Bernd Zimanowski

Fragmentation of basaltic melt in the course of explosive volcanism

With the aim to enhance interpretation of fragmentation mechanisms during explosive volcanism from size and shape characteristics of pyroclasts experimental studies have been conducted using remelted volcanic rock (olivine-melilitite). The melt was fragmented and ejected from a crucible by the controlled release of pressurized air volumes (method 1) or by controlled generation of phreatomagmatic explosions (Molten Fuel Coolant Interaction (MFCI); method 2). Both methods were adjusted so that the ejection history of the melt was identical in both cases. The experiments demonstrate that exclusively during MFCI, angular particles in the grain size interval 32 to 130 μm are generated that show surface textures dominated by cracks and pitting. The physical process of their generation is described as a brittle process acting at cooling rates of >106 K/s, at stress rates well above 3 GPa/m2, and during ∼700 μs. In this time period the emission of intense shock waves in the megahertz range was detected, releasing kinetic energy of >1000 J. By both experimental methods, three more types of particles were produced in addition, which could be identified and related to the acceleration and ejection history of the melt: spherical particles, elongated particles, and Peles hair. Abundance and grain size distribution of these particles were found to be proportional to the rate of acceleration and the speed of ejection but were not influenced by the experimental method used. Peles hair occurred at ejection speeds of >75 m/s.

Quantitative experiments on phreatomagmatic explosions

Abstract An experimental set-up for controlled generation of thermal explosions (Fuel Coolant Interactions) has been built by an interdisciplinary research group of volcanologists, physicists and engineering scientists: The TEE-Haus (Thermal Explosion Experiment). During the last 3 years more than 500 experimental runs were performed. Aim of the project was not to build a “mini volcano” but to produce igniteable mixtures of water and hot melt on a small scale in order to investigate basic physical aspects. Such small scale mixtures, however, most probably also occur during phreatomagmatic events in nature. Mixtures of water and magma on a large scale may be modelled by combination of small elements. Earlier experimental studies of thermal explosions were carried out with the use of metal melts nearly exclusively. Therefore, a carbonate melt (approx. 0.3 kg of 1:1 mass ratio Na2CO3 / K2CO3) was used for initial experimental series because of easy handling and more “magma similarity”. By injection of water into this melt explosive mixtures were produced and ignited (self-triggering). Explosive interactions occurred in a wide melt-temperature range (720 to 1040°C). At melt temperatures exceeding 1000°C, CO2 increasingly degassed. Presence of non-condensible gas bubbles in the melt strongly reduced the explosivity. The intensity of explosions only slightly increased with higher melt temperatures but strongly increased with higher water injection velocities, because of more intensive mixing. A minimum injection velocity was found to be approx. 0.7 m s−1. Experiments with silicate melts, derived from diverse remelted volcanic rocks were carried out with a slightly modified set-up. Explosions were ignited by an additional trigger: a shock wave of approx. 8 J. In volcanic systems, shock waves of this magnitude are abundant (volcanic tremor, etc.). Strong explosions with silicate melts were produced at melt temperatures between 1350 and 1750°C. The produced explosions exerted repulsion forces of up to 24,000 N with a typical pulse duration of 1 ms (carbonate melt) resp. 1.5 ms (silicate melts). The ejection velocity was found to be in a range between 200 and 400 m s−1. Explosion energy values up to 500 J were calculated. The maximum explosion pressure was calculated to be in the range between 10 and 100 MPa. Only part of the melt (so-called interactive melt) reacts explosively with injected water. Most of the melt is passively ejected by and after the thermal explosion. Different fragmentation processes result in specific grain size and grain shape populations that can be distinguished from each other. Particles resulting from the fine fragmentation process that precedes and causes the thermal explosion are characterized by angular to subrounded shapes and grain sizes ranging from 20 μm to 180 μm. The amount of interactive melt is proportional to the intensity of the explosion. The maximum amount of water vapourized at the explosive interaction (so-called interactive water) was estimated. Thus, the water/melt mass ratio for explosive interactions could be calculated. The values range between 1 25 and 1 35 (carbonate melt) resp. between 1 6 and 1 25 (silicate melts).

Identifying magma–water interaction from the surface features of ash particles

The deposits from explosive volcanic eruptions (those eruptions that release mechanical energy over a short time span) are characterized by an abundance of volcanic ash. This ash is produced by fragmentation of the magma driving the eruption and by fragmenting and ejecting parts of the pre-existing crust (host rocks). Interactions between rising magma and the hydrosphere (oceans, lakes, and ground water) play an important role in explosive volcanism, because of the unique thermodynamic properties of water that allow it to very effectively convert thermal into mechanical energy. Although the relative proportion of magma to host-rock fragments is well preserved in the pyroclastic rocks deposited by such eruptions, it has remained difficult to quantitatively assess the interaction of magma with liquid water from the analysis of pyroclastic deposits. Here we report the results of a study of natural pyroclastic sequences combined with scaled laboratory experiments. We find that surface features of ash grains can be used to identify the dynamic contact of magma with liquid water. The abundance of such ash grains can then be related to the water/magma mass ratios during their interaction.

