Steven Carey

Quantitative models of the fallout and dispersal of tephra from volcanic eruption columns

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 heightHB where the column density equals that of the atmosphere. AboveHB the column rises further to a heightHT due to excess momentum. BetweenHT andHB 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 columns 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.

The intensity of plinian eruptions

Peak intensities (magma discharge rate) of 45 Pleistocene and Holocene plinian eruptions have been inferred from lithic dispersal patterns by using a theoretical model of pyroclast fallout from eruption columns. Values range over three orders of magnitude from 1.6 × 106 to 1.1 × 109 kg/s. Magnitudes (total erupted mass) also vary over about three orders of magnitude from 2.0 × 1011 to 6.8 × 1014 kg and include several large ignimbrite-forming events with associated caldera formation. Intensity is found to be positively correlated with the magnitude when total erupted mass (tephra fall, surges and pyroclastic flows) is considered. Initial plinian fall phases with intensities in excess of 2.0 × 108 kg/s typically herald the onset of major pyroclastic flow generation and subsequent caldera collapse. During eruptions of large magnitude, the transition to pyroclastic flows is likely to be the result of high intensity, whereas the generation of pyroclastic flows in small magnitude eruptions may occur more often by reduction of magmatic volatile content or some transient change in magma properties. The correlation between plinian fall intensity and total magnitude suggests that the rate of magma discharge is related to the size of the chamber being tapped. A simple model is presented to account for the variation in intensity by progressive enlargement of conduits and vents and excess pressure at the chamber roof caused by buoyant forces acting on the chamber as it resides in the crust. Both processes are fundamentally linked to the absolute size of the pre-eruption reservoir. The data suggest that sustained eruption column heights (i.e. magma discharge rates) are indicators of eventual eruption magnitude, and perhaps eruptive style, and thus are key parameters to monitor in order to assess the temporal evolution of plinian eruptions.

Plinian and co-ignimbrite tephra fall from the

A study of pyroclastic deposits from the 1815 Tambora eruption reveals two distinct phases of activity, i.e., four initial tephra falls followed by generation of pyroclastic flows and the production of major co-ignimbrite ash fall. The first explosive event produced minor ash fall from phreatomagmatic explosions (F-1 layer). The second event was a Plinian eruption (F-2) correlated to the large explosion of 5 April 1815, which produced a column height of 33 km with an eruption rate of 1.1 × 108 kg/s. The third event occurred during the lull in major activity from 5 to 10 April and produced minor ash fall (F-3). The fourth event produced a 43-km-high Plinian eruption column with an eruption rate of 2.8 × 108 kg/s during the climax of activity on 10 April. Although very energetic, the Plinian events were of short duration (2.8 h each) and total erupted volume of the early (F-1 to F-4) fall deposits is only 1.8 km3 (DRE, dense rock equivalent). An abrupt change in style of activity occurred at end of the second Plinian event with onset of pyroclastic flow and surge generation. At least seven pyroclastic flows were generated, which spread over most of the volcano and Sanggar peninsula and entered the ocean. The volume of pyroclastic flow deposits on land is 2.6 km3 DRE. Coastal exposures show that pyroclastic flows entering the sea became highly fines depleted, resulting in mass loss of about 32%, in addition to 8% glass elutriation, as indicated by component fractionation. The subaqueous pyroclastic flows have thus lost about 40% of mass compared to the original erupted mixture. Pyroclastic flows and surges from this phase of the eruption are stratigraphically equivalent to a major ash fall deposit (F-5) present beyond the flow and surge zone at 40 km from the source and in distal areas. The F-5 fall deposit forms a larger proportion of the total tephra fall with increasing distance from source and represents about 80% of the total at a distance of 90 km and 92% of the total tephra fall from the 1815 eruption. The field relations indicate that the 20-km3 (DRE) F-5 deposit is a co-ignimbrite ash fall, generated largely during entrance of pyroclastic flows into the ocean. Based on the observed 40% fines depletion and component fractionation from the flows, the large volume of the F-5 co-ignimbrite ash requires eruption of 50 km3 (DRE, 1.4 × 1014 kg) pyroclastic flows.

Temporal variations in column height and magma discharge rate during the 79 A.D. eruption of Vesuvius

