Showing papers on "Pyroclastic rock published in 1992"

A comparison of pyroclastic flow and debris avalanche mobility

Abstract: Many pyroclastic flows have runout distances longer than expected from basic frictional arguments. In these cases the ratio of the height descended H and the runout L are as small as ∼0.2, a feature which has been attributed to fluidization. However, significant fluidization by gases is now considered unlikely during emplacement of debris avalanches, where H/L and volume are inversely related. We determine whether pyroclastic flows are more mobile than debris avalanches of the same volume and if different emplacement mechanisms are needed to describe their runout. H/L and volume data were compiled for 34 nonvolcanic and 40 volcanic debris avalanche deposits, as well as 15 pyroclastic flow deposits. Systematic differences in the volume versus H/L plots were explored by statistical analysis. Results show an inverse relationship between H/L and volume for both nonvolcanic and volcanic debris avalanche deposits, as expected, and also for pyroclastic flow deposits. Regression lines for plots of pyroclastic flow and volcanic debris avalanche data are statistically indistinguishable and the non-volcanic debris avalanche line is parallel to the other two. We argue that the nonvolcanic and volcanic debris avalanche differences are due to material properties rather than emplacement mechanisms. Therefore since pyroclastic flows and volcanic debris avalanches have indistinguishable H/L versus volume, it is unlikely that pyroclastic flows have a significantly different primary emplacement mechanism. Based on the data in hand, we conclude that it is unnecessary to invoke gas fluidization to explain the mobility of the types of pyroclastic flows examined or the excess runout of large-volume debris avalanches.

The plinian eruptions of 1912 at Novarupta, Katmai National Park, Alaska.

Abstract: The three-day eruption at Novarupta in 1912 consisted of three discrete episodes. Episode I began with plinian dispersal of rhyolitic fallout (Layer A) and contemporaneous emplacement of rhyolitic ignimbrites and associated proximal veneers. The plinian column was sustained throughout most of the interval of ash flow generation, in spite of progressive increases in the proportions of dacitic and andesitic ejecta at the expense of rhyolite. Accordingly, plinian Layer B, which fell in unbroken continuity with purely rhyolitic Layer A, is zoned from >99% to ∼15% rhyolite and accumulated synchronously with emplacement of the correspondingly zoned ash flow sequence in Mageik Creek and the Valley of Ten Thousand Smokes (VTTS). Only the andesiterichest flow units that cap the flow sequence lack a widespread fallout equivalent, indicating that ignimbrite emplacement barely outlasted the plinian phase. On near-vent ridges, the passing ash flows left proximal ignimbrite veneers that share the compositional zonation of their valley-filling equivalents but exhibit evidence for turbulent deposition and recurrent scour. Episode II began after a break of a few hours and was dominated by plinian dispersal of dacitic Layers C and D, punctuated by minor proximal intraplinian flows and surges. After another break, dacitic Layers F and G resulted from a third plinian episode (III); intercalated with these proximally are thin intraplinian ignimbrites and several andesite-rich fall/flow layers. Both CD and FG were ejected from an inner vent <400 m wide (nested within that of Episode I), into which the rhyolitic lava dome (Novarupta) was still later extruded. Two finer-grained ash layers settled from composite regional dust clouds: Layer E, which accumulated during the D-F hiatus, includes a contribution from small contemporaneous ash flows; and Layer H settled after the main eruption was over. Both are distinct layers in and near the VTTS, but distally they merge with CD and FG, respectively; they are largely dacitic but include rhyolitic shards that erupted during Episode I and were kept aloft by atmospheric turbulence. Published models yield column heights of 23–26 km for A, 22–25 km for CD, and 17–23 km for FG; and peak mass eruption rates of 0.7–1x108, 0.6–2x108, and 0.2–0.4x108 kg s-1, respectively. Fallout volumes, adjusted to reflect calculated redistribution of rhyolitic glass shards, are 8.8 km3, 4.8 km3, and 3.4 km3 for Episodes I, II, and III. Microprobe analyses of glass show that as much as 0.4 km3 of rhyolitic glass shards from eruptive Episode I fell with CDE and 1.1 km3 with FGH. Most of the rhyolitic ash in the dacitic fallout layers fell far downwind (SE of the vent); near the rhyolite-dominated ignimbrite, however, nearly all of Layers E and H are dacitic, showing that the downwind rhyolitic ash is of ‘co-plinian’ rather than co-ignimbrite origin.

