A vesicularity index for pyroclastic deposits
TL;DR: In this article, the vesicularity of juvenile clasts in pyroclastic deposits is measured for the 16-32 mm size fraction by water immersion techniques and converted to vesicleities using measured dense-rock equivalent densities.
Abstract: The vesicularity of juvenile clasts in pyroclastic deposits gives information on the relative timing of vesiculation and fragmentation, and on the role of magmatic volatiles versus external water in driving explosive eruptions. The vesicularity index and range are defined as the arithmetic mean and total spread of vesicularity values, respectively. Clast densities are measured for the 16–32 mm size fraction by water immersion techniques and converted to vesicularities using measured dense-rock equivalent densities. The techniques used are applied to four case studies involving magmas of widely varying viscosities and discharge rates: Kilauea Iki 1959 (basalt), Eifel tuff rings (basanite), Mayor Island cone-forming deposits (peralkaline rhyolite) and Taupo 1800 B.P. (calc-alkaline rhyolite). Previous theoretical studies suggested that a spectrum of clast vesicularities should be seen, depending on the magma viscosity, eruption rate, and the presence and timing of magma: water interaction. The new data are consistent with these predictions. In magmatic “dry” eruptions the vesicularity index lies uniformly in the range 70%–80% regardless of magma viscosity. For high viscosities and eruption rates the vesicularity ranges are narrow (< 25%), but broaden to between 30% and 50% as the viscosity and eruption rates are lowered and the volatiles and magma can de-couple. In phreatomagmatic “wet” eruptions, widely varying clast vesicularities reflect complex variations in the relative timing of vesiculation and water-induced fragmentation. Magma:water interaction at an early stage greatly reduces the vesicularity indices (< 40%) and broadens the ranges (as high as 80%), whereas late-stage interaction has only a minor effect on the index and broadens the range to a limited extent. Clast vesicularity represents a useful third parameter in addition to dispersal and fragmentation to characterise pyroclastic deposits.
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TL;DR: The history of the Taupo Volcanic Zone (TVZ) can be divided into two categories: the old TVZ from 2.0 Ma to 0.34 Ma and the young TVZ between 0.9 and 0.6 Ma as discussed by the authors.
734 citations
TL;DR: A model in which a fragmentation criterion, based on a rate-limited crossing of the glass transition, is incorporated into a multiphase fluid-dynamic description of magma ascent demonstrates the feasibility of strain-induced brittle fragmentation of magMA in volcanic eruptions.
Abstract: Explosive eruptions are the most powerful and destructive type of volcanic activity These eruptions are characterized by magma fragmentation, the process through which a bubbly or foamy magma is transformed into a gas-pyroclast dispersion Although magma fragmentation has been investigated both experimentally and theoretically, and the basic transport phenomena that occur in a volcanic conduit have been modelled, the underlying mechanism responsible for magma fragmentation is still poorly understood This lack of understanding seriously limits our ability to forecast volcanic hazards, preventing reliable discrimination between conditions that lead to explosive and effusive eruptions Here I develop a model in which a fragmentation criterion, based on a rate-limited crossing of the glass transition, , is incorporated into a multiphase fluid-dynamic description of magma ascent The numerical results of this model demonstrate the feasibility of strain-induced brittle fragmentation of magma in volcanic eruptions, and reconcile experimental with theoretical studies as well as with the observed volcanic products of large explosive eruptions
410 citations
TL;DR: This paper proposed a classification scheme that unifies terminology for all primary volcaniclastic deposits, assigns initial depositional mechanism as the basis for classifying them, and refines the grain-size classes used to pigeonhole samples.
Abstract: We propose a classification scheme that unifies terminology for all primary volcaniclastic deposits, assigns initial depositional mechanism as the basis for classifying them, and refines the grain-size classes used to pigeonhole samples. By primary volcaniclastic deposits and rocks, we mean the entire range of fragmental products deposited directly by explosive or effusive eruption. This definition thus focuses on the primary transport and deposition of particles, rather than those processes by which the particles form or the nature of the fluid in which they are carried. We favor this approach for all primary volcaniclastic deposits because they typically contain assemblages of clasts formed by different processes and/or at different times that are subsequently brought together during eruption.
408 citations
TL;DR: In this article, the authors proposed that the degree to which magma is fragmented is determined by factors controlling bubble coalescence: magma viscosity, temperature, bubble size distribution, bubble shapes, and time.
