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The nature of processes leading to magma fragmentation and the generation of pyroclasts during explosive volcanic eruptions is a fundamental question in volcanology. To investigate formation of pyroclasts under well-constrained conditions we have constructed a shock-tube type facility operating at high temperatures and pressures. This apparatus makes possible, for the first time, experimental fragmentation of actual magma samples due to explosive decompression, under controlled laboratory conditions.

The experimental facility consists of a high-pressure high-temperature section and a low-pressure low-temperature section separated by a steel diaphragm. The high pressure-temperature section was constructed from heat-treated Nimonic 105 alloy, has an internal diameter of 20 mm, an external diameter 40 mm and a length of 450 mm. It was designed to operate under pressures up to 20 MPa, and temperatures up to 950 °C. The distance between the upper surface of the sample (l=50 mm, d=17 mm) and the diaphragm is 210 mm. The position of a sample in the high pressure-temperature section corresponds to the hot zone of the section heated by an external Kanthal® -wound resistance furnace. The low-pressure low-temperature section consists of a 120 mm long tube with internal diameter 20 mm which vents into a large tank (volume = 0.7 m³). After diaphragm disruption, the head of a rarefaction wave propagates into the high pressure section towards the sample. This wave incidents on the upper surface of sample, reflects and causes a pressure drop on the sample surface. In turn the decompression of the upper surface of the sample causes the propagation of a release wave through the sample. The tensile stresses thus applied to the porous sample can cause its disruption if they are higher than the tensile strength of the material, and fragmentation can occur. Fragments generated will be outbursted by the gas expanding into the low pressure tank.

Samples of Mount St. Helens gray dacite similar to cyptodome material (provided by R. Hoblitt, 17 mm, length 50 mm) were used in the experiments. Investigation of sample properties indicates that density = 1600 kg/m³, total volume vesicularity (porosity) = 34 %, open volume porosity (water incorporation method) = 20 %. The experimental procedure consisted of placing a sample in the high pressure-temperature section, heating at 15 °C/min, and slow saturation/pressurization by Argon at temperature. The gas fills the connected pores of the samples as well as the space between the sample and the diaphragm. After the diaphragm disruption, samples decompress rapidly. Most of the experiments were performed at an initial pressure differential of about 120 bar and a temperature range of 20 - 885 °C. Two sets each consisting of 7 experiments were performed at 17° - 19°C, 750° - 765 °C and 810° - 825 °C.

The above experimental conditions yielded successful fragmentation of Mount St. Helens gray dacite. The generated fragments of dacite had angular shapes and consisted of shards of crystals and vesicular glass (Fig. 3.9-2). The angular shape of fragments was observed for all investigated temperatures suggesting brittle fragmentation of vesicular dacite.
 

Fig. 3.9-2: SEM images of experimentally produced fragments of the Mount St. Helens grey dacite by rapid decompression (expt. # 118; P0=12.6 MPa, T0= 810° -825 °C). The temperature bounds refer to the ends and the center of the sample, respectively. Particles consist of crystals and glassy vesicular matrix. They exhibit angular shapes implying brittle fragmentation. (b) A detail of a single fragment illustrating the texture of the experimentally fragmented Mt. St. Helens grey dacite. The characteristic size is comparable to the phenocryst size. Note the highly irregular shape of the vesicles.

Frequency grain size distribution curves have simple or bimodal distributions (Fig. 3.9-3). The largest sizes are comparable to sample diameter. Median diameters of fragments derived from a set of experiments at 810° - 825 °C were in the range -3.1 to -1.85 phi units while fragments from experiments at lower temperature (750° -765 °C) were finer (median diameter -2.3 to -1.5 phi units). Experiments performed at room temperature yielded finer fragments (median diameters -0.85 to -1.4). Higher temperature experiments (810° - 825 °C) yield fragments with broader ranges of sorting coefficients (1.35 - 2.05) than fragments from experiments at 750° -765 °C (1.3 - 1.7) and 17° -19 °C (1.3 - 1.5). The fragments obtained in experiments are within the limits of curves suggested by Walker (1971) for the designation of fallout pyroclastics and pyroclastic flow deposits. A plot of median diameter versus sorting coefficient of fragments is shown in Fig. 3.9-3. This plot demonstrates that coarser fragments corresponding to experiments at higher temperature were more variable in sorting coefficient vs. median diameter than fragments of low temperature experiments.This figure demonstrates the increase of fragments size with increase of temperature.
 

Fig. 3.9-3: (Left) Frequency - grain size distribution curves (wt% of pyroclasts versus fragment size in units, where φ = -log 2 d (in mm)) for two experiments conducted at similar initial pressure differentials (12.5-12.6 MPa) but contrasting temperatures (expt. # 107, T = 752° - 767 °C; expt. # 118, T = 810° - 825 °C). (Right) Sorting coefficient (σψ ; = (φ84 - φ16)/2; characterising dispersion) versus median diameter (MdΨ ; derived from cumulative frequency curves) showing the fields of experimental data for two sets of experiments conducted at differing temperatures but similar pressures. These overlapping fields illustrate an increase in fragment size with increasing temperature.

SEM study of fragments did not reveal any essential foaming or additional vesiculation of dacite particles after rapid decompression and preceeding fragmentation when compared with the pre-experimental state. The disk-like shape of some fragments and their angular shape of them support the hypothesis that fragmentation is accomplished mainly by tensile stresses acting on the dacite during propagation of the release wave through the sample after rapid depressurization of its upper end.

The present results have important consequences for the evaluation of certain volcanic hazards. Our observation that pyroclast formation and outburst occurs over a large temperature range (extending down to very low temperatures) must be taken into account in the consideration of problems of rapid decompression of the interior of lava domes. If porous lava inside a lava dome has sufficient gas pressure inside pores and undergoes sufficiently rapid decompression due to landslide or lava dome collapse then it may generate explosive outbursts of pyroclastics, even at temperatures well below "magmatic" and below those normally thought to be associated with major volcanic hazards. Thus, relatively cool volcanic edifices may be very hazardous if subjected to such unloading events.