Volcán Quizapu, Chilean Andes

Abstract: Quizapu is a flank vent of the basalt-to-rhyodacite Holocene stratocone, Cerro Azul, and lies at the focus of a complex Quaternary volcanic field on the Andean volcanic front. The Quizapu vent originated in 1846 when 5 km3 of hornblende-dacite magma erupted effusively with little accompanying tephra. Between ∼ 1907 and 1932, phreatic and strombolian activity reamed out a deep crater, from which 4 km3 of dacite magma identical to that of 1846 fed the great plinian event of 10–11 April 1932. Although a total of >9 km3 of magma was thus released in 86 years, there is no discernible subsidence. As the pre-plinian crater was lined by massive lavas, 1932 enlargement was limited and the total plinian deposit contains only ∼ 0.4 wt % lithics. Areas of 5-cm and 1-cm isopachs for compacted 1932 fallout are about half of those estimated in the 1930's, yielding a revised ejecta volume of ∼9.5 km3. A strong inflection near the 10-cm isopach (downwind ∼110 km) on a plot of log Thickness vs Area1/2 reflects slow settling of fine plinian ash — not of coignimbrite ash, as the volume of pyroclastic flows was trivial ( 95% of the ejecta are dacitic pumice (67–68% SiO2); minor andesitic scoria and frothier rhyodacite pumice (70% SiO2) accompanied the dominant dacite. Phenocrysts (pl>hb∼opx>mt>ilm∼cpx) are similar in both abundance and composition in the 1846 (effusive) and 1932 (plinian) dacites. Despite the contrast in mode of eruption, bulk compositions are also indistinguishable. The only difference so far identified is a lower range of δ D values for 1846 hornblende, consistent with pre-eruptive degassing of the effusive batch.

Crumbling of dacite dome lava and generation of pyroclastic flows at Unzen volcano

Abstract: RECENT modelling of volcanic eruptions has shown that the efficiency of subsurface degassing of magmas determines whether magma erupts explosively or effuses quietly1,2. Slow uprise of magma is often accompanied by effective degassing, leading to the extrusion of lava flows and domes. Although lava dome extrusion is one of the less explosive modes of eruption, it is often accompanied by explosive pyroclastic activities3–5. The 1991 eruption of Unzen volcano provided an opportunity to observe at close range several types of small-scale pyroclastic flow (glowing avalanches) originating from lava domes. Most of the pyroclastic flows are of Merapi type, caused by blocks falling from a collapsing dome; others are of Pelean type originating in an explosion from the side of a dome. The lavas apparently show variable degrees of self-explosivity. We suggest that variable degrees of degassing of the magma produced a wide range of excess pore pressures in the extruded lava domes, resulting in both Merapi-type and Pelean-type pyroclastic flows from the domes.

Giant debris avalanches from the Colima Volcanic Complex, Mexico: Implications for long-runout landslides (>100 km) and hazard assessment

Abstract: At least two giant volcanic debris-avalanche deposits are associated with the Colima Volcanic Complex located in the western part of the Trans-Mexican volcanic belt. One avalanche originated from Volcan de Colima (3820 m), probably 4300 yr ago. A much larger avalanche originated from Nevado de Colima (4240 m) 18500 yr ago; it traveled more than 120 km from its source and covered an area of at least 2200 km 2 , almost twice the area of any previously described avalanche deposit. This older debris avalanche is the second largest (by volume) known, has the longest travel distance yet reported, and has one of the lowest height/length ratios (0.04). The avalanche material was probably hot during emplacement and, after initial slope failure, may have behaved similarly to a pyroclastic flow. Such large long-runout landslides from volcanic or nonvolcanic constructs present a serious geologic hazard that must be considered in risk assessment.

Submarine volcanism; eruption styles, products, and relevance to understanding the host-rock successions to volcanic-hosted massive sulfide deposits

Abstract: The increasing hydrostatic pressure of the water column with increasing water depth in subaqueous environments limits the ability of superheated volatiles to expand instantaneously against the ambient pressure. Explosive submarine eruptions are only likely in water depths less than 1 km, and generally less than 500 m, thus refuting models of highly explosive, very deep water calderas popularly proposed as the host volcanic centers for the kuroko volcanic-hosted massive sulfide (VHMS) deposits of Japan. Large volumes of blocky breccias can be produced by quench fragmentation during submarine lava eruptions, and such deposits may have been mistaken for pyroclastic deposits in many cases in the past. Pyroclastic debris may occur in the host-rock successions of some VHMS deposits and may even be very voluminous. However, facies characteristics and theoretical constraints indicate that these are usually the products of mass-flow deposition from shallow-water or basin margin volcanic centers. The maximum water depths for submarine explosive eruptions coincide approximately with the minimum pressures and water depths required to prevent boiling of mineralizing hydrothermal fluids in the stockwork before the fluids reach the sea floor. The key elements in evaluating the prospectivity of ancient volcanic successions for VHMS deposits appear to be deep-water sediments and lavas or shallow intrusions in an extensional basin setting. Pyroclastic debris, in many cases at least, appears to be an accidental, externally introduced component. There is little evidence that explosive submarine calderas are essential as host volcanic centers to VHMS deposits.

