caa a22 4500 477062571 CHVBK 20180405111405.0 cr unu---uuuuu 170330e19961101xx s 000 0 eng 10.1007/s004450050138 doi (NATIONALLICENCE)springer-10.1007/s004450050138 White James D. L. Department of Geology, University of Otago, P.O. Box 56, Dunedin, New Zealand Fax: +64 3 479-7527 e-mail: jwhite@gandalf.otago.ac.nz, NZ aut Pre-emergent construction of a lacustrine basaltic volcano, Pahvant Butte, Utah (USA) [Elektronische Daten] [James D. L. White]  The subaqueous phases of an eruption initiated approximately 85 m beneath the surface of Pleistocene Lake Bonneville produced a broad mound of tephra. A variety of distinctive lithofacies allows reconstruction of the eruptive and depositional processes active prior to emergence of the volcano above lake level. At the base of the volcano and very near inferred vent sites are fines-poor, well-bedded, broadly scoured beds of sideromelane tephra having local very low-angle cross-stratification (M1 lithofacies). These beds grade upward into lithofacies M3, which shows progressively better developed dunes and cross-stratification upsection to its uppermost exposure approximately 10 m below syneruptive lake level. Both lithofacies were emplaced largely by traction from relatively dilute sediment gravity flows generated during eruption. Intercalated lithofacies are weakly bedded tuff and breccia (M2), and nearly structureless units with coarse basal layers above strongly erosional contacts (M4). The former combines products of deposition from direct fall and moderate concentration sediment gravity flows, and the latter from progressively aggrading high-concentration sediment gravity flows. Early in the eruption subaqueous tephra jetting from phreatomagmatic explosions discontinuously fed inhomogeneous, unsteady, dilute density currents which produced the M1 lithofacies near the vent. Dunes and crossbeds which are better developed upward in M3 resulted from interaction between sediment gravity flows and surface waves triggered as the explosion-generated pressure waves and eruption jets impinged upon and occasionally breached the surface. Intermingling of (a) tephra emplaced after brief transport by tephra jets within a gaseous milieu and (b) laterally flowing tephra formed lithofacies M2 along vent margins during parts of the eruption in which episodes of continuous uprush produced localized water-exclusion zones above a vent. M4 comprises mass flow deposits formed by disruption and remobilization of mound tephra. Intermittent, explosive magma-water interactions occurred from the outset of the Pahvant eruption, with condensation, entrainment of water and lateral flow marking the transformation from eruptive to "sedimentary" processes leading to deposition of the mound lithofacies. Springer-Verlag Berlin Heidelberg, 1996 Key words Surtseyan volcanism nationallicence Subaqueous volcanism nationallicence Hydrovolcanism nationallicence Sediment-gravity flow nationallicence Sideromelane nationallicence Basaltic volcanism nationallicence https://doi.org/10.1007/s004450050138 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/s004450050138 text/html Onlinezugriff via DOI NATIONALLICENCE 100 1- White James D. L. Department of Geology, University of Otago, P.O. Box 56, Dunedin, New Zealand Fax: +64 3 479-7527 e-mail: jwhite@gandalf.otago.ac.nz, NZ aut Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer