallization histories and crystal size distributions for both melt components were calculated by accounting for the nucleation and growth of crystals. The crystal sizes were found to decrease with increasing rates of heat loss. The crystallization time defined as the time to crystallize 99% of the unit volume also decreased strongly with increasing rates of heat loss up to a critical rate. The critical rate was found to be somewhat smaller than the heat loss rate for the beginning of glass formation. At larger than critical rates, crystallization time increased again and for rates larger than the glass formation rate, crystallization time became infinite. The residual melt composition was found to increasingly deviate from the equilibrium composition with increasing rates of heat loss. But as long as the loss rate was less than the critical rate the crystallization path reverted to the eutectic composition during the final crystallization. For supercritical rates, no such reversion was observed. We compared the critical rate with estimates of the rates of heat loss in magmatic intrusions based on the Stefan solution for a freezing half space. It was found that rates of heat loss should be supercritical at distances of up to 0.5 m from the margin of an intrusion. In this region, non-equilibrium effects are expected to dominate and the texture of the crystallized rock should be characterized by small crystals and by glass. The glass and the crystals should be of non-equilibrium composition. Non-equilibrium effects should be negligible only at distances of more than 5 m from the margin where the rates of heat loss are less than 10 −2 times critical. At these distances, the crystallized rock should have an equigranular texture and an equilibrium composition.

Cooling-induced crystallization of microlite crystals in two basaltic pumice clasts

Abstract Microlites in pumice fragments can record the rate of magma decompression and ascent, but only if none grow while those fragments cool in the atmosphere. For highly viscous silicate melts, such crystallization is unlikely, but more basic melts are known to crystallize rapidly, and thus could partially crystallize during cooling and overprint decompression textures. To examine whether postfragmentation crystallization can occur, we examined two basaltic pumice clasts from the sub-Plinian April 1999 eruption of Shishaldin volcano, Alaska. Radial sectioning shows that microlite content doubles from rim to core in both, mainly from growth of plagioclase. Dendrite magnetite also increases greatly in content, but only within the larger pumice clast. Such radial textures demonstrate that crystallization occurred after fragmentation and before deposition (no welding occurred). Using a conductive cooling model coupled with a model for temperature in the eruption column, we estimate that rims of the pumice clasts cool to their glass-transition temperature in ~100-200 s, but their cores take ~500-2000 s to cool, which translates into cooling rates of ~0.2 to 2.5 °C/s. Using a conservative assumption that all plagioclase nucleated before cooling began, we estimate that both short and long axes grew at ~4.8 (±2.7) × 10-7 cm/s. Such rates match those determined experimentally for basaltic melts at similar cooling rates. Magnetite grew only at the slowest cooling rates, and the rate of bulk magnetite crystallization equals that of plagioclase. Our results demonstrate that groundmass crystallization can occur in basic melts on the timescale of explosive eruptions, and so pumice clasts from such eruptions must be viewed with caution before being used to infer eruption dynamics.

Satellite and Ground Based Thermal Observation of the 2014 Effusive Eruption at Stromboli Volcano

As specifically designed platforms are still unavailable at this point in time, lava flows are usually monitored remotely with the use of meteorological satellites. Generally, meteorological satellites have a low spatial resolution, which leads to uncertain results. This paper presents the first long term satellite monitoring of active lava flows on Stromboli volcano (August–November 2014) at high spatial resolution (160 m) and relatively high temporal resolution (~3 days). These data were retrieved by the small satellite Technology Experiment Carrier-1 (TET-1), which was developed and built by the German Aerospace Center (DLR). The satellite instrument is dedicated to high temperature event monitoring. The satellite observations were accompanied by field observations conducted by thermal cameras. These provided short time lava flow dynamics and validation for satellite data. TET-1 retrieved 27 datasets over Stromboli during its effusive activity. Using the radiant density approach, TET-1 data were used to calibrate the MODVOLC data and estimate the time averaged lava discharge rate. With a mean output rate of 0.87 m3/s during the three-month-long eruption, we estimate the total erupted volume to be 7.4 × 106 m3.

Constraints on the formation of submarine lava flows from numerical model calculations

Abstract To constrain life time and flow rates in a submarine lava tube (pillow), 3D finite-element calculations were carried out for a simplified fixed geometry model tube. The principal parameters included into the model are: a temperature-dependent Newtonian viscosity of the lava, whereby viscosity is also dependent on composition, temperature and crystallinity of the lava, cooling by seawater, and the release of latent heat of crystallization. Time-dependent model runs suggest the longevity of the tube flow is constrained by the competition of advective heat flow along the tube axis and the conduction of heat towards the margins of the flow. In order to keep the lava flowing, a minimum critical pressure gradient along the tube is required. Otherwise, the amount of thermal energy brought in by the influx of hot lava is less than the amount lost at the outer boundaries, and the tube will solidify. Providing an exponentially decaying driving pressure at the tube inlet, this critical pressure gradient has been calculated for different tube lengths and diameters and is compared to critical pressure values resulting from a more simplistic solution, assuming constant viscosity and pure conductive or convective cooling. An analysis of the difference between pressure gradients obtained through the numerical and simplified model is used to derive an appropriate power law for the critical pressure gradient in bodies cooled by convection and conduction including the effect of temperature-dependent viscosity. That gradient scales with the tube radius R as R3.85. This law is valid for tube-style lava flows and lava viscosities in the basaltic range.

Theoretical modeling of eruption plumes on Mars under current and past climates

We have used a combined conduit transport/eruption column model to explore the evolution of volcanic eruption plumes on Mars under different atmospheric conditions. In the calculations we consider a volatile phase composed of H2O, CO2, and SO2 and take into account that the magmatic water erupted at the vent may condense as the eruption column rises into the Martian atmosphere. As two end-member models, we explore the eruption of rhyolitic and basaltic melt compositions containing different amounts of volatiles as well as having different eruption temperatures. Under current Martian atmospheric conditions eruption plumes are found to rise as high as 100 km for a mass eruption rate of 5×107 kg s−1, which is consistent with model calculations by Wilson and Head [1994]. In contrast, under a dense atmosphere (105 Pa on the Martian surface) which may have existed earlier in Martian history, the same eruption plume reaches only about 25 km height. All magmatic water released during the eruption is found to freeze as it rises in the eruption column, which means that fallout from the plume will contain water ice which can be subsequently deposited in near surface layers. This ice may then suddenly melt due to higher surface heat flow or shallow intrusions leading to rapid release of water on the flanks of volcanoes. Only if the atmosphere were hotter in the past could the water in the eruption plume condense and produce rain rather than ice. Furthermore, the calculations show that smaller micron-size particles would be distributed globally from eruption plumes under current Martian conditions but would not have been as widely dispersed from plumes erupted into an earlier dense atmosphere.

Constraining the uncertainties of volcano thermal anomaly monitoring using a Kalman filter technique

Abstract The activity status of a volcano can be evaluated from the remotely measured radiant power (RP). The measured RP contains noise due to reasons such as atmospheric effects and instrument characteristics. Here we first show how to estimate the uncertainty of each single RP measurement. To additionally reduce the temporal noise of the RP time series we apply a Kalman filter. The Kalman filter is able to recursively analyse an unevenly sampled time series. To validate our filtering scheme, we applied it to an eruption of Etna in 2002, as well as to the eruption of Nyamuragira in 2010. In the case of the Etna eruption, the denoised time series agrees well with in situ observations of a waxing and waning flow. For the case of Nyamuragira, the enhanced time series of RP shows more fluctuation probably due to cloud coverage. Thus, we propose a multiple instrument approach that increases the temporal resolution of the RP time series and reduces the associated noise.