Abstract: Fragmentation, or the "coming apart" of magma during a plinian eruption, remains one of the least understood processes in volcanology, although assumptions about the timing and mechanisms of fragmentation are key parameters in all existing eruption models. Despite evidence to the contrary, most models assume that fragmentation occurs at a critical vesicularity (volume percent vesicles) of 75–83%. We propose instead that the degree to which magma is fragmented is determined by factors controlling bubble coalescence: magma viscosity, temperature, bubble size distribution, bubble shapes, and time. Bubble coalescence in vesiculating magmas creates permeability which serves to connect the dispersed gas phase. When sufficiently developed, permeability allows subsequent exsolved and expanded gas to escape, thus preserving a sufficiently interconnected region of vesicular magma as a pumice clast, rather than fully fragmenting it to ash. For this reason pumice is likely to preserve information about (a) how permeability develops and (b) the critical permeability needed to insure clast preservation. We present measurements and calculations that constrain the conditions (vesicularity, bubble size distribution, time, pressure difference, viscosity) necessary for adequate permeability to develop. We suggest that magma fragments explosively to ash when and where, in a heterogeneously vesiculating magma, these conditions are not met. Both the development of permeability by bubble wall thinning and rupture and the loss of gas through a permeable network of bubbles require time, consistent with the observation that degree of fragmentation (i.e., amount of ash) increases with increasing eruption rate.
372 citations
Cites background from "A vesicularity index for pyroclasti..."
...…vesicularities (volume percent vesicles) of 70–80% have led to the assumption that fragmentation occurs when bubble-to-melt volume ratios are between 3 :1 and 5 :1 (vesicularities of 75–83%), corresponding to a range of packing limits of spherical bubbles (Sparks 1978; Houghton and Wilson 1989)....
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...An increasing amount of vesicularity data in the literature reveals, however, that pumice preserves a range of vesicularity from F60 to 93% (Whitham and Sparks 1986; Houghton and Wilson 1989; Klug and Cashman 1991; Gardner et al. 1991; Thomas et al. 1994)....
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TL;DR: In this paper, the first measurement of bubble nucleation in hydrated rhyolitic melts in response to pressure release was reported, and the results demonstrate the importance of heterogeneous nucleation.
326 citations
Cites methods from "A vesicularity index for pyroclasti..."
...When the gas volume approaches 75-80% of the gas-melt mixture, fragmentation of the melt occurs [3, 4 ]....
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References
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TL;DR: In this article, a numerical method has been developed to determine bubble growth rates during volcanic eruptions of basaltic and rhyolitic tephras, and the numerical solutions consider both diffusional and decompressional growth and the effects of magma ascent rates (0-400 cm s−1), magma viscosity (102 to 108 poise), gas solubility, gas content (0.25-5%), and gas diffusivity (10−6 to 10−9 cm2 s− 1) on growth rates.
901 citations
TL;DR: In this paper, an improved empirical method for the plotting of field data and the calculation of tephra fall volumes is presented, where two new quantitative parameters are proposed which describe the rates of thinning of the deposit (bt the thickness half distance) and the maximum clast size (bc the clast half distance).
Abstract: An improved empirical method for the plotting of field data and the calculation of tephra fall volumes is presented. The widely used “area” plots of ln(thickness) against ln(isopach area) are curved, implying an exponential thinning law. Use of ln(thickness)−(area)1/2 diagrams confirm the exponential dependence of many parameters (e.g. thickness, maximum and median clast size) with distance from source, producing linear graphs and allowing volumes to be calculated without undue extrapolation of field data. The agreement between theoretical models of clast dispersion and observation is better than previously thought. Two new quantitative parameters are proposed which describe the rates of thinning of the deposit (bt the thickness half-distance) and the maximum clast size (bc the clast half-distance). Many deposits exhibit different grainsize and thickness thinning rates, with the maximum clast size diminishing 1–3 times slower than the thickness. This implies that the entrained grainsize population influences the morphologic and granulometric patterns of the resulting deposit, in addition to the effects of column height and wind-speed. The grainsize characteristics of a deposit are best described by reference to the half-distance ratio (bc/bt). A new classification scheme is proposed which plots the half-distance ratio against the thickness half-distance and may be contoured in terms of the column height.
704 citations
"A vesicularity index for pyroclasti..." refers methods in this paper
...In particular, dispersal has been used to classify pyroclastic fall deposits and infer their eruption rates using models of eruption column behaviour (Walker 1973; Pyle 1989 ), while grain-size parameters are frequently used to assess the role of steam explosivity in pyroclastic eruptions (Walker and Croasdale 1972; Self and Sparks 1978)....
