Gray Pumice Research Articles

Pre-eruptive volatile (H 2O, F, Cl and S) contents of phonolitic magmas feeding the 3550-year old Avellino eruption from Vesuvius, southern Italy

The behaviour of H 2O, F, Cl and S prior to and during the Plinian phase of the 3550-year old Avellino eruption was determined by the study of unheated primary glass inclusions and matrix glasses. The fall deposit can be divided into two sub-units on the basis of an abrupt change in colour: a phonolitic white layer at the base and a tephriphonolitic banded grey pumice layer at the top. The original tephriphonolitic magma is always found mixed with the phonolitic magma. Primary glass inclusions were only found in feldspar, amphibole and garnet associated with the white pumice. Glass inclusions in the products of the Avellino Plinian eruption contain a mean of 3.10±0.74 wt.% H 2O, 0.95±0.15 wt.% F, 0.59±0.02 wt.% Cl, and 560±55 ppm S. The concentration of CO 2 was below the detection limit. Coexisting matrix glasses contain similar amounts of halogens but significantly less water (∼0.5 wt.%) and sulphur (∼100 ppm), suggesting that during the eruption the halogens did not behave as volatiles. Owing to the lack of primary glass inclusions and its hybrid nature, similar conclusions can only be hypothesised for the tephriphonolitic magma on the basis of volatile element distribution in the matrix glasses. Taking into account both our data and data from parental mafic melt inclusions by a previous study by P. Marianelli and co-workers, it has been possible to reconstruct the behaviour of volatiles in the Avellino magmatic reservoir in two stages: (1) variation of volatiles during crystal fractionation; and (2) exsolution of an “excess” gas phase when volatiles reach their solubility limit in silicate melts. The agreement between our data and the solubility limit of chlorine, sulphur and probably water in phonolitic melts at the postulated pressure and temperature (i.e., 100 MPa and 850°C), suggests that such a phase exists. Compared to the other volatiles, fluorine does not exhibit a behaviour (i.e., flat distribution) consistent with the attainment of its solubility limit in phonolites. Using the petrologic method, the minimum output of volatiles during the Plinian phase of the Avellino eruption was hypothesised to be about 2×10 6 tons of H 2SO 4 and 25×10 6 tons of H 2O. Due to both the presence of an “excess” vapour phase prior to eruption and the assumptions inherent in the petrologic method, the total calculated amounts should be considered as very conservative estimates of volatile outputs during the eruption.

Behaviour of chlorine prior and during the 79 A.D. Plinian eruption of Vesuvius (southern Italy) as inferred from the present distribution in glassy mesostases and whole-pumices

This study deals with the distribution of chlorine in glassy mesostases and whole-pumices from the 79 A.D. Plinian eruption (Somma–Vesuvius volcanic complex, Italy). This explosive event produced a prominent Plinian fall deposit followed by flows and surges. The fall deposit can be divided into two sub-units on the basis of an abrupt change in colour at approximately mid-level: a phonolitic white pumice layer at the base and a tephriphonolitic grey pumice layer at the top. Due to its hybrid nature (a mixture of k-tephritic, tephriphonolitic and phonolitic magmas), information on chlorine behaviour in tephriphonolitic magma has been inferred only by means of mass balance calculations. In the white pumices, chlorine concentrations show constant whole-pumice values, whereas glassy mesostases display significant compositional variations. These variations have been linked to the cryptocrystallisation of leucite in glassy mesostases, which affected the original melt compositions just before and during the eruptive event. In this framework, whole-pumice appears to better represent the pre-eruptive melt compositions. Using chlorine concentration in whole-pumices, a three-stage model of chlorine behaviour prior and during the eruptive event is predicted: (i) free variation during Rayleigh fractionation, according to a system with variance greater than zero; (ii) exsolution of volatile chlorine compounds (e.g., metal chlorides), when chlorine reaches its solubility limit in silicate melt, in coexisting hyper-saline and in dilute immiscible fluids; the variance of the system is zero at a given temperature and pressure; (iii) residual syn-eruptive variable enrichment of chlorine in the melt due to cryptocrystallization of leucite, suggesting a very minor loss of chlorine in the gas phase during the eruption. Although chlorine does not behave as a volatile element during the eruption, it is present in the volcanic plume. This is due to the postulated `excess' fluid phase containing chlorine that formed in the magma chamber prior to the eruption. The homogeneous distribution of chlorine in whole pumices, in contrast with a well-established chemical and isotopic layering in Vesuvian magmas prior to Pompei eruption, suggests that the trace element zonation is not directly linked to chlorine distribution in silicic melts.

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

Vesiculation of May 18, 1980, Mount St. Helens magma

The May 18, 1980, eruption of Mount St. Helens, Washington, produced both white and gray pumice, similar in composition but varying in phenocryst, microlite, and vesicle content. The white pumice has fewer phenocrysts, no microlites, and higher vesicularity, and is thus less dense than the gray. In addition, vesicles in the white pumice are larger and more interconnected than those in the gray. Both white and gray have effective (crystal-free) vesicularities (85.7% and 72.2%, respectively), close to the traditionally accepted of 75%-77%. Given that the two pumice types were erupted concurrently for several phases of the eruption and therefore probably had similar eruption paths, we suggest that differences in vesicularity and vesicle size distributions result from the presence or absence of microlites. The presence of microlites in the gray pumice not only increased the magma viscosity and effective vesicularity, but appears to have aided bubble nucleation and hindered bubble expansion and coalescence. Thus, magmas with microlites may fragment at a lower bulk vesicularity than those without microlites. Fragmented microlite-bearing clasts are also likely to expand less after fragmentation and therefore more closely preserve the bubble distribution and structure at the time of magma fragmentation.

The Avellino plinian eruption of Somma-Vesuvius (3760 y.B.P.): the progressive evolution from magmatic to hydromagmatic style

A detailed stratigraphic analysis of the Avellino plinian deposit of the Somma-Vesuvius volcano shows a complicated eruptive sequence controlled by a combination of magmatic and hydromagmatic processes. The role of external water on the eruptive dynamics was most relevant in the very early phase of the eruption when the groundwater explosively interacted with a rising, gas-exolving magma body creating the first conduit. This phase generated pyroclastic surge and phreatoplinian deposits followed by a rapidly increasing discharge of a gas-rich, pure magmatic phase which erupted as the most violent plinian episode. This continuing plinian phase tapped the magma chamber, generating about 2.9 km 3 of reverse-graded fallout pumice, more differentiated at the base and more primitive at the top (white and gray pumice). A giant, plinian column, rapidly grew up reaching a maximum height of 36 km. The progressive magma evacuation at a maximum discharge rate of 10 8 kg/s that accompanied a decrease of magmatic volatile content in the lower primitive magma allowed external water to enter the magma chamber, resulting in a drastic change in the eruptive style and deposit type. Early wet hydromagmatic events were followed by dry ones and only a few, subordinated magmatic phases. A thick, impressive sequence of pyroclastic surge bedsets of over 430 km 2 in area with a total volume of about 1 km 3 is the visible result of this hydromagmatic phase.

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.

Petrology and dynamics of the Waimihia mixed magma eruption, Taupo Volcano, New Zealand

The Waimihia pumice deposit was erupted from Taupo volcano about 3.3 ka BP. It comprises a plinian tephra fall deposit divisible into two volumetrically subequal fall units and a late-stage, volumetrically minor, near-vent ignimbrite. About 7.5km 3 of magma were erupted at an average rate of c. 8 × 10 8 kg s −1 . Whole rock, glass and mineral compositions define three crystal-poor magma types: rhyolite, rhyodacite and andesite. The proportions of the three magmas varies with stratigraphic position in the two fall units. The lower Waimihia fall unit shows a steady upward increase in rhyodacite from <1% to 5.5% and a near absence of andesite (<0.05%). Rhyodacite in the upper Waimihia fall unit decreases upward from 28% to 2% while andesite increases (0.2% to 0.8%) before decreasing (0.3%). The composition of the ignimbrite coincides with that of the top part of the upper fall unit (consistent with its stratigraphic position), suggesting that partial column collapse occurred towards the close of activity. Rhyolite dominates the total Waimihia deposit ( c . 92%) and is similar to other post-22 ka Taupo rhyolites in terms of major element, trace element and Sr isotope composition, mineralogy and Fe-Ti oxide temperature and oxygen fugacity. The rhyodacite ( c. 7.3%) formed by hybridization of rhyolite and andesite prior to the eruption, and occurs as grey pumices and with rhyolite in streaky pumices. Andesite ( c. 0.3%) occurs as black scoria, sometimes containing traces of rhyodacite; it is an unusual high TiO 2 tholeiitic andesite whose 87 Sr/ 86 Sr ratio of 0.7062 is indistinguishable from that of the rhyolite. The heterogeneous pumice and scoria clasts were formed in a second magma mixing process that was active when the magmas were being transported to the surface during the eruption. The order in which the magmas were erupted, and the decoupling of the peaks in rhyodacite and andesite production, are explained by withdrawal from a three-layer sill that had formed following the injection of c. 0.16 km 3 of andesite into a ≥ 7.5 km 3 sillshaped rhyolitic magma chamber.

