Pumice Fall Deposits Research Articles

Dilute gravity current and rain-flushed ash deposits in the 1.8 ka Hatepe Plinian deposit, Taupo, New Zealand

Two groups of poorly sorted ash-rich beds, previously interpreted as rain-flushed ashes, occur in the ca. AD 180 Hatepe Plinian pumice fall deposit at Taupo volcano, New Zealand. Two ash beds with similar dispersal patterns and an aggregate thickness of up to 13 cm make up the lowermost group (A). Group A beds extend 45 km north-east of the vent and cover 290 km2. In the southern part of the group A distribution area, a coarse ash to lapilli-size Plinian pumice bed (deposit B) separates the two group A beds. The scarcity of lapilli (material seen elsewhere from the still-depositing pumice fall) in group A beds indicates that they were rapidly transported and deposited. However, this rapid transportation and deposition did not produce cross-bedding, nor did it erode the underlying deposits. It is proposed that thick (>600 m) but dilute gravity currents generated from the collapsing outer margin of the otherwise buoyant Hatepe Plinian eruption column deposited the group A beds. The upper ash beds (group C) consist of one to seven layers, attain an aggregate thickness of ≤35 cm, and vary considerably in thickness and number of beds with respect to distance from vent. Group C beds contain variable amounts of ash mixed with angular Plinian pumices and are genuine rain-flushed ashes. Several recent eruptions at other volcanoes (Ukinrek Maars, Vulcan, Rabaul, La Soufrere de Guadeloupe and Soufriere, St Vincent) have produced gravity currents similar in style, but much smaller than those envisaged for group A deposits. The overloaded margins of otherwise buoyant eruption plumes generated these gravity currents. Laboratory studies have produced experimental gravity current analogues. Hazards from dilute gravity currents are considerable but often overlooked, thus the recognition of gravity current deposits will contribute to more thorough volcanic hazard assessment of prehistoric eruption sequences.

Chemical variations of the A.D. 79 pumice deposits of Vesuvius

The pumice-fall deposits of the A.D. 79 Vesuvius eruption are subdivided into a lower white and an upper grey pumice. Major and trace element variations show significant differences between them. The more evolved phonolitic white pumice exhibits well-developed, stratigraphically controlled chemical gradients in all major and trace elements whereas the tephriphonolitic grey pumice is compositionally less variable and lacks such gradients. Incompatible elements are mostly enriched in the white pumice, whereas the grey pumice shows higher contents of compatible elements such as Ti and Mg. The difference in chemical composition between white and grey pumice is interpreted in terms of fractional crystallization of clinopyroxene, sanidine and leucite. The well-developed gradients of the white pumice indicate a stable layering within the upper phonolitic magma body, which is thought to originate from diffusion processes of highly incompatible elements. The lack of a similar stratigraphy-related zonation within the grey pumice indicates a more homogeneous tephriphonolitic magma resulting from continuous convective mixing within the lower part of the magma chamber. The transition from white to grey pumice is characterized by a boundary zone containing, besides white and grey pumice clasts, a third type of pumice, «the boundary pumice», which is light grey in color, this pumice is internally homogeneous, without evidence of physical mingling between «white» and «grey» glass, and is typically characterized by intermediate MgO, K 2 O and Ba contents. It is interpreted to represent a third magma component in the magma chamber, the interface magma layer, resulting from mixing and diffusion processes across the phonolitic/tephriphonolitic boundary

Paleointensities for 10–22 ka from volcanic rocks in Japan and New Zealand

Eight volcanic rocks from Japan and New Zealand in the age range 10–50 ka B.P. were studied using the Thellier method paleointensity experiment and yielded five successful results for 10–22 ka. Samples from the Shikotsu, Kuttara and Daisen volcanoes in Japan were taken from pyroclastic flows (both welded and non-welded) and a pumice fall deposit. One of the two New Zealand rocks is a rhyolite lava from the Taupo Volcanic Zone. Four sites out of seven located on rocks that are not usually subjected to Thellier's experiment yielded successful results, which indicates that volcanic products such as pyroclastic flows and rhyolite lavas are as good a material as basalt and andesite lavas for application of the paleointensity experiment. One paleointensity from New Zealand at 9.9 ka is very large, at about 100 μT, and provides support for the paleointensity high found in Europe and Japan [1,2]. The other four paleointensities for 14–22 ka are smaller than the present-day value, and this agrees with the idea of a broad paleointensity minimum for the period 10–50 ka that was suggested by McElhinny and Senanayake [3]. It is also noted that the variation curve of absolute paleointensity from the volcanic rocks is reasonably similar to the relative paleointensity curves from marine and lake sediments which were summarized by Tauxe [4].