The 79 A.D. plinian eruption of Vesuvius ejected ∼4 km3 (ORE) of phonolitic magma over a period of ∼19 hr. A change in magma composition during the eruption is marked by a sharp transition from white, evolved phonolitic pumice to denser, overlying gray pumice, at mid-level within the fall deposit. Deposition of the upper, gray pumice fall was interrupted six times by the emplacement of pyroclastic surges and flows. Reverse size grading is conspicuous in the fall deposit. Measurements of maximum pumice and lithic diameters have been used to construct isopleths for eight chronostratigraphic levels within the fall deposit. The temporal evolution of eruption column height and magma discharge rate have been evaluated from these isopleths, using a theoretical model of pyroclast dispersal from explosive eruptions. During ejection of the white pumice, the column height rose from 14 to 26 km, as the magma discharge rate increased to 7.7 × 107 kg/s. Shortly after onset of the gray pumice fall, the column reached its maximum altitude of 32 km, with a discharge rate of 1.5 × 108 kg/s. Subsequent generation of surges and pyroclastic flows was associated with fluctuations in column height, supporting an origin by column collapse. At the white-gray boundary in the fall deposit, pumice density increases abruptly from 0.60 g/cm3 in the white pumice to 1.10 g/cm3 at the base of the gray pumice. Higher in the gray fall, the density decreases continuously to 0.60 g/cm3. The variation in pumice density is attributed primarily to differences in volatile content of two magmas which were tapped and mixed in varying proportions during ascent and eruption.

Computer simulation of transport and deposition of the campanian Y-5 ash

Analyses of grain-size and modal composition of the Campanian tuff ash layer (Y-5) from 11 deep-sea cores have been carried out. This layer represents ash fall that has been correlated with the 38,000 y.b.p. Campanian ignimbrite (Thunell et al., 1979), a deposit formed by the largest eruption documented in the Mediterranean region during the late Pleistocene (Barberi et al., 1978). The bulk deposit is bimodal in grain-size and dominated by glass shards. The calculated mean grain-size of the coarse mode of the individual size distributions decreases with distance from the source and progressively approaches a near-constant fine mode of approximately 13 microns. Distal samples are unimodal in grain-size. These data combined with a set of vertical profiles of wind (10 year average) have been used as input to a computer model that simulates fallout of tephra. Modelling indicates that the downwind variation of grain-size of the coarse mode can be accurately reproduced with transport of ash between 5 and 35 km. The observed fine mode of the deposit cannot, however, be generated by transport of ash as individual particles at these elevations. Such transport would result in deposition of virtually all of the fine ash beyond the studied area. Deposition of fine ash within the studied distance of 1600 km from source can only occur by fallout as particle aggregates from a high eruption plume or as individual particles from co-ignimbrite ash clouds with a maximum elevation of 3 km. The large volume of ash in the fine mode (>70 wt.%) and the irregularity in azimuth of low-level winds argue against major low-level transport of co-ignimbrite ash. Rather, the ash may have been derived from both a plinian eruption column and high-altitude clouds of co-ignimbrite ash, with settling of fine ash as particle aggregates.

The roseau ash: Deep-sea tephra deposits from a major eruption on Dominica, lesser antilles arc

Abstract Two extensive marine tephra layers recovered by piston coring in the western equatorial Atlantic and eastern Caribbean have been correlated by electron microprobe analyses of glass shards and mineral phases to the Pleistocene Roseau tuff on Dominica in the Lesser Antilles arc. Tephra deposition and transport to the deep sea was primarily controlled by two processes related to two different styles of eruptive activity: a plinian airfall phase and a pyroclastic flow phase. A plinian phase produced a relatively thin (1–8 cm) airfall ash layer in the western Atlantic, covering an area of 3.0 × 105 km2 with a volume of 13 km3 (tephra). The majority of the airfall tephra was transported by antitrade winds at altitudes of 6–17 km. Aeolian fractionation of crystals and glass occurred during transport resulting in an airfall deposit enriched in crystals relative to the source. Mass balance calculation based on crystal/glass fractionation indicates an additional 12 km3 of airfall tephra was deposited outside the observed fall-out envelope as dispersed ash. Discharge of pyroclastic flows into the sea along the west coast of Dominica initiated subaqueous pyroclastic debris flows which descended the steep western submarine flanks of the island. 30 km3 of tephra were deposited by this process on the floor of the Grenada Basin up to 250 km from source. The Roseau event represents the largest explosive eruption in the Lesser Antilles in the last 200,000 years and illustrates the complexity of primary volcanogenic sedimentation associated with a major explosive eruption within an island arc environment.

Influence of convective sedimentation on the formation of widespread tephra fall layers in the deep sea

The formation of widespread volcanic ash-fall layers in deep sea sediments was investigated experimentally to examine the settling behavior of tephra (20–180 µm diameter) as it travels from the atmosphere into water. Using a fallout mass flux rate that was constrained by measurements of distal fallout from the 1980 eruption of Mount St. Helens (0.2 g/cm 2 /hr), the experiments show that particle settling in the water column is dramatically accelerated by the formation of diffuse vertical gravity currents. The currents form as a result of convective instabilities that develop in a surface boundary layer when the local particle concentration becomes large. At the air-water interface, the settling velocity of particles drops abruptly and the concentration of particles increases because of the differential in mass flux above and below the fluid surface. The implication of the experiments is that deposition of distal tephra fall layers in deep-sea sediments may be dominantly controlled by diffuse vertical gravity currents, as opposed to passive settling of individual particles through the water column. This process greatly reduces the residence time of fine ash in the ocean and diminishes the role of ocean currents in influencing the distribution patterns of individual tephra layers. Support for this mechanism comes from observations of greatly accelerated tephra settling rates in the South China Sea following the 1991 eruption of Mount Pinatubo in the Philippines.

The 1982 eruptions of El Chichon volcano, Mexico (2): Observations and numerical modelling of tephra-fall distribution

Tephra fallout from the A-1 (March 29, 0532 UT), B (April 4, 0135 UT), and C (April 4, 1122 UT) 1982 explosive eruptions of El Chichon produced three tephra fall deposits over southeastern Mexico. Bidirectional spreading of eruption plumes, as documented by satellite images, was due to a combination of tropospheric and stratospheric transport, with heaviest deposition of tephra from the ENE tropospheric lobes. Maximum column heights for the eruptions of 27, 32, and 29 km, respectively, have been determined by comparing maximum lithic-clast dispersal in the deposits with predicted lithic isopleths based on a theoretical model of pyroclast fallout from eruption columns. These column heights suggest peak mass eruption rates of 1.1 × 108, 1.9 × 108, and 1.3 × 108 kg/s. Maximum column heights and mass eruption rates occured early in each event based on the normal size grading of the fall deposits. Sequential satellite images of plume transport and the production of a large stratospheric aerosol plume indicate that the eruption columns were sustained at stratospheric altitudes for a significant portion of their duration. New estimates of tephra fall volume based on integration of isopach area and thickness yield a total volume of 2.19 km3 (1.09 km3 DRE, dense rock equivalent) or roughly twice the amount of the deposit mapped on the ground. Up to one-half of the erupted mass was therefore deposited elsewhere as highly dispersed tephra.

Sedimentation of tephra by volcanic plumes. Part 2: controls on thickness and grain-size variations of tephra fall deposits

A model for sedimentation from turbulent suspensions predicts that tephra concentration decreases exponentially with time in an ascending volcanic column and in the overlying umbrella cloud. For grain-size distributions typical of plinian eruptions application of the model predicts for thickness variations in good agreement with the exponential thinning observed in tephra fall deposits. The model also predicts a proximal region where fallout from the plume margins results in a more rapid decrease in thickness so that the deposit shows two segments on a thickness versus distance plot. Several examples of deposits with two segments are known. The distance at which the two segments intersect is a measure of eruption column height. The thickness half-distance (∼ equivalent to the dispersal index of Walker) is strongly correlated with column height, but is also weakly dependent on grain-size distribution of the ejecta. For a dispersal index of 500 km2 (the plinian/subplinian boundary of Walker) column heights between 14 and 18 km are calculated. For ultraplinian deposits with D>50000 km2 column heights of at least 45 km are implied. Model grain-size distributions of the deposits have sorting values comparable to those observed in tephra fall deposits formed from eruption columns in a weak or negligible cross-wind. Median diameter decreases exponentially with distance as is observed. Sorting (σφ) improves with distance as is observed in plinian deposits in a weak wind. However, tephra fall deposits formed in strong winds do not show improved sorting with distance and proximal deposits are typically somewhat better sorted than the model calculations. Differences are attributed to the influence of wind which disperses particles further than predicted in our model and which has an increasing influence as particle size decreases.

The 1982 eruptions of El Chichon volcano, Mexico (3): Physical properties of pyroclastic surges

Two major pyroclastic surges generated during the 4 April 1982 eruption of El Chichon devastated an area of 153 km2 with a quasi-radial distribution around the volcano. The hot surge clouds carbonized wood throughout their extent and were too hot to allow accretionary lapilli formation by vapor condensation. Field evidence indicates voidage fraction of 0.99 in the surge cloud with extensive entrainment of air. Thermal calculations indicate that heat content of pyroclasts can heat entrained air and maintain high temperatures in the surge cloud. The dominant bed form of the surge deposits are sand waves shaped in dune forms with vertical form index of 10–20, characterized by stoss-side erosion and lee-side deposition of 1–10 cm reversely graded laminae. A systematic decrease in maximum lithic diameter with distance from source is accompanied by decrease in wavelength and amplitude. Modal analysis indicates fractionation of glass and pumice from the surge cloud relative to crystals, resulting in loss of at least 10%–25% of the cloud mass due to winnowing out of fines during surge emplacement. Greatest fractionation from the −1.0–0.0−∅ grain sizes reflects relatively lower pumice particle density in this range and segregation in the formative stages of the surge cloud. Extensive pumice rounding indicates abrasion during bed-load transport. Flow of pyroclastic debris in the turbulent surge cloud was by combination of bed-load and suspended-load transport. The surges are viewed as expanding pyroclastic gravity flows, which entrain and mix with air during transport. The balance between sedimentation at the base of the surge cloud and expansion due to entrainment of air contributed to low cloud density and internal turbulence, which persisted to the distal edge of the surge zone.