Greg A. Valentine

Stratified flow in pyroclastic surges

Stratified flow theory is applied to pyroclatic surges in an effort to gain insight into transport dynamics during explosive eruptions. Particle transport is assumed to be by turbulent suspension, and calculations contained herein show that this is likely for many cases including the 18 May 1980 blast at mount St. Helens. The discussion centers on the Rouse number (Pn), which represents a ratio of particle settling velocity to scale of turbulence; the Brunt-Väisälä frequency (N), which is the maximum possible frequency of internal waves; the Froude number (Fr), representing the ratio of inertial forces to gravitational forces; and the Richardson number (Ri), a ratio of buoyant restoring forces to turbulent mixing forces. The velocity or flow power dependence of bed-form wavelength in surge deposits is related to a velocity dependence of wavelength of internal waves in the turbulent surge. This produces a decrease in dune wavelength with increasing distance from vent. Migration direction of bed forms is related toFr as it is defined for a continuously stratified flow. Proximal to distal facies variations in surge deposits reflect increasingPn andRi as the flows move away from their sources. This produces the progression from sandwave to massive to planar facies with increasing distance from vent. Where the long axis of topography is at low angles to the flow direction, massive facies in topographic lows may from concurrently with sandwave facies on highs, due to the higher particle concentration in the lows. Where long axis of topography is at high angles to flow direction, denser lower parts of the surge may be dammed or “blocked”. Blocked material tends to form massive flows that may move down slope independent of the overriding surge. A model incorporating turbulent transport, stratified flow, and time evolution of pyroclastic surges is proposed for deposits which have been attributed to both pyroclastic flow and pyroclastic surge transport by various workers. During the initial high energy (waxing) phase of the eruptive event,Pn is sufficiently low that only coarse, but poorly sorted, material is deposited to form relatively coarse bottom layers. As the event wanes, remaining finer material is deposited through a thin bed load to produce overlying bedded and cross-bedded veneer deposits. Throughout most of the event, blocking occurs to produce relatively thick and massive deposits in valley bottoms.

Revised conceptual model for maar-diatremes: Subsurface processes, energetics, and eruptive products

Diatremes are debris-filled structures beneath maars that result from many magma-water (phreatomagmatic) explosions during a monogenetic volcano’s lifetime. A long-standing model requires deepening explosions, due to water table drawdown, that eject progressively deeper-seated country rock from the explosion sites, while the overlying diatreme and its surface crater widen due to subsidence. A revised model is proposed wherein explosions can take place at any level within a diatreme at a given time, most effectively venting material from near-surface explosions. Deep-seated country rock lithics in tephra deposits record stepwise vertical mixing of material by upward-directed debris jets and downward subsidence, rather than direct ejection from deep explosions. Juvenile and lithic clasts erupted during a given explosion may have had a complex history within the diatreme and need not directly reflect fragmentation or brecciation during the explosion that ejects them.

Scoria cone construction mechanisms, Lathrop Wells volcano, southern Nevada, USA

Scoria cones are commonly assumed to have been constructed by the accumulation of ballistically ejected clasts from discrete, relatively coarse-grained Strombolian bursts and subsequent avalanching such that the cone slopes are at or near the angle of repose for loose scoria. The cone at the hawaiitic Lathrop Wells volcano, southern Nevada, contains deposits that are consistent with these processes during early cone-building phases; these early deposits are composed mainly of coarse lapilli and fluidal bombs and are partially welded, indicating relatively little cooling during flight. However, the bulk of the cone is composed of relatively fine-grained (ash and lapilli) planar beds with no welding, even within a few tens of meters of the vent. This facies is consistent with deposition by direct fallout from sustained eruption columns of relatively well-fragmented material, primarily mantling cone slopes and with a lesser degree of avalanching than is commonly assumed. A laterally extensive fallout deposit (as much as 20 km from the vent) is inferred to have formed contemporaneously with these later cone deposits. This additional mechanism for construction of scoria cones may also be important at other locations, particularly where the magmas are relatively high in volatile content and where conditions promote the formation of abundant microlites in the rising mafic magma.