Cyclic formation of debris avalanches at Mount St Augustine volcano

Abstract: VOLCANIC debris avalanches have been seen at many volcanoes since the 1980 eruption of Mount St Helens, but typically only one or two avalanche deposits are identified at each eruptive centre, suggesting that catastrophic slope failures are rare or even unique events in the lifetime of a volcano1–4 Here we present a series of radiocarbon dates from volcanic deposits showing that the summit edifice of Mount St Augustine, a 1,220-m-high active volcano on Augustine Island in the Cook Inlet area of south-central Alaska, has repeatedly collapsed and regenerated, averaging 150–200 years per cycle, during the past 2,000 years. The unprecedented frequency of summit edifice failure was made possible by sustained lava effusion rates over 10 times greater than is typical of plate-margin volcanoes.

Distinguishing strongly rheomorphic tuffs from extensive silicic lavas

Abstract: High-temperature silicic volcanic rocks, including strongly rheomorphic tuffs and extensive silicic lavas, have recently been recognized to be abundant in the geologic record. However, their mechanisms of eruption and emplacement are still controversial, and traditional criteria used to distinguish conventional ash-flow tuffs from silicic lavas largely fail to distinguish the high-temperature versions. We suggest the following criteria, ordered in decreasing ease of identification, to distinguish strongly rheomorphic tuffs from extensive silicic lavas: (1) the character of basal deposits; (2) the nature of distal parts of flows; (3) the relationship of units to pre-existing topography; and (4) the type of source. As a result of quenching against the ground, basal deposits best preserve primary features, can be observed in single outcrops, and do not require knowing the full extent of a unit. Lavas commonly develop basal breccias composed of a variety of textural types of the flow in a finer clastic matrix; such deposits are unique to lavas. Because the chilled base of an ashflow tuff generally does not participate in secondary flow, primary pyroclastic features are best preserved there. Massive, flow-banded bases are more consistent with a lava than a pyroclastic origin. Lavas are thick to their margins and have steep, abrupt flow fronts. Ashflow tuffs thin to no more than a few meters at their distal ends, where they generally do not show any secondary flow features. Lavas are stopped by topographic barriers unless the flow is much thicker than the barrier. Ash-flow tuffs moving at even relatively slow velocities can climb over barriers much higher than the resulting deposit. Lavas dominantly erupt from fissures and maintain fairly uniform thicknesses throughout their extents. Tuffs commonly erupt from calderas where they can pond to thicknesses many times those of their outflow deposits. These criteria may also prove effective in distinguishing extensive silicic lavas from a postulated rock type termed lava-like ignimbrite. The latter have characteristics of lavas except for great areal extents, up to many tens of kilometers. These rocks have been interpreted as ash-flow tuffs that formed from low, boiling-over eruption columns, based almost entirely on their great extents and the belief that silicic lavas could not flow such distances. However, we interpret the best known examples of lava-like ignimbrites to be lavas. This interpretation should be tested through additional documentation of their characteristics and research on the boiling-over eruption mechanism and the kinds of deposits it can produce. Flow bands, flow folds, ramps, elongate vesicles, and probably upper breccias occur in both lavas and strongly rheomorphic tuffs and are therefore not diagnostic. Pumice and shards also occur in both tuffs and lavas, although they occur throughout ash-flow tuffs and generally only in marginal breccias of lavas. Dense welding, secondary flow, and intense alteration accompanying crystallization at high temperature commonly obliterate primary textures in both thick, rheomorphic tuffs and thick lavas. High-temperature silicic volcanic rocks are dominantly associated with tholeiitic flood basalts. Extensive silicic lavas could be appropriately termed flood rhyolites.

Subaqueous Explosive Eruption and Welding of Pyroclastic Deposits

Abstract: Silicic tuffs infilling an ancient submarine caldera, at Mineral King in California, show microscopic fabrics indicative of welding of glass shards and pumice at temperatures >500 degrees C. The occurrence indicates that subaqueous explosive eruption and emplacement of pyroclastic materials can occur without substantial admixture of the ambient water, which would cause chilling. Intracaldera progressive aggradation of pumice and ash from a thick, fast-moving pyroclastic flow occurred during a short-lived explosive eruption of approximately 26 cubic kilometers of magma in water >/=150 meters deep. The thickness, high velocity, and abundant fine material of the erupted gas-solids mixture prevented substantial incorporation of ambient water into the flow. Stripping of pyroclasts from upper surfaces of subaqueous pyroclastic flows in general, both above the vent and along any flow path, may be the main process giving rise to buoyant-convective subaqueous eruption columns and attendant fallout deposits.