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.

Chronology and pyroclastic stratigraphy of the May 18, 1980, Eruption of Mount St. Helens, Washington

Many timed observations make it possible to subdivide the 9‐hour Plinian eruption of Mount St. Helens on May 18, 1980, into six phases, defined by eruption style. The phases are correlated with stratigraphic subunits of ashfall tephra and pyroclastic flow deposits. The suite of pyroclastic deposits indicates that the eruption became more pumice‐rich and compositionally diverse with time, perhaps owing to concurrent eruption of less evolved, gas‐poor parts of the magma body with the more evolved, gas‐rich parts. The paroxysmal phase I (0832–0900) consisted of landslides, lithic pyroclastic flows of a lateral blast and other explosions, and a weak pre‐Plinian column. Phase I pyroclastic deposits include lithic ash flow deposits intercalated with and overlying the voluminous debris avalanche deposit and basal pumice lapilli tephra that underlies a pisolitic ash layer. The early Plinian phase II (0900–1215) consisted of vertical ejection of tephra with an early pulse of small pyroclastic flows on the upper flanks (1010–1035), a brief period of lithic ash ejection (1035–1100), and a pumice‐rich pulse that accompanied growth in height of the eruption column (1100–1215). Deposits include minor pyroclastic flows on the crater rim and a reversely graded sequence of proximal tephra that include the lower pumice lapilli layer, the lower lithic ash layer, and the middle pumice lapilli layer, all of which consist of evolved white dacitic pumice (63–64% SiO2). During the early ash flow phase III (1215–1500) the height of the eruption column decreased, vertical ejection of tephra ceased, and pyroclastic flows were fed from intermittent fountains. Phase III deposits consist of a poorly exposed sequence (.≤12 m) of ash flow tuff that consist of many thin flow units (≤2 m each) containing pumiceous white dacite (63–64% SiO2) and denser, gray silicic andésite (61–62% SiO2), and fine‐grained ash cloud deposits interbedded with a nongraded middle pumice ash layer. The climactic phase IV (1500–1715) developed in two stages: fountain‐fed pyroclastic flows, followed by a short pulse (1625–1715) of vigorous vertical ejection of tephra. These stages were accompanied by the peak seismic energy release and peak eruption column height, respectively. Climactic deposits consist of a thick (≤35 m) sequence of thick, lapilli‐rich ash flow sheets (4–12 m each) with white and gray pumice, and streaky scoria bands (60% SiO2) in pumice breccia clasts, and the reversely‐graded, upper pumice lapilli layer that is interbedded with fine‐grained ash cloud deposits. During the late ash flow phase V (1715–1815) eruption intensity waned but included a brief episode of small pyroclastic flows (1745–1815). Phase V deposits consist of small distributary lobes of ash flow tuff containing white and gray pumice, and minor fine‐ash deposits. Phase VI activity (1815 to May 19, 1980) consisted of a low‐energy ash plume, with transient increases in intensity, while seismicity continued at depth. Sustained vertical discharge of phase II prodeced evolved dacitewith high S/Cl ratios. Ash flow activity of phase III is attributed to decreases in gas content, indicated by reduced S/Cl ratios and increased clast density of the less evolved, gray pumice. Climactic events are attributed to vent clearing and exhaustion of the evolved dacite.

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

Research Article| August 01, 1987 Temporal variations in column height and magma discharge rate during the 79 A.D. eruption of Vesuvius STEVEN CAREY; STEVEN CAREY 1Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island 02881 Search for other works by this author on: GSW Google Scholar HARALDUR SIGURDSSON HARALDUR SIGURDSSON 1Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island 02881 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1987) 99 (2): 303–314. https://doi.org/10.1130/0016-7606(1987)99<303:TVICHA>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation STEVEN CAREY, HARALDUR SIGURDSSON; Temporal variations in column height and magma discharge rate during the 79 A.D. eruption of Vesuvius. GSA Bulletin 1987;; 99 (2): 303–314. doi: https://doi.org/10.1130/0016-7606(1987)99<303:TVICHA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

6B. Gray pumice drifited from Fukutokuoka-no-ba to the Ryukyu Islands.

6B. Gray pumice drifited from Fukutokuoka-no-ba to the Ryukyu Islands.

The white trachytic tuff of Roccamonfina Volcano (Roman Region, Italy)

The White Trachytic Tuff (WTT) is a compositionally-zoned, trachytic, pyroclastic-flow deposit which erupted from Roccamonfina volcano about 300,000 years ago. It was principally emplaced as unwelded, pumice-rich flow units with an estimated volume of 10 km3. These now cover the flanks of the volcano on all sides except the west, behind the highest rim of Roccamonfina's summit caldera; the caldera was probably in existence prior to the WTT activity. Eruption of the WTT generally initiated the leucite-free, second stage of Roccamonfina's development, following a long history of leucite-bearing volcanism, but minor leucite-bearing lavas and pyroclastics overlie the WTT as well. The WTT was in turn followed by progressively more basic, leucite-free magmas (latite, trachybasalt, and basalt). During the course of the eruption, the WTT evolved from white, crystal-poor pumices containing 66% SiO2 and 1.2% CaO, to grey pumices containing higher crystal contents, 60% SiO2, and 3% CaO. Early pumices are also relatively enriched in Mn, Na, Zn, Ga, Rb, Y, Zr, Nb, Cs, La, Ce, Yb, Lu, W, Hf, Th, and U, and depleted in Ti, Fe, Mg, K, Sc, V, Cr, Co, Sr, Ba, Nd, Sm, Eu, Tb, Dy, and Ta. The pumices are essentially bimodal in composition, with several minor intermediate types including megascopic, physical mixtures of the white and grey varieties. Certain WTT pumices, including all analyzed intra-caldera samples, are relatively enriched in Pb, Th, Zr, Rb, Ga, Zn, and Cs compared to the rest of the suite. These enrichments may reflect local assimilation of carbonates or more complex exchange processes at the magma chamber margin. All WTT pumices contain the phases sanidine, plagioclase, clinopyroxene, biotite, titanomagnetite, sphene, and apatite; grey varieties also contain magnesian olivine crystals which are probably xenocrysts. The white, crystal-poor types show relatively simple mineralogies with little compositional variability or zoning among crystals of a single phase. Other pumice types, and dark, trachyandesitic inclusions separated from white pumices, show a large compositional spectrum of individually homogeneous crystals. These compositionally diverse crystals and inclusions are interpreted as a result of widespread mixing between the trachytic magmas and more basic magmas prior to or during the WTT eruption. Major-element crystal-fractionation models can successfully derive the early trachytes from the late trachytes by 50–85% separation of a syenitic assemblage of all phases. The predicted phase proportions and compositions closely match cumulate syenite nodules found at Roccamonfina. Trace element models are permissive of syenitic fractionation within the large uncertainties allowed by published partition coefficients.

The pyroclastic deposits of the 1875 eruption of Askja, Iceland

The 1875 explosive eruption of Askja, Iceland was part of a series of regional volcanic and tectonic events which took place in the northern rift zone in 1874 and 1875. These events were marked by regional seismicity, graben formation and a basaltic fissure eruption at Sveinagja, and the plinian eruption of Askja on 28-29 March. Crustal rifting caused basaltic magma to be mixed with rhyolitic magma, triggering the plinian eruption. A caldera, Oskjuvatn, was formed in Askja measuring 3 x 4 km and 267 m deep. Six distinguishable pyroclastic layers can be recognized. The main eruption began with a small sub-plinian pumice eruption forming layer B. The next phase produced a fine-grained, poorly sorted pumice and ash deposit with well developed stratification (layer C), which contains base surge beds near source and is interpreted as phreatomagmatic in origin. The main plinian phase of the eruption lasted 6 h and formed a coarse-grained, poorly bedded pumice-fall deposit (layer D) which contains 75% of the total ejecta. Late-stage explosions formed a layer of lithic clasts (layer E). Isopach and grain-size isopleth maps show that the vents migrated from south to north along a line 1.5 km long in the area now occupied by Oskjuvatn. The intensity and column height of the eruption increased with time as shown by reverse grading and an increasing dispersal index in successive layers. Most of the ejecta is composed of white rhyolitic pumice and ash. Lithics consist of rhyolitic obsidian, partially fused trondhjeimite, and basalt fragments: layer D contains 2.1 mass % lithics. All layers contain abundant grey pumice clasts consisting of intimate mixtures of dark brown basaltic and brown rhyolitic glasses. The mass percentage of mixed pumice in layer D is 4.7, of which 40 % is basaltic glass. These mixed pumice clasts are concentrated at distances of 30-80 km in layer D by aeolian sorting. A grey, crystal-rich, andesitic pumice occurs as inclusions in the white pumice. Layer D shows a systematic decrease in median grain diameter, but no change in cr^ with distance from source. Layer C shows no change in median grain diameter, but a decrease in with distance from source. Phreatomagmatic deposits such as layer C can be readily distinguished from plinian deposits on a Md$ against cr^ diagram, on a against a* (skewness) diagram and on the F against D plot of Walker (1973). The downwind, coarse-tail grading in layer C is attributed to fall-out of fine ash as clumps and aggregates. The total grain-size distributions of both layers D and C show bimodality. In layer D a minor mode in the ash size classes reflects secondary processes of fragmentation by collisions in the vent and column, whereas the major mode is due to disruption of magma by expanding gases. In layer C the fine mode is dominant and represents extensive fragmentation by explosive interaction with water. Field and grain-size studies of layer D show that impact breakage is of major importance near source.

Gravity instability in the Holocene Big and Little Glass Mountain rhyolitic obsidian flows, northern California

Field relations on the surfaces of the Holocene Big and Little Glass Mountain rhyolitic obsidian flows in northern California suggest that regularly spaced areas of coarsely vesicular pumice are diapirs which rose from the base of the flow in response to a density inversion inherent in the flow stratigraphy. The flows contain three lithologic units, defined on the bases of color, texture, and specific gravity, which are arranged in the following vertical sequence: a basal layer of coarsely vesicular, brownish grey pumice ( ρ∼- 0.8 to 2.0), a central core of black, glassy obsidian ( ρ ∼- 2.2), and a surface crust of white to pinkish grey, finely vesicular pumice ( ρ ∼- 2.0). The flows were modeled as multilayered fluid systems with unstable density stratifications, and the spacings of coarse pumice diapirs were equated with the dominant wavelengths of viscous folding theory. Wavelengths of 45 m, 56 m, and 104 m were calculated using measurements of unit thicknesses and estimates of viscosity ratios for the three lobes. Dome spacings of 43 m, 70 m, and 60 m were measured on the same three flow lobes. Discrepancies between predicted and measured spacings are attributed to local variations in thicknesses and viscosities of the flow units. Elongation and surface folding of diapirs are cited as evidence that they emerged before the flows stopped moving.

Two Plinian Pumice-Fall Deposits from Somma-Vesuvius, Italy

Two of the major pumice-fall deposits of Somma-Vesuvius are described. One, the Pompei Pumice, resulted from the classic eruption of A.D. 79, which is taken as the type example of a Plinian eruption. The Pompei Pumice is dispersed south and southeast of the volcano and has been traced to a distance of 72 km. The other, the Avellino Pumice, is probably a thousand years older. It is dispersed to the east and northeast and has been traced to a distance of 50 km. The two deposits are remarkably similar in character, each consisting of a lower part of white pumice and an upper part of mafic gray pumice having a similar grain-size distribution. The distinction between the two deposits is based on (a) the differing proportions of felsic crystals, mafic crystals, and lithic fragments in selected sieve grades; (b) the abundance of nepheline crystals in the Avellino Pumice and their absence from the Pompei; and (c) the greater abundance of crystals in the Avellino Pumice and of lithic debris in the Pompei. The change in composition of the erupting magmas was accompanied by an increase in the vigor of each eruption, as shown by an increase in the density and size of the pumice, in the content and size of lithic fragments, and in the area of dispersal. This is correlated with an increasing depth of origin in a compositionally zoned magma chamber. Granulometric analyses of some ninety samples show that the deposits are well sorted, the sorting improving slightly with increasing distance from the source. Maps showing the median terminal fall velocity, based on the grain-size data, enable limiting values to be derived for the height of the eruptive column and the wind strength. The volume of each deposit has been estimated from the isopach maps, namely, 2.6 km 3 for the Pompei Pumice and 2.1 km 2 for the Avellino Pumice.