Coarse-tail vertical and lateral grading in pyroclastic flow deposits of the Latera Volcanic Complex (Vulsini, central Italy): origin and implications for flow dynamics

Abstract Coarse-tail vertical and lateral grading are common features of pyroclastic flow deposits of the Latera Volcanic Complex (Vulsini Volcanoes, central Italy), and are described for four representative flow units. Lithic clasts show normal vertical and lateral grading in all four units. Pumice clasts show reverse vertical and lateral grading within three flow units; normal vertical and lateral grading of scoria clasts has been observed in one flow unit. The origin of vertical and lateral grading of lithic and pumice clasts is related to mechanisms operating during the transport process within the high particle concentration basal avalanche of pyroclastic flows. Vertical grading results from the balance between gravitational and dispersive forces, and is transferred to a lateral grading by vertical velocity gradients within a nonturbulent flow zone of pyroclastic flows. The pyroclastic flows are modeled as Bingham-type fluids, a framework that explains some of the basic deposit features even though it is a highly simplified treatment of the flows. Plug flow zones in the flows were a relatively minor part of their thicknesses. Evidence for the flows having had high densities, probably within a factor of two of the final deposit density, is presented. Finally, their origin by “single pulse” or “progressive formation” is discussed.

Turbulent transport and deposition of the Ito pyroclastic flow: Determinations using anisotropy of magnetic susceptibility

The Ito pyroclastic flow erupted about 22,000 years ago from Aira caldera in southern Kyushu, Japan. Flow directions were determined by anisotropy of magnetic susceptibility (AMS), which measured the preferential alignment of magnetite microphenocrysts, usually ≤0.25 mm in diameter. The microphenocrysts are aligned with the long axis of the clasts during strain and fracture in the vent. The grains are then deposited parallel to the flow direction with an imbrication. There is no evidence of rolling of clasts or nonflow parallel lineations, and thus AMS can be used to determine flow directions that occurred immediately before deposition. Beyond 30 km from the center of the caldera, flow directions were predominantly down paleogradient, indicating expanded flow and that the depositional system was gravity driven and largely decoupled from the transport system. Within 30 km, measured flow directions are random, indicating that sedimentation occurred from a depositional system that was closely coupled to the turbulent transport system. Individual flow directions at all sites except one varied more than the analytical error, demonstrating that a variety of flow directions existed even within a small area of the flow. This implies that the depositional system was turbulent.

Mechanisms of low-flux intraplate volcanic fields—Basin and Range (North America) and northwest Pacific Ocean

We compare two intraplate, Pliocene-Pleistocene volcanic fields in different tectonic settings—the central Basin and Range and the northwest Pacific Ocean. Both fields are characterized by widely scattered, small-volume, alkali basaltic volcanoes; within the fields, each volcano apparently originates from a separate, volatile-enriched parental melt from the upper mantle. There is no evidence at either field for locally anomalous heat flow or ongoing introduction of new fluids into the upper mantle such as might occur above a subducting slab. We conclude that the volcanic fields reflect deformation-driven collection of already existing partial melts in a heterogeneous upper mantle. Deformation-driven melt collection may be an important mechanism for other diffuse intraplate volcanic fields, and this is consistent with a tectonically controlled, low-flux end member for intraplate fields where magmatism is a passive response to regional deformation. Differences in the degree of fractionation and contamination between the two fields are inferred to be related to flexure-induced vertical variations in the orientation of principal stresses in the northwest Pacific Ocean, which cause stalling of ascending dikes in the lithosphere.

Entrainment of Country Rock during Basaltic Eruptions of the Lucero Volcanic Field, New Mexico

As magma rises through the lithosphere it may entrain wall rock debris. The entrainment process depends on the local hydrodynamic regime of the magma (e.g., velocity, temperature, bulk density), the extent of interaction of magma with groundwater, and the mechanical properties of the wall rocks. Wall rock entrainment results in local flaring of dikes and conduits, which in turn affects the hydrodynamics of magma ascent and eruption. We studied upper-crustal xenoliths erupted from small-volume basaltic volcanoes of the Lucero volcanic field (west-central New Mexico) in order to assess the relative importance of various entrainment mechanisms during a range of eruptive styles, including strongly hydrovolcanic, Strombolian, and effusive processes. Total xenolith volume fractions ranged from 0.3-0.9 in hydrovolcanic facies to

Decreasing magmatic footprints of individual volcanoes in a waning basaltic field

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