The Ilchulbong tuff cone, Cheju Island, South Korea

Abstract: The Ilchulbong mount of Cheju Island, South Korea, is an emergent tuff cone of middle Pleistocene age formed by eruption of a vesiculating basaltic magma into shallow seawater. A sedimentological study reveals that the cone sequence can be represented by nine sedimentary facies that are grouped into four facies associations. Facies association I represents steep strata near the crater rim composed mostly of crudely and evenly bedded lapilli tuff and minor inversely graded lapilli tuff. These facies suggest fall-out from tephra finger jets and occasional grain flows, respectively. Facies association II represents flank or base-of-slope deposits composed of lenticular and hummocky beds of massive or backset-stacked deposits intercalated between crudely to thinly stratified lapilli tuffs. They suggest occasional resedimentation of tephra by debris flows and slides during the eruption. Facies association III comprises thin, gently dipping marginal strata, composed of thinly stratified lapilli tuff and tuff. This association results from pyroclastic surges and cosurge falls associated with occasional large-scale jets. Facies association IV comprises a reworked sequence of massive, inversely graded and cross-bedded (gravelly) sandstones. These facies represent post-eruptive reworking of tephra by debris and stream flows. The facies associations suggest that the Ilchulbong tuff cone grew by an alternation of vertical and lateral accumulation. The vertical buildup was accomplished by plastering of wet tephra finger jets. This resulted in oversteepening and periodic failure of the deposits, in which resedimentation contributed to the lateral growth. After the eruption ceased, the cone underwent subaerial erosion and faulting of intracrater deposits. A volcaniclastic apron accumulated with erosion of the original tuff cone; the faulting was caused by subsidence of the subvolcanic basement within the crater.

Morphology and emplacement of an unusual debris-avalanche deposit at Jocotitlán volcano, Central Mexico

Abstract: A pre-historic collapse of the northeastern flank of Jocotitlan Volcano (3950 m), located in the central part of the Trans Mexican Volcanic Belt, produced a debris-avalanche deposit characterized by surficial hummocks of exceptional size and conical shape. The avalanche covered an area of 80 km2, had an apparent coefficient of friction (H/L)_of 0.11, a maximum runout distance of 12 km, and an estimated volume of 2.8 km3. The most remarkable features of the Jocotitlan debris avalanche deposit are: the several steep (29–32°) conical proximal hummocks (up to 165 m high), large tansverse ridges (up to 205 m high and 2.7 km long) situated at the base of the volcano, and the steep 15–50 m thick terminal scarp. Proximal conical hummocks and parallel ridges that can be visually fitted back to their pre-collapse position on the mountain resulted from a sliding mode of emplacement. Steep primary slopes developed as a result of the accumulation of coarse angular clasts at the angle of repose around core clasts that are decameters in size. Distal hummocks are commonly smaller, less conical, and clustered with more diffuse outlines. Field evidence indicates that the leading distal edge of the avalanche spilled around certain topographic barriers and that the distal moving mass had a yield strength prior to stopping. In the NE sector, the avalanche was suddenly confined by topographically higher lacustrine and volcaniclastic deposits which as a result were intensely thrust-faulted, folded, and impacted by large clasts that separated from the avalanche front. Post-emplacement loading also induced normal faulting of these soft, locally water-rich sediments. The regional tectonic pattern, N-NE direction of flank failure, and the presence of a major normal fault which intersects the volcano and is parallel to the orientation of the Acambay graben located 10 km to the N suggest a genetic relationship between the extensional tectonic stress regime and triggering of catastrophic slope failure. The presence of a 3-m-thick sequence of pumice and obsidian-rich pyroclastic surge and fall tephra directly overlying the debris-avalanche deposit indicates that magma must have been present within the edifice just prior to the catastrophic flank failure. The breached crater left by the avalanche has mostly been filled by dacitic domes and lava flows. The youngest pryroclastic surge deposits on the upper flanks of the volcano have an historical C14 age of 680±80 yearsBp (Ad 1270±80). Thus Jocotitlan volcano, formerly believed to be extinct, should be considered potentially active. Because of its close proximity to Mexico-City (60 km), the most populous city in the world, reactivation could engender severe hazards.