支しゃくおよびクッタラ火山のテフロクロノロジー

The surrounding area of the Shikotsu and the Kuttara volcanoes, and Ishikari-Lowland, in the southwestern Hokkaido, are covered with thick pyroclastic deposits of the late Pleistocene. These pyroclastic deposits have been subdivided into units according to facies changes. consequently each unit does not necessarily represented a single eruption. In the present paper, the products of one eruption (tephra formations) are separated from one another by recognition of intervening “volcanic ash soils” which are considered to be the most suitable for indicating quiet period, owing to its low depositional rate and generality.In the Ishikari-Lowland, the previously defined two groups of tephra formations (En-c and Spfl · Spfa 1; Spfa 7, 8, 9 and 10) are reinterpreted to represent the products of two eruptions, because the volcanic ash soils are absent among them. And three tephra formations are newly recognized. Twenty two tephra formations constitute the late Pleistocene pyroclastic deposits in the Ishikari-Lowland.The pyroclastic deposits distributed around the Kuttara volcano are subdivided into twelve tephra formations and one scoria fall group. The tephra formations derived from the Kuttara volcano are as follows in ascending order : Kt-8, 7, 6, 5, 4, Kt-Hy, Kt-3, Kt-Tk scoria fall group, Kt-2, Kt-1. The Kt-8 …-1 are mainly composed of plinian pumice fall deposits and pumice flow deposits generated by large and moderate scale explosive eruptions.The correlation of the tephra formations in the surrounding area of the Kuttara volcano and the Ishikari-Lowland is attempted with several petrographic parameters and stratigraphic position. The following tephra formations are correlated each other : Kt-8 and Aafa 1, Kt-5 and Mpfa 2b, Kt-4 and Mpfa 2a, Kt-3 and Spfa 4, Kt-Tk and Spfa 3, Kt-1 and Spfa 2. In addition, it is proved that the following tephra formations spread to the Ishikari-Lowland : Kt-7, Kt-Hy, Nj-Os.On the Shikotsu volcano, three explosive eruptions preceded a large scale caldera forming eruption (Spfl · Spfa 1 : Ca. 32 ka), and after a short repose time, present post caldera activity started. The Kuttara Volcano was the site of seven explosive eruptions during the period from 70-40 ka ; then the stratovolcano was formed by erupting Kt-Tk scoria fall group, and the summit caldera was formed by the last two explosive eruption (Kt-1 and 2).

恵庭a降下軽石層の降下年代とその降下前後の古気候

恵庭a降下軽石層の降下年代とその降下前後の古気候

The Ottaviano eruption of Somma-Vesuvio (8000 y B.P.): a magmatic alternating fall and flow-forming eruption

The Ottaviano eruption occurred in the late neolithic (8000 y B.P.). 2.40 km 3 of phonolitic pyroclastic material (0.61 km 3 DRE) were emplaced as pyroclastic flow, surge and fall deposits. The eruption began with a fall phase, with a model column height of 14 km, producing a pumice fall deposit (LA). This phase ended with short-lived weak explosive activity, giving rise to a fine-grained deposit (L1), passing to pumice fall deposits as the result of an increasing column height and mass discharge rate. The subsequent two fall phases (producing LB and LC deposits), had model column heights of 20 and 22 km with eruption rates of 2.5 × 10 7 and 2.81 × 10 7 kg/s, respectively. These phases ended with the deposition of ash layers (L2 and L3), related to a decreasing, pulsing explosive activity. The values of dynamic parameters calculated for the eruption classify it as a sub-plinian event. Each fall phase was characterized by variations in the eruptive intensity, and several pyroclastic flows were emplaced (F1 to F3). Alternating pumice and ash fall beds record the waning of the eruption. Finally, owing to the collapse of a eruptive column of low gas content, the last pyroclastic flow (F4) was emplaced.

Modeling of the ascent of magma during the plinian eruption of Vesuvius in A.D. 79

The ascent of magma during the A.D. 79 eruption of Vesuvius was studied by a steady-state, one-dimensional, and nonequilibrium two-phase flow model. The gas exsolution process was modeled by assuming a chemical equilibrium between the exsolved and dissolved gas, whereas the magma density and viscosity were modeled by accounting for the crystal content in magma. The exsolution, density, and viscosity models consider the effect of different compositions of the white and gray magmas. By specifying the conduit geometry and magma composition, and employing the model to search for the maximum discharge rate of magma which is consistent with the specified geometry and magma composition, the model was then used to establish the two-phase flow parameters along the conduit. It was found that for all considered conditions the magma pressure in the conduit decreases below the lithostatic pressure near the magma fragmentation level, and that in the deep regions of the conduit the white magma pressure is larger and the gray magma pressure is lower than the lithostatic one. The exsolution and fragmentation levels were found to be deeper for the white than for the gray magma, and the changing composition during the eruption causes an increase of the exit pressure and decrease of the exit gas volumetric fraction. The model also predicted a minimum conduit diameter which is consistent with the white and gray magma compositions and mass flow-rates. The predictions of the model were shown to be consistent with column collapses during the gray eruption phase, large presence of carbonate lithics in the gray pumice fall deposit, and magma-water interaction during a late stage of the eruption.

A case of no-wind plinian fallout at Pululagua caldera (Ecuador): implications for models of clast dispersal

The caldera of Pululagua is an eruptive centre of the Northern Volcanic Zone of the South American volcanic arc, located about 15 km north of Quito, Ecuador. Activity leading to formation of the caldera occurred about 2450 b.p. as a series of volcanic episodes during which an estimated 5–6 km3 (DRE) of hornblende-bearing dacitic magma was erupted. A basal pumice-fall deposit covers more than 2.2x104 km2 with a volume of about 1.1 km3 and represents the principal and best-preserved plinian layer. Circular patterns of isopachs and pumice, lithic and Md isopleths of the Basal Fallout (BF) around the caldera indicate emplacement in wind-free conditions. Absence of wind is confirmed by an ubiquitous, normally graded, thin ash bed at the top of the lapilli layer which originated from slow settling of fines after cessation of the plinian column (co-plinian ash). The unusual atmospheric conditions during deposition make the BF deposit particularly suitable for the application and evaluation of pyroclast dispersal models. Application of the Carey and Sparks' (1986) model shows that whereas the 3.2-, 1.6-, and 0.8-cm lithic isopleths predict a model column height of about 36 km, the 6.4-cm isopleth yields and estimate of only 21 km. The 4.9- and 6.4-cm isopleths yield a column height of 28 km using the model of Wilson and Walker (1987). The two models give the same mass discharge rate of 2x108 kg s-1. A simple exponential decrease of thickness with distance, as proposed by Pyle (1989) for plinian falls, fits well with the BF. Exponential decrease of size with distance is followed by clasts less than about 3 cm, suggesting, in agreement with Wilson and Walker (1987), that only a small proportion of large clasts reach the top of the column. Variations with distance in clast distribution patterns imply that, in order to obtain column heights by clast dispersal models, the distribution should be known from both proximal and distal zones. Knowledge of only a few isopleths, irrespective of their distance from the vent, is not sufficient as seemed justified by the method of Pyle (1989).

