Impact of the lateral blast on the spatial pattern and grain size characteristics of the 18 May 1980 Mount St. Helens fallout deposit

摘要: The 18 May 1980 eruption of Mount St. Helens started with a lateral blast that fed pyroclastic surge, which then uplifted to form co-blast plume. Thirty minutes later, Plinian activity at the vent and fluctuated in intensity for ~9 h. resulting fallout deposit, documented > 600 km from vent, presents some striking features: (1) displacement overall deposit north (2) secondary thickness mass maximum ~300 km (3) total grain size distribution dominated by fine ash (62 wt % < 63 µm), (4) individual distributions vary dramatically crosswind direction strongly bimodal south skewed unimodal north. Results new deconvolution show they are combination coarse subpopulation decreases distance constant mean ∼15 µm. Relative proportions each asymmetrically directions, preponderant toward one dominating both reach their absolute maxima on axis. Componentry analyses selected samples blast-derived material is greatly enriched deposit. These results indicate plume dispersed fine-grained over great distances subpopulation. Comparison reanalysis data atmospheric wind fields satellite images spreading cloud suggests contrasting transport depositional processes (early) (later) vent-derived plumes. displaced north; it had high overshoot height, eastward dispersion via strong winds low stratosphere (~10–15 km). plumes were lower most southeast as high-velocity shifted just before late climactic eruptive phase. Fine (fine subpopulation) was deposited continuously throughout an increase sedimentation rate where there thickness. probably settled enhanced mechanisms, including not only aggregation but also gravitational convective instabilities base, hydrometeor formation destruction, entrainment small particles larger ones. Finally, we half (by mass) medial area plume, significant proportion nonjuvenile, has implications magmatic budget this eruption.