Topic
Pyroclastic fall
About: Pyroclastic fall is a research topic. Over the lifetime, 552 publications have been published within this topic receiving 22322 citations.
Papers published on a yearly basis
1 / 2
Papers
TL;DR: In this paper, a theoretical model of clast fallout from convective eruption columns has been developed which quantifies how the maximum clast size dispersal is determined by column height and wind strength.
Abstract: A theoretical model of clast fallout from convective eruption columns has been developed which quantifies how the maximum clast size dispersal is determined by column height and wind strength. An eruption column consists of a buoyant convecting region which rises to a heightH
B
where the column density equals that of the atmosphere. AboveH
B
the column rises further to a heightH
T
due to excess momentum. BetweenH
T
andH
B
the column is forced laterally into the atmosphere to form an upper umbrella region. Within the eruption column, the vertical and horizontal velocity fields can be calculated from exprimental and theoretical studies and consideration of mass continuity. The centreline vertical velocity falls as a nearly linear function over most of the column's height and the velocity decreases as a gaussian function radially away from the centreline. Both column height and vertical velocity are strong functions of magma discharge rate. From calculations of the velocity field and the terminal fall velocity of clasts, a series of particle support envelopes has been constructed which represents positions where the column vertical velocity and terminal velocity are equal for a clast of specific size and density. The maximum range of a clast is determined in the absence of wind by the maximum width of the clast support envelope. The trajectories of clasts leaving their relevant support envelope at its maximum width have been modelled in columns from 6 to 43 km high with no wind and in a wind field. From these calculations the shapes and areas of maximum grain size contours of the air-fall deposit have been predicted. For the no wind case the theoretical isopleths show good agreement with the Fogo A plinian deposit in the Azores. A diagram has been constructed which plots, for a particular clast size, the maximum range normal to the dispersal axis against the downward range. From the diagram the column height (and hence magma discharge rate) and wind velocity can be determined. Historic plinian eruptions of Santa Maria (1902) and Mount St. Helens (1980) give maximum heights of 34 and 19 km respectively and maximum wind speeds at the tropopause of m/s and 30 m/s respectively. Both estimates are in good agreement with observations. The model has been applied to a number of other plinian deposits, including the ultraplinian phase of theA.D. 180 Taupo eruption in New Zealand which had an estimated column height of 51 km and wind velocity of 27 m/s.
696 citations
TL;DR: In this paper, a classification of pyroclastic fall deposits is proposed based on measurements made on the resulting pyro-clastic-fall deposits, the significant parameters being the area of dispersal and degree of fragmentation of the material.
Abstract: A classification scheme is proposed based on measurements made on the resulting pyroclastic fall deposits, the significant parameters being the area of dispersal and degree of fragmentation of the material. An empirical measure of the first is the area enclosed by the 0.01 Tmax isopach (where Tmax is the maximum thickness of the deposit), called D, which ranges from less than 10 km2 for deposits of strongly cone-building type to more than 1000 km2 for deposits of strongly sheet-forming type. An empirical measure of the second is the percentage of material finer than 1 mm in the deposit, or more simply at the point where the 0.1 Tmax isopach crosses the dispersal axis. The latter value, called F, varies from less than 20 for deposits in which fragmentation was mainly achieved by the tearing apart of magma, to more than 80 where it was largely due to thermal shock resulting from the quenching of lava by water.
572 citations
TL;DR: In this article, a contoured diagram is given based on 1,600 samples to facilitate comparison of mechanical analyses of pyroclastic fall and flow deposits, and a histogram or cumulative curve where the weight percentages are plotted against the fall velocity are shown to be more meaningful than those against the grain size.
Abstract: Pyroclastic fall and flow deposits occupy two distinct fields on an $$Md_{\phi}/\sigma_{\phi}$$ plot (Inman parameters), and a contoured diagram is given based on 1,600 samples to facilitate comparison of mechanical analyses. Analyses which plot where the fields overlap include rain-flushed ashes and thin flow deposits. Among factors influencing $$\sigma_{\phi}$$ of fall deposits is the wind: a strong wind will reduce its value. Another is the characteristics of the initial population-the entire assemblage of fragments coming from the vent-which is quite different for crystals than for pumice or lithic components. Each component in a polycomponent deposit has a different grain-size distribution due to this and subsequent air sorting. Histograms or cumulative curves where the weight percentages are plotted against the fall velocity are shown to be more meaningful than those against the grain size, and a quantity V is defined analogous to $$\phi$$. Ignimbrites are remarkably homogeneous, but two departures ...
512 citations
TL;DR: In the Pelean phase of an eruptive act, pumice-fall and fine ash-fall deposits are produced by the same act as mentioned in this paper, and they and the ignimbrite constitute the several and varied products of a Pelean-phase eruption.
Abstract: Many ignimbrite flow units show a reverse grading of large pumice clasts and a normal grading of large lithic clasts. Each ignimbrite flow unit has a basal layer finer grained than the body of the ignimbrite, with a ground surge deposit commonly underlying the ignimbrite, and a fine ash-fall deposit commonly overlying it. These two types of deposit, although not an integral part of the ignimbrite, are produced by the same eruptive act, and they and the ignimbrite constitute the several and varied products of a Pelean-phase eruption. Volcanic eruptions in which ignimbrite is generated show the following sequence of events so often that it may be regarded as the normal one: (1) a highly explosive, often Plinian, phase, producing a pumice-fall deposit; (2) a Pelean phase; and (3) an effusive phase, producing a lava flow. This sequence is believed to represent the tapping of progressively deeper levels in the magma chamber and the escape during the eruption of magma of progressively lower gas content.
422 citations
TL;DR: The 26.5-ka Oruanui eruption, from Taupo volcano in the central North Island of New Zealand, is the largest known ‘wet’ eruption, generating 430km3 of fall deposits, 320 km3 of pyroclastic density-current (PDC) deposits (mostly ignimbrite) and ∼420 km 3 of primary intracaldera material, equivalent to ∼530 km3 magma as discussed by the authors.
327 citations
Related Topics (5)
Performance Metrics
| Year | Papers |
|---|---|
| 2025 | 2 |
| 2024 | 8 |
| 2023 | 14 |
| 2022 | 17 |
| 2021 | 9 |
| 2020 | 11 |