Some remarks on volcanic vent evolution during plinian eruptions

The magma mass discharge rate governs the eruption magnitude of plinian eruptions, and depends strongly on conduit geometry. The presence of lithic clasts in pumice deposits results largely from conduit erosion, and the magma discharge rate and the lithic abundance in the resulting deposit are parameters linked by the overall vent widening rate. Many pumice fall deposits have lithic-enriched basal and top sections; pumice flow deposits tend to carry more lithics than the preceding fall layer. Theoretical models of vent evolution for a cylindrical conduit with radius R indicate an R 4 dependency of the magma discharge rate, which would lead to rapidly decreasing lithic abundances from bottom to top in plinian fall deposits, and from fall to flow deposits, contrary to observations. To simulate observed lithic abundance patterns, a model of conduit evolution with a stepped geometry was developed, in which the lower conduit section remains at approximately constant radius, while the upper conduit section widens. The mass discharge rate during an eruption then increases with R 2 top , and predicted lithic abundance patterns agree qualitatively with field data. Field evidence for conduit widening in the upper conduit section only is presented from plinian pumice beds from Nisyros, Greece. The change in eruption style from plinian fall to surge and ash flow is sometimes accompanied by a dramatic change in lithic abundance and lithic type. In these cases, this transition in deposition style is not the result of a gradual increase in mass discharge rate because of overall vent widening, but is caused by catastrophic rupture of the lower vent section during incipient caldera collapse, with resulting changes in magma discharge rates, lithic abundance and possibly water/magma interaction.

The Te Rere and Okareka eruptive episodes — Okataina Volcanic Centre, Taupo Volcanic Zone, New Zealand

Abstract The Te Rere and Okareka eruptive episodes occurred within the Okataina Volcanic Centre at c. 21 000 and 18 000 yr B.P., respectively. The widespread rhyolitic pumice fall deposits of Te Rere Ash (volume 5 km3) and Okareka Ash (6 km3) are only rarely exposed in near‐source areas, and locations of their vent areas have been uncertain. New exposures and petrographic and chemical analyses show that the Te Rere episode eruptions occurred from multiple vents, up to 20 km apart, on the Haroharo linear vent zone. The Okareka episode eruptions occurred from vents since buried beneath the Tarawera volcanic massif. Eruption of the rhyolitic Okareka pumice fall was immediately preceded by a small basaltic scoria eruption, apparently from vents close to those for the following rhyolite eruptions. Dacitic mixed pumices scattered within the rhyolite pumice layers immediately overlying the scoria were formed by mixing of the basalt and rhyolite magmas. The Te Rere and Okareka pyroclastic eruptions were both foll...

Magma mixing and convective compositional layering within the Vesuvius magma chamber

The pumice-fall deposits of the last two Plinian eruptions of Vesuvius-a.d. 79 “Pompei” and 3700 b.p. “Avellino”-show a marked vertical compositional variation from white phonolite at the base to grey tephritic phonolite at the top. In both Avellino and Pompei sequences a compositional gap separates white from grey pumice. Grey and white pumice have distinct Sr and Nd isotopic compositions (grey pumice: 87Sr/36Sr=0.70749-56, 143Nd/144Nd=0.512507 for Pompei; 0.70760-69, 0.512504 for Avellino; white pumice: 0.70757-78 for Pompei; 0.70729-42 for Avellino). K-feldspar separated from both grey and white pumice has, in all cases, a “white” 87Sr/86Sr ratio (0.70766-79 for Pompei, 0.70728-33 for Avellino). The observed variations are interpreted as reflecting a pre-eruptive zonation of the magma chamber. Although mineralogical evidence of interaction between magma and calcareous country rocks exists in both eruptions, crustal contamination has not significantly modified the isotopic signatures of the erupted products. Petrographic and isotopic evidence of syneruptive magma mingling occur in Pompei grey pumice as well as in Avellino white and grey pumice, but they do not fully explain all the observed geochemical and isotopic variations. These variations are related to the complex refilling history of the magmatic system and result by fractional crystallization and mixing processes acting within the magma chamber. Preliminary data from other Plinian and subplinian sruptions of the Somma-Vesuvius point out the repeticive behaviour of 87Sr/86Sr variation in the last 25 000 years, hence suggesting a single magma chamber and continuity of the feeding system.

