Auto-granulation of particle surfaces during Surtseyan eruptions

[Abstract unavailable

Linking distal volcaniclastic sedimentation and stratigraphy with the growth and development of stratovolcanoes, Ruapehu volcano, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of PhD in Earth Sciences at Massey University, New Zealand

Large, long-lived stratovolcanoes are inherently unstable, and commonly experience large -scale flank collapse. The resulting debris avalanches permanently alter the edifice and the valleys they impact. New mapping reveals that at least six hitherto unknown debris avalanches occurred from Mt. Ruapehu, New Zealand. They collectively inundated >1,200 km2 and ranged between 1.3 and >3 km3 in volume, the latter being the largest debris avalanche known from the volcano. Constriction of the sliding debris avalanches into deep river valleys enhanced basal erosion, incorporation of water-saturated substrate and formation of a basal lubrication zone. This led to runouts of up to 100 km, 2 - 3 times longer than expected for equivalent unconfined dry landslides. Two of the seven river catchments affected by debris avalanches were truncated from the volcano by proximal debris choking. The debris avalanches commonly coincided with warming from glacial into interglacial periods and rapid deglaciation of Mt. Ruapehu. A loss of ice-armouring of the slopes and increased water saturation likely weakened the edifice. At least two of the debris avalanches were triggered by intrusion of new magma into the mountain. The highly resistant debris-avalanche deposits form distinctive plateaus at the highest topographic elevations along present eroding river valleys, in places reflecting earlier drainage pathways. Deposit ages and those from lower climate-controlled (non-volcanic) fluvial aggradational terraces allowed calculation of regional uplift rates, which varied between 1.3 ± 0.5 mm yr-1 to 5 ± 1.3 mm yr-1 over the last c. 125 ka. Each major flank failure led to decompression of the Mt. Ruapehu magmatic system, triggering pulses of numerous large -scale eruptions and syn-eruptive lahars. Ar- Ar dating of lava clasts within the debris avalanche deposits provided evidence of volcanic episodes that are not exposed on the present edifice. The oldest deposits from Mt. Ruapehu are now identified at =340,000 ka and show that a complex multi -stage storage magma system was operating, similar to that of the present day. Hornblende -bearing xenoliths from these lavas show that a magmatic crustal underplate at >40 km depth existed beneath the volcano by ~486.5 ± 37.6 ka. Combined, samples from the mass -flow deposits and the cone lavas show more complex variation over time than previously thought, but generally reflect a progressively increasing heat flux and a shift of the magma -storage system from the lower crust to mid- and upper -crustal levels

Diversity of soluble salt concentrations on volcanic ash aggregates from a variety of eruption types and deposits

Ash aggregation is a common phenomenon in particle-laden environments of volcanic eruption plumes and pyroclastic density currents. Many of these initially fragile aggregates gain sufficient mechanical strength to remain intact after atmospheric transport and deposition. Several processes contribute to ash aggregate stability, including electrostatic and hydrostatic bonding, ice formation, and cementation by salt precipitates. Here, we compare leachate chemistry from aggregates from a variety of eruption and sedimentation conditions, ranging from dry magmatic eruptions with immediate deposition, to eruptions through seawater. The leachate data shows that the broad window of opportunity for aggregation and aggregate break-up may be used to qualitatively constrain suspended ash concentration and its temporal evolution. We show that aggregation rate and aggregate stability largely depend on the availability of external water and salt source. In particular, high humidity and extensive salt precipitation in seawater environments, such as during the Surtseyan eruptions, promote high aggregation rates and aggregate stability, with accordingly accentuated proximal deposition and aggregate concentration in the deposits. On the other hand, low humidity and salt concentrations during dry magmatic eruptions promote less aggregation and more efficient aggregate break-up, explaining the rarity of aggregates in the deposits. These results have strong implications for the ash budget in volcanic plumes and associated models of plume dispersal and related hazards

In situ granulation by thermal stress during subaqueous volcanic eruptions

Some of the most complex volcanic thermodynamic processes occur when erupting magma interacts with water. In shallow water, “Surtseyan” eruptions are spectacular, and they efficiently fragment magma into fine ash particles. The aviation hazard from these eruptions depends the amount of transportable fine ash that is generated and whether it is aggregated into particle coatings or accretions. To investigate both mechanisms, we analyzed ash-encased lapilli from the Surtseyan eruptions of Capelinhos (Azores, 1957–1958) and Hunga Tonga–Hunga Ha’apai (Tonga, 2014–2015) using X-ray computed microtomography and electron microscopy. We discovered pyroclasts that were not coated, sensu stricto, but had enveloping ash produced by in situ granulation of the particle surface. This was caused by thermal stress as pyroclasts briefly traveled through water and were quenched during eruption. In situ granulation is thus an important secondary disruption process in shallow subaqueous eruptions. Our results imply that ash encasement is not always evidence of particle aggregation and accretion, but it may also result from new ash formation. Shallow-water conditions produce the most efficient ash-generation conditions, leading to the greatest hazard to downwind populations and air traffic