A laboratory study of explosive volcanic eruptions

Abstract: This paper describes a series of laboratory experiments in which buoyant mixtures of methanol and ethylene glycol (MEG) are injected as a downward propagating jet into a tank of fresh water. As the MEG mixes with water, it becomes denser than the water. If the MEG mixes with sufficient water before its initial momentum is exhausted, the jet fluid may become dense and continue downward into the tank as a convecting plume. If the jet does not have sufficient initial momentum, then a collapsing fountain develops, and the material in the jet rises back toward the top of the tank and spreads laterally as a gravity current. Subsequent mixing can cause some of the material in the gravity current to become dense, separate from the current, and sink into the tank. Although the direction of gravity is reversed, these experiments simulate many of the important dynamical features of eruption columns which can develop during explosive volcanic eruptions. In the volcanic situation, a hot, dense, and dusty mixture of gas, ash, and clasts is erupted from a vent at high speed. If sufficient ambient air is entrained into the jet, then the mixture may become buoyant through heating and expansion of the air; it therefore continues rising high into the atmosphere. Otherwise, the material collapses back to the ground. These experiments allow one to investigate systematically the different styles of behavior of the erupted material as the eruption conditions change. Four different styles of behavior have been identified, with transitions from one style to the next as the initial momentum flux of the jet is decreased: (1) Plinian style convecting columns; (2) the periodic release of discrete convecting clouds originating close to the vent, from a collapsed fountain; (3) coignimbrite eruption columns centered some distance from the vent which are generated when a fraction of a pyroclastic flow becomes buoyant; and (4) pyroclastic flows in which the majority of the material remains relatively dense and therefore spreads laterally from the vent. Using a simple theoretical model of the laboratory experiments, an analytical expression describing the conditions necessary for collapse of the analogue laboratory columns has been derived. This is successfully compared with the laboratory experiments. The simple analysis predicts that column collapse may be induced by (1) increasing the density of the erupted material; (2) decreasing the maximum buoyancy the mixture can attain on mixing with ambient; (3) increasing the erupted mass flux for a given momentum flux; or (4) decreasing the initial momentum flux for a given mass flux, as may occur if the vent is eroded. These results are consistent with earlier studies (Wilson et al., 1980; Wilson and Walker, 1987; Bursik and Woods, 1991). Although the analogue experimental system is somewhat simplified, the observations of the periodic release of convecting thermals just after column collapse suggest a mechanism for the complex grading which is commonly found just below the collapse horizons in fall deposits, for example the Fogo A deposit (Walker and Croasdale, 1970) and the Vesuvius A.D. 79 deposit (Carey and Sigurdsson, 1987). In conjunction with partial column collapse, this periodicity also suggests a means by which flow deposits may become interspersed with fall deposits; this feature has been observed in a number of cases including the Taupo deposit (Wilson and Walker, 1985).

Facies architecture of mineralized submarine volcanic sequences; Cambrian Mount Read Volcanics, western Tasmania

Abstract: The Middle to Late Cambrian Mount Read Volcanics, western Tasmania, comprise compositionally and texturally diverse lavas and volcaniclastic rocks, most of which were emplaced in submarine environments below wave base. The facies architecture reflects the contrasting character and geometry of primary volcanic and volcaniclastic facies which are strongly controlled by eruption style and eraplacement processes. In the northern Mount Read Volcanics the principal volcanic facies are (1) silicic, intermediate, and mafic lavas; (2) juvenile volcaniclastic deposits, generated in association with the extrusion of lava flows, or else produced by explosive eruptions; and (3) synvolcanic intrusions involving silicic and mafic magmas emplaced into unconsolidated host sequences. Distribution and lithofacies characteristics suggest that deposits from both intrabasinal submarine and extrabasinal subaerial and/or shallow-marine volcanic centers are present. The volcanic facies are interbedded with a sedimentary facies association comprising black mudstone and/or graded bedded sandstone of mixed volcanic and Precambrian basement provenance. The predicted facies geometry of the principal volcanic facies has guided our approach to correlation in the Mount Read Volcanics. Mass-fiow-emplaced pumiceous volcaniclastic facies provide the best framework for correlation, because these are produced in large volumes, erupted infrequently, eraplaced rapidly, and are widely distributed. Distinctive units of this type that occur near Hellyer (in the Southwell Subgroup) and 40 km to the south at White Spur-Howards Road (White Spur Formation) could each be part of the same regionally extensive volcaniclastic facies association. A similar volcaniclastic facies association hosts the Hercules and Rosebery massive sulfide deposits and clearly demonstrates that such associations are prospective for this style of mineralization.