The volcanic ash problem

Abstract Explosive volcanic eruptions are the result of intensive magma and rock fragmentation, and they produce volcanic ash, which consists of fragments

Thermal conductivity of a volcanic rock material (olivine-melilitite) in the temperature range between 288 and 1470 K

Experimental studies have been performed on the thermal properties of an olivine-melilitite in solid and partly molten states in a temperature range between 288 and 1470 K. Densities (ρ), heat capacities (Cp), and thermal diffusivities (α) were measured by dilatometry, heat flux differential scanning calorimetry, and laser flash analysis and used to calculate the thermal conductivities (λ). In the solid state (288 to 1220 K) ρ decreased from 3091 to 2993 kg/m3, Cp increased from 0.76 to 1.41 J g−1 K−1, α varied between 5.1 and 8.7 10−3 cm2/s, and λ varied between 1.75 and 2.54 W m−1 K−1. In the partly molten state (1300 to 1470 K) Cp increased from 1.56 to 1.70 J g−1 K−1, α decreased from 2.5 to 2.2 10−3 cm2/s, and λ decreased from 1.16 to 1.09 W m−1 K−1. The importance of these data for the physical description of processes acting during explosive volcanic eruptions is discussed.

Phreatomagmatic Explosions in Subaqueous Volcanism

Pyroclastic deposits, produced during subaqueous volcanic eruptions, point to the existence of explosive processes. Magma/water interaction is a possible source of these explosions. Under atmospheric pressure a thermohydraulic explosion mechanism was identified that can explain the high kinetic energy release of phreatomagmatic explosion and the formation of typical subaerial phreatomagmatic pyroclastic deposits. The applicability of this mechanism under subaqueous physical conditions is discussed. Whereas the efficacy of heat energy transfer from magma to water in general increases with rising hydrostatic pressure, the conditions for the formation of a critical magma-water premix volume increasingly decline. Our analysis indicates that subaqueous volcanic thermohydraulic explosions should become increasingly improbable at water depths exceeding 100 m and practically impossible at water depths in excess of 1 km. Pyroclastic deposits found at greater depths, bearing signatures of phreatomagmatic origin therefore should be the result of comparably low energy magma-water interaction.

Dynamic mingling of magma and liquefied sediments

Abstract Hydrodynamic mingling of magma and liquefied sediments is generally accepted to represent the key process in the formation of some peperites. Experimental studies on simulant liquids and calculations based on recent empirical findings in the field of polymer research were undertaken to investigate the effectiveness of this process. These studies show that for a wide range of shear rates a laminar flow behaviour of the system magma–liquefied sediment can be expected, i.e. turbulent mingling is not a realistic scenario. Formation of peperitic fabrics with grain sizes on a cm scale can hydrodynamically be explained under realistic intrusion velocities. In addition to the hydrodynamics, cooling processes must also be considered during peperite formation. Calculations of the cooling history of the juvenile magmatic component under realistic cooling conditions demonstrate a significant limitation of the hydrodynamic mingling time. The consequence for peperite formation under normal intrusion conditions is that domains of magmatic melt and domains of liquefied sediment smaller than 10 cm are not probable. However, as smaller domain sizes (peperitic grain sizes) exist in nature, we conclude that either those peperites represent the products of explosive events or that hydrodynamic mingling was accompanied by additional fragmentation processes.

On the formation of deep-seated subterranean peperite-like magma–sediment mixtures

Abstract In addition to large amounts of fragmented country rocks, maar–diatreme volcanoes commonly eject juvenile lapilli and cauliflower bombs containing many dispersed xenoliths and/or xenocrysts derived from the country rocks surrounding the diatremes. Formation of these lapilli and bombs is attributed to thermohydraulic explosions in the root zones of the diatremes and their consequences. The thermohydraulic explosions release shock waves which are capable of intensely fragmenting the surrounding country rocks. Vaporisation of water superheated during thermohydraulic explosions and its expansion to lower pressure causes ejection of major parts of the fragmented country rocks. Surviving sections of fragmented country rocks may collapse or slide into the partially evacuated root zone and thus form a mass-flow deposit subsequent to each explosion. When, prior to a new explosion, magma rises from the underlying feeder dyke into the root zone, it intrudes this clastic debris, inflates it and may mingle with the debris to form a peperite. Subsequent explosions can fragment and eject fragments of this peperite in the form of juvenile ash grains, juvenile lapilli and bombs containing xenoliths or xenocrysts from the country-rock debris. Lack of further explosions, but continued rise and intrusion of magma will cause emplacement of such inflated peperite masses as plugs.

On the first experimental phreatomagmatic explosion of a kimberlite melt

Abstract Detailed field investigations of kimberlite pipes of the Upper Cretaceous Gibeon Kimberlite Field in southern Namibia revealed geologic features which do neither agree with extensive nor with explosive vesiculation. In the pipe Hanaus 2, vesicle-free magmatic kimberlite intruded the diatreme as a plug. Here we report on the first experiments in which phreatomagmatic explosions were produced with remolten natural, non-fragmental magmatic kimberlite from the Hanaus 2 central plug as a best fit model kimberlite magma system. After water was injected into the melt under controlled conditions, the resulting water–melt mix was triggered by a low-energy shock wave ( 400 m/s.

Conduit flow experiments help constraining the regime of explosive eruptions

Accepted for publication in (Geophysical Research Letters). Copyright (2009) American Geophysical Union.