Revisions to the stratigraphy and volcanology of the post‐0.5 Ma units and the volcanic section of VC‐1 core hole, Valles Caldera, New Mexico

Reexamination of the Continental Scientific Drilling Project VC‐1 core, new field data, and new isotopic ages necessitate revision of the previously described geology of the youngest eruptive units from the Valles caldera. The Banco Bonito rhyolite lava flow and minor attendant pyroclastic, autoclastic, and reworked deposits about 0.9 km3 dense rock equivalent volume (DRE), are recognized as the product of the youngest eruption from the caldera. The best estimate of the age of this event is 170–240 ka (1σ). A slightly earlier eruption produced the El Cajete pumice fall deposit, followed by the Battleship Rock ignimbrite and the VC‐1 Rhyolite, totalling about 1.8 km3 DRE. We suggest that the VC‐1 Tuffs, ignimbrites recognized only in the VC‐1 core hole, are not a separate unit as previously proposed but are, in fact, the Battleship Rock ignimbrite. A complex volcaniclastic breccia dating from about the time of the South Mountain Rhyolite lava flow (529 ka) is the lowermost volcanic unit in the VC‐1 core. The young, post‐500 ka eruption products from the SW part of the Valles caldera are all composed of very similar low‐silica rhyolite and are thus distinct from earlier, postcaldera high‐silica rhyolites included in the Valles Rhyolite Formation. This distinction marks a major change in the magmatic evolution of the Valles caldera. We propose that recognition of this change necessitates revisions to the stratigraphy of the upper part of the Tewa Group.

Pyroclastic deposits of the November 13, 1985 eruption of Nevado del Ruiz volcano, Colombia

The November 13, 1985 eruption of Nevado del Ruiz produced a series of pyroclastic flows and surges that eroded channels on the surface of the summit glacier and generated lahars which descended down most of the rivers that drain the volcano. The stratigraphy of the proximal pyroclastic deposits indicates that there were at least four episodes to the eruption. Episode I, deposited an unusual surge consisting of small pieces of ice mixed with ash and exhibiting planar stratification. Ballistically emplaced fragments are also intercalated with this unit. During Episode II, at least two pyroclastic flows were erupted. Their deposits contain the most evolved pumice of the entire eruption; SiO 2 content of matrix glass ranges between 74.5 and 74.9%. Episode III is marked by the emplacement of a welded tuff with an average SiO 2 content of about 66% in the matrix glass. The final Episode IV was characterized by the development of a high-altitude eruption column and the emplacement of several nonwelded pyroclastic flows. Banded pumice are common in the pyroclastic flow as well as in the pumice fall deposits. Co-existing dark and light pumice bands differ in SiO 2 content by 3.5% and in general are similar to the composition of the welded pumice from Episode III. The compositional zonation of the pyroclastic deposits from Episode I to IV suggests that a nearsurface compositionally-stratified portion of the magma body was tapped during Episode II. During Episodes III and IV the main body of magma was involved although the coexistence of the compositionally distinct pumice clasts at similar stratigraphic levels argues for mixing of magma from different levels in the chamber during the eruptive process.

テフロクロノロジーからみた赤城火山最近２０万年間の噴火史

Akagi volcano situated in the North Kanto district of central Japan is a Quaternary volcano. The eruptive history of this volcano during the last 200, 000 years is clarified by the tephrochronological study.The plinian pumice fall deposits derived from Akagi volcano are as follows in ascending order; the Moka Pumice (MoP), Akagi-Mizunuma Pumice-16. …-12, -10…-1 (MzP-16…-12, -10…-1), Namekawa Pumice-2, -1 (Nm-2, -1), Yunokuchi Pumice (UP) and Kanuma Pumice (KP) (Fig. 1). Stratigraphy, distributions and petrographic characteristics of these tephras are described (Figs. 5, 6, 8, 9 and Tables 1, 3).The MoP pumice fall deposit covering the most part of the eastern part of the North Kanto district, erupted in the penultimate glacial stage preceding the Last Interglacial Stage. Moreover, the stratigraphic relations of the MzP-10…-1, Nm-2, -1 and UP with the well dated widespread tephras, which are the K1P-7 (ca. 130 ka), DPm, On-Pm I (ca. 80 ka), K-Tz (ca. 75-80 ka), Aso-4 (ca.. 70 ka) and DKP (ca. 45-48 ka), are clarified (Fig. 7). These data can give the chronological framework for the eruptive history.The MzP series, Nm-2, Nm-1 and UP erupted during the stage called the younger stratovolcano (YS) of the Akagi volcano in previous work (Fig. 10). The total volume of the plinian pumice fall deposits from the MzP-10 to the UP amounts to 28 km3. This corresponds to the discharge rate of the pumice equivalent to 0.33 km3/1, 000 years and the frequency of the plinian eruption 0.15/1, 000 years.Before the formation of the central cone, it occurred the most eruptive episode of Akagi volcano. This is represented by the members : the KP pumice fall deposit (ca. 31-32 ka) and the Mizunuma lithic (chert lapilli) fall deposit (CLP). The volume of the KP deposit which amounts to 25 km3, is the largest volume of the plinian pumice fall deposits derived from Akagi volcano.

Lead isotope and trace element variation in Tenerife pumices: evidence for recycling within an ocean island volcano

AbstractThree voluminous Quaternary phonolitic pumice fall deposits erupted from the compositionally-zoned Tenerife magma chamber exhibit variability in Sr and Pb isotope ratios. It has been previously argued that the Sr isotope variations are due to syn-eruptive interaction between magma and hydrothermal fluids (Palacz and Wolff, 1989). Pb compositions are not correlated with Sr, and are believed to reflect magmatic values. Pb isotope ratios exhibit regular variation with degree of fractionation, and one zoned deposit is heterogeneous in Pb. The highest values seem to characterize the most fractionated upper parts of the zoned system. This is unlikely to be a consequence of magmatic recharge. Isotopic and trace element behaviour is instead consistent with combined assimilation and fractional crystalliza- tion, involving the recycling of material containing relatively radiogenic Pb, from within the volcanic edifice. Assimilation of sediment intercalated within the submarine portion of the pile is ruled out by the isotopic data. The most probable contaminant is a felsic igneous rock. Early trachytes reported by Sun (1980) have the required Pb isotope compositions and may approximately represent the assimilant.

The recent pumice eruptions of Mt. Pelée volcano, Martinique. Part I: Depositional sequences, description of pumiceous deposits

Abstract Mount Pelee is one of the most active volcanoes of the Lesser Antilles arc, with more than twenty eruptions over the last 5000 years. Both nuee ardente-type eruptions, which are well known, and pumice eruptions, although little known, are very common in the stratigraphic record. The four younger pumice eruptions, P4 (2440 y.B.P.), P3 (2010 y.B.P.), P2 (1670 y.B.P.) and P1 (650 y.B.P.) can be used to reconstruct the eruption sequences. The various pumiceous deposits can be described as fine lithic ash layer, Plinian fall deposits, pumice and ash flow deposits with associated ash cloud fall deposits, and pumice surge deposits. Three kinds of depositional sequences have been defined. The distinctions between them are based on the occurrence of an initial Plinian phase and the generation of intraflow pyroclastic surges. The pumice eruptions of Mt. Pelee are small in intensity and magnitude, as expressed by the dispersal of their products and by the total mass of erupted material which is estimated to be less than 1 km3 in each case. The pumice fall deposits have dispersal characteristics of small Plinian eruptions, close to the sub-Plinian type. Nevertheless, the probability of an occurrence of a new pumice eruption at Mt. Pelee is high, and the widespread distribution of pumice deposits around the volcano suggests that such an eruption is a major volcanic risk during the present stage of activity.

６世紀における榛名火山の２回の噴火とその災害

Haruna volcano, situated in the central part of Japan, erupted twice in the 6th century, from Futatsu-dake crater. The first eruption, which occurred in the early part of the 6th century, was set set off by a low-temperature phreatomagmatic eruption. The initially ejected very fine ash accumulated as accretionary lapilli and muddy rainfalls. Later, the eruption changed to hot pyroclastic flow effusions, which contained many essential lithics. These pyroclastic flow effusions included small-scale phreatic eruptions. The ash had formed ash clouds that then accumulated on each pyroclastic flow deposit. This tephra sequence was named the Haruna-Shibukawa tephra formation(Hr-S).These pyroclastic flow encroached on an older village, Nakasuji, situated on the eastern flank of Haruna volcano. The pyroclastic flow (S-5) burned and destroyed many houses. Because its deposit was very thinly laminated, it took the form of a hot pyroclastic surge, which spread over the eastern side of Haruna volcano, causing widespread damage there before changing to mud flows and floods and damaging rice fields in the area.The second eruption, which occurred in the middle or later part of the 6th century, is characterized by plinian eruptions and pyroclastic flow effusions. This tephra sequence was named the Haruna-lkaho tephra formation (Hr-I). The pumice ejected in the plinian eruptions was deposited, in a layer about 3cm thick, on Soma city, 200km from the vent.An older village, Kuroimine, situated about 10km from the vent, was buried by a layer of pumice about 200cm thick. Because pumice oxidized by the flames of burning houses is observed from the bottom to near the top of the pumice fall deposit, we can confirm that the greater part of the pumice accumulated during a period of hours. A house was crushed by the coarser part of the pumice fall deposit (1-6). The pyroclastic flows, which caused columns to collapse, moved and accumulated along the valleys before changing into mud flows and floods. They also caused heavy damage to rice fields and farms.In Gunma Prefecture, it may well be that villages, rice fields, and farms damaged by volcanic eruptions in the same way as Nakasuji village and Kuroimine village were damaged will be discovered. The data in relation to past volcanic hazards, obtained by joint research between archaeology and volcanology, will contribute to predicting volcanic disasters.

Age of the Kumakura Pumice Fall Deposit Erupted from Kusatsu-Shirane Volcano, Central Japan

Age of the Kumakura Pumice Fall Deposit Erupted from Kusatsu-Shirane Volcano, Central Japan

Lithic breccia and ignimbrite erupted during the collapse of Crater Lake Caldera, Oregon

The climactic eruption of Mount Mazama (6845 y.B.P.) vented a total of ∼50 km 3 of compositionally zoned rhyodacitic to basaltic magma from: (a) a single vent as a Plinian pumice fall deposit and the overlying Wineglass Welded Tuff, and (b) ring vents as ignimbrite and coignimbrite lithic breccia accompanying the collapse of Crater Lake caldera. New field and grain-size data for the ring-vent products are presented in this report. The coarse-grained, poorly bedded, clast-supported lithic breccia extends as far as 18 km from the caldera center. Like the associated ignimbrite, the breccia is compositionally zoned both radially and vertically, and silicic, mixed, and mafic types can be recognized, based on the proportion of rhyodacitic pumice. Matrix fractions in silicic breccias are depleted of fines and are lithic- and crystal-enriched relative to silicic ignimbrite due to vigorous gas sorting during emplacement. Ignimbrite occurs as a proximal veneer deposit overlying the breccia, a medial (∼ 8 to ∼ 25 km from the caldera center), compositionally zoned valley fill as much as > 110 m thick, and an unzoned distal (⪖ 20 km) facies which extends as far as 55 km from the caldera. Breccia within ∼ 9 km of the caldera center is interpreted as a coignimbrite lag breccia formed within the deflation zone of the collapsing ring-vent eruption columns. Expanded pyroclastic flows of the deflation zone were probably vertically graded in both size and concentration of blocks, as recently postulated for some turbidity currents. An inflection in the rate of falloff of lithic-clast size within the lithic breccia at ∼ 9 km may mark the outer edge of the deflation zone or may be an artifact of incomplete exposure. The onset of ring-vent activity at Mt. Mazama was accompanied by a marked increase in eruptive discharge. Pyroclastic flows were emplaced as a semicontinuous stream, as few ignimbrite flow-unit boundaries are evident. As eruption from the ring vents progressed, flow-runout distance and the extent of breccia deposition decreased due to (a) greater internal flow friction, and (b) decreasing eruption column heights. Effect (b) probably resulted from a progressive decrease in magmatic gas content and discharge rate. Waning discharge may have been promoted by the tapping of more viscous, crystal-rich magma, collapse of conduit walls, and declining caldera collapse rate.

34. Grain size distribution and mode of emplacement for Haruna, Futatsudake pumice fall deposits.(Abstracts of Papers Presented at the Fall Meeting of the Society)

34. Grain size distribution and mode of emplacement for Haruna, Futatsudake pumice fall deposits.(Abstracts of Papers Presented at the Fall Meeting of the Society)