Sedimentology and event timing of a catastrophic volcaniclastic mass flow, Volcan Hudson, Southern Chile

Abstract: Eruption of Volcan Hudson, Aisen region, Southern Chile on 12 August 1971 melted up to 80% of ice within the 9-km-wide Hudson Caldera. Rapid mobilisation of abundant fluvio-glacial sediment within the adjacent Huemules valley led to the generation of a catastrophic mass flow which swept westwards along the valley, eventually entering the fjord some 40 km downstream. The mass flow, developing from one source and being unaffected by tributary input, was characterised by three events within the flood hydrograph: (1) a precessional flood possibly linked to breakage of an early formed dammed lake; (2) a main mass flow which varied in its rheological behaviour from plug-flow to hyperconcentrated flow; and (3) a recessional flood at the tail of the main flow which diluted towards a concentrated streamflow. The main debris flow displayed characteristics of pulsing, internal segregation/density stratification, significant downstream sediment assimilation, cohesive/frictional freezing with ensuing fluidisation and development of an advancing runout phase. Consideration of these sedimentary characteristics and spatial/temporal flow transformations are used to propose a schematic model of event timing within this catastrophic volcaniclastic flow.

Reconstruction of the tectonic, volcanic, and sedimentary setting of strongly deformed Zn-Cu massive sulfide deposits at Benambra, Victoria

Abstract: The Wilga and Currawong massive sulfide deposits at Benambra are hosted by a strongly deformed, Upper Silurian rhyolite-dacite-andesite-basalt-sediment succession, dominated by silicic volcanics and sedimentary rocks. This host succession developed in an ensialic back-arc or intra-arc basin of limited extensional origin on the active Gondwana continental margin. Strong deformation is related to compressional and transcurrent crustal movements during closing of the depositional basin at the end of the Silurian and to subsequent steep faulting.The massive sulfide deposits are essentially synvolcanic because (1) they have the same three generations of tectonic structures and the same intensity of deformation as their host rocks, suggesting that the ores predate the earliest compressional deformation, (2) the ores are constrained to a Late Silurian age similar to their host rocks because both the host rocks and this earliest deformation are Late Silurian in age, (3) there is a tight stratigraphic control on the location of the massive sulfides, and (4) the stratigraphy, depositional setting, and alteration are similar to many other less-deformed volcanic-associated massive sulfide districts.Deformation and alteration inhibit detailed stratigraphic correlation and modify rock textures, but do not destroy facies relationships, which allow sedimentological and volcanological interpretation. Sedimentary facies associations indicate that mineralization occurred in the basin center after fault-controlled subsidence from a mixed subaerial and subaqueous environment with active rhyolitic volcanism, through a marine shelf environment with mixed limestone-volcaniclastic sedimentation, to a moderate- to deep-water environment with mudstone and turbidite sedimentation and rhyolitic to basaltic volcanism.Juvenile elastic and nonelastic volcanic rocks are closely associated, subequal in volume, and comprise 50 to 70 percent of the volcano-sedimentary sequence. Evaluation of fragmentation and emplacement processes indicates that the juvenile volcaniclastics are mainly in situ, resedimented and intrusive hyaloclastites, rather than pyroelastics. Classification of primary contact relationships of the volcanics into passive, slightly disruptive and highly disruptive types, and the internal facies organization of the volcanic units, indicate that the volcanics form extrusive domes, extrusive tabular flows, partially emergent cryptodomes and sills, and entirely intrusive sills. Shallow sills and cryptodomes emplaced into wet subsea-floor sediments are texturally similar to, but more abundant than, lavas in the environment of mineralization. These shallow intrusions are distinguished by disruptive upper contacts including sediment-veined and sediment-matrix hyaloclastite. Their recognition is critical because the mineralized stratigraphic interval can only be correlated using volcanics and sediments emplaced on the sea floor. Based on extrusive units, the massive sulfides occur within 100 m above the last major extrusive rhyolites and within 50 m below the first major basalt lava.Integrating the tectonic, structural, sedimentological, and volcanological data shows that the massive sulfides formed in siltstone, in the proximal (near-vent) facies association of a low profile, mainly nonexplosive, moderate to deep-water submarine volcano composed of turbiditic sediments interleaved with numerous rhyolitic to basaltic sills, lavas, and associated hyaloclastites. The ores can be related in space and time to the advanced stage of extension and subsidence in a continental margin incipient rift, when relatively deep-marine conditions were attained, generation of voluminous rhyolitic crustal melts waned, and basaltic magma started to penetrate up deep extensional faults to the sea floor. The spatial association of the last rhyolite extrusions, the massive sulfides, and the first basalt extrusions, suggests that these rocks are genetically related and that all were fed via the same extensional fault system. This setting of silicic volcanic-associated massive sulfides at Benambra provides a model for the many deposits related to mainly nonexplosive volcanism and an alternative to the recently popular submarine pyroclastic caldera model.