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TL;DR: In this paper, the ascent and emplacement of basaltic magma on the earth and moon is modeled by the application of geological and physical observations and constraints, provided that allowance is made for the coalescence of gas bubbles.
Abstract: The ascent and emplacement of basaltic magma on the earth and moon is modeled by the application of geological and physical observations and constraints. Relatively simple mathematical models of the motion of gas/liquid mixtures are shown to be adequate in the treatment of basaltic eruptions, provided that allowance is made for the coalescence of gas bubbles and that realistic geological and petrochemical constraints are applied to the numerical values of variables. Because gas exsolution from magmas on the earth and moon commonly occur at depths of less than 2 km, it is generally convenient to consider separately the rise of bubble-free magmatic liquid at depth in a planetary crust and the more complex motions occurring near the surface with gas exsolution.
690 citations
"A vesicularity index for pyroclasti..." refers background in this paper
...Vesiculation processes in magmas have been considered by several authors (Sparks 1978 (which see for a review of earlier studies); Wilson et al. 1980; Wilson and Head 1981; Sparks and Brazier 1982)....
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TL;DR: In this paper, the authors considered the flow of gas/pyroclast dispersions and high viscosity magma through various magma chamber/conduit/vent geometries.
Abstract: Summary
Plinian air-fall deposits and ignimbrites are the principal products of explosive eruptions of high viscosity magma. In this paper, the flow of gas/pyroclast dispersions and high viscosity magma through various magma chamber/conduit/vent geometries is considered. It is argued that after the first few minutes of an eruption magma fragmentation occurs at a shallow depth within the conduit system. Gas pressures at the fragmentation level are related to exsolved gas contents by consideration of the exsolution mechanism.
The sizes of blocks found near vents imply that gas velocities of 200 to 600 m s−1 commonly occur. These velocities are greater than the effective speed of sound in an erupting mixture (90-200 m s−1) and the transition from subsonic to supersonic flow is identified as occurring at the depth at which the conduit has its minimum diameter. The range of values of this minimum diameter (∼ 5 to ∼ 100 m) is estimated from observed and theoretically deduced mass-eruption rates.
The energy and continuity equations are solved, taking account of friction effects, for numerous geometries during the evolution, by wall erosion, of a conduit. Conduit erosion ceases, near the surface, when an exit pressure of one atmosphere is reached. Eruption velocities are found to depend strongly on exsolved magma gas content and weakly on radius of conduit and friction effects. Assuming water as the main volatile phase, velocities of 400-600 m s−1 for plinian events imply magma water contents of 3-6 per cent by weight.
Three scenarios are presented of eruptions in which: (1) conduit radius increases but gas content remains constant; (2) conduit radius increases and gas content decreases with time; and (3) conduit radius remains fixed and gas content decreases. These models demonstrate that the reverse grading commonly observed in plinian air-fall deposits is primarily a consequence of conduit erosion, which always results in increasing eruption intensity and eruption column height with time. The models also show that a decrease in gas content as deeper levels in a magma chamber are tapped or an increasing vent radius as conduit walls are eroded leads to the prediction of a progression from air-fall activity through ignimbrite formation to cessation of eruption and caldera collapse.
633 citations
TL;DR: In this article, the authors show that non-explosive eruption of lava appears to result from rapid, sub-surface gas release from magma ascending as a permeable foam.
Abstract: Silicic magma can erupt quietly, as vapour-poor lava, despite petrological evidence that it once contained ample dissolved water to drive violent venting of tephra. Non-explosive eruption of lava appears to result from rapid, sub-surface gas release from magma ascending as a permeable foam. The degassed foam then collapses during extrusion. Conditions of shallow ascent, rather than pre-eruption volatile concentrations, control eruptive behaviour.
582 citations
"A vesicularity index for pyroclasti..." refers background in this paper
...after a "passive" or non-explosive degassing episode (e.g. Eichelberger et al. 1986 )....
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...There are three possible processes which terminate vesiculation: 1. non-explosive degassing, either by streaming of bubbles through ponded magma held at shallow depths (Blackburn et al. 1976) or by collapse of vesiculated froth at depth ( Eichelberger et al. 1986 ) 2. fragmentation by the rupturing of vesicles 3. interaction with external water which chills and fragments the magma....
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