Incorporation and redistribution of locally derived lithic fragments within a pyroclastic flow

Abstract: The lower Miocene Peach Springs Tuff exposed in the Newberry Mountains, California, was deposited within a paleovalley trending S65°W. Exposures within the paleovalley contain lithic breccia intercalated with ash-rich ignimbrite. The clast assemblage of the lithic breccias matches the rock types of the paleovalley walls, and therefore the clasts were not derived from a distant eruptive vent. Flow direction, breccia bed thickness, grain-size data, and sedimentary textures indicate that the lithic breccias were deposited from density currents within the pyroclastic flow that moved down tributaries and into the main paleovalley to be intermingled with the ash-rich pyroclastic flow. A model is proposed whereby a turbulent boundary layer at the base of the pyroclastic flow is induced by surface roughness of the substrate and incorporates loose material from the substrate to produce a high-density ground layer that decouples from the lower-density, ash-rich pyroclastic flow. After decoupling has occurred, the high-density, lithic-rich ground layer moves independently from the ash-rich pyroclastic flow. The lithic breccia horizons have many characteristics of proximal (lag) breccias, and caution must be used when inferring distance from vent in ancient ignimbrites based on the occurrence of coarse breccias.

An Appalachian isochron: A kaolinized Carboniferous air-fall volcanic-ash deposit (tonstein)

Abstract: The Fire Clay tonstein is a kaolinized, airfall volcanic ash bed that was deposited in a widespread late Carboniferous peat-forming mire. Eleven samples from Kentucky and West Virginia, spanning a distance of 200 km, and two samples from Tennessee and Virginia indicate a characteristic mineralogical signature, as compared with other Appalachian tonsteins, consisting of well-crystallized kaolinite, beta-quartz crystal paramorphs, sanidine, ilmenite, zircon, and brookite. Detrital illite and quartz are rarely present or are in very small amounts, which indicates rapid deposition in a mire. Several normal graded cycles in this tonstein suggest repeated episodes of pyroclastic activity that produced a composite ash layer. A high-silica alkalic rhyolitic source is suggested by the geochemistry of immobile elements and by electron-probe analyses of glass inclusions in volcanic quartz from the Fire Clay tonstein. The rare-earth-element plots (chondrite normalized) of the tonstein show a pronounced negative Eu anomaly and relatively high concentrations of Zr and Th, which are both indicative of a rhyolitic source. Probe analyses of the Fire Clay glass inclusions from four states indicate a chemically identical high-silica rhyolite with peraluminous affinities. 40 Ar/ 39 Ar sanidine plateau dating indicates an age of 312 ± 1 Ma for the Fire Clay tonstein, which is consistent with previous 40 Ar/ 39 Ar dates for this tonstein. This age is in agreement with a late Westphalian B age in the European Carboniferous chronostratigraphy on the basis of an age of 311 Ma for the Westphalian B/C boundary. A new isopachous map of the Fire Clay ash-fall deposit indicates an area of 37,000 km 2 and a probable source to the present-day southwest. The deposit has a minimum preserved compacted volume of 2.8 km 3 , which corresponds to an original uncompacted volume of about 20 km 3 . This preserved volume indicates an ultraplinian volcanic explosion. Pindell and Dewey (1982) proposed an Andean-type arc in this block during the late Carboniferous, prior to South American-North American plate collision. We hypothesize an associated back-arc caldera system in the Yucatan block to explain the high-silica, potassic rhyolitic ash that gave rise to the Fire Clay tonstein.

Effects of topography on facies and compositional zonation in caldera-related ignimbrites

Abstract: Large-scale fluid dynamical processes during explosive eruptions within calderas are examined numerically by solving the full set of two-phase hydrodynamic equations with a topographic barrier, representing the rim of a caldera. The effect of the caldera rim on eruption dynamics depends on the relative locations of the rim and the impact zone where tephra collapsing from the eruption column strikes the ground. The distance of the impact zone from the vent is proportional to the collapse (fountain) height of the eruption column. Three significantly different eruption patterns have been observed in the simulations: (1) If the impact zone is outside the caldera rim, relatively continuous pyroclastic flow occurs outside the caldera. (2) If the impact zone is on or near the caldera rim, an initial pyroclastic current flow out of the caldera and is followed by a lapse in outflow during which the cladera fills up with ash. (3) If the impact zone is inside the rim, all initial pyroclastic flows are contained within the caldera unless the flows have sufficiently high initial densities and velocities to carry them over the rim. In most cases, recirculation of pyroclasts into the base of the column causes fountain height to decrease dramaticallymore » with time due to the {open_quotes}choking{close_quotes} effect of the ash. This recycling of ash in turn reduces the ability of pyroclastic flows to surmount the rim. The numerical models suggest several processes that cause the formation of multiple cooling and flow units in deposits outside a caldera from a single eruption of steady discharge. Compositional gaps may occur in outflow ignimbrite due entirely to interaction of eruption and emplacement dynamics with topography; sharp compositional gradients within a magma chamber are not necessarily implied by compositional gaps in outflow units. 35 refs., 10 figs., 1 tab.« less

The Tharsis-Montes, Mars - Comparison of Volcanic and Modified Landforms

Abstract: The three Tharsis Montes shield volcanos, Arsia Mons, Pavonis Mons, and Ascraeus Mons, have broad similarities that have been recognized since the Mariner 9 reconnaissance in 1972. Upon closer examination the volcanos are seen to have significant differences that are due to individual volcanic histories. All three volcanos exhibit the following characteristics: gentle (less than 5 deg) flank slopes, entrants in the northwestern and southeastern flanks that were the source for lavas extending away from each shield, summit caldera(s), and enigmatic lobe-shaped features extending over the plains to the west of each volcano. The three volcanos display different degrees of circumferential graben and trough development in the summit regions, complexity of preserved caldera collapse events, secondary summit-region volcanic construction, and erosion on the lower western flanks due to mass wasting and the processes that formed the large lobe-shaped features. All three lobe-shaped features start at elevations of 10 to 11 km and terminate at 6 km. The complex morphology of the lobe deposits appear to involve some form of catastrophic mass movement followed by effusive and perhaps pyroclastic volcanism.

Mixing of rhyolite, trachyte and basalt magma erupted from a vertically and laterally zoned reservoir, composite flow P1, Gran Canaria

Abstract: The 14.1 Ma composite welded ignimbrite P1 (45 km3 DRE) on Gran Canaria is compositionally zoned from a felsic lower part to a basaltic top. It is composed of four component magmas mixed in vertically varying proportions: (1) Na-rhyolite (10 km3) zoned from crystal-poor to highly phyric; (2) a continuously zoned, evolved trachyte to sodic trachyandesite magma group (6 km3); (3) a minor fraction of Na-poor trachyandesite (<1 km3); and (4) nearly aphyric basalt (26 km3) zoned from 4.3 to 5.2 wt% MgO. We distinguish three sites and phases of mixing: (a) Mutual mineral inclusions show that mixing between trachytic and rhyolitic magmas occurred during early stages of their intratelluric crystallization, providing evidence for long-term residence in a common reservoir prior to eruption. This first phase of mixing was retarded by increasing viscosity of the rhyolite magma upon massive anorthoclase precipitation and accumulation. (b) All component magmas probably erupted through a ring-fissure from a common upper-crustal reservoir into which the basalt intruded during eruption. The second phase of mixing occurred during simultaneous withdrawal of magmas from the chamber and ascent through the conduit. The overall withdrawal and mixing pattern evolved in response to pre-eruptive chamber zonation and density and viscosity relationships among the magmas. Minor sectorial variations around the caldera reflect both varying configurations at the conduit entrance and unsteady discharge. (c) During each eruptive pulse, fragmentation and particulate transport in the vent and as pyroclastic flows caused additional mixing by reducing the length scale of heterogeneities. Based on considerations of magma density changes during crystallization, magma temperature constraints, and the pattern of withdrawal during eruption, we propose that eruption tapped the P1 magma chamber during a transient state of concentric zonation, which had resulted from destruction of a formerly layered zonation in order to maintain gravitational equilibrium. Our model of magma chamber zonation at the time of eruption envisages a basal high-density Na-poor trachyandesite layer that was overlain by a central mass of highly phyric rhyolite magma mantled by a sheath of vertically zoned trachyte-trachyandesite magma along the chamber walls. A conventional model of vertically stacked horizontal layers cannot account for the deduced density relationships nor for the withdrawal pattern.

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...

Stratigraphic-volcanic setting of massive sulfide deposits in the Cambrian Mount Read Volcanics, Tasmania

Abstract: The Cambrian Mount Read Volcanics of western Tasmania host the volcanic-hosted massive sulfide (VHMS) deposits at Hellyer, Que River, Rosebery, Hercules, and Mount Lyell. The volcanics were erupted along, and partly onto, the western margin of a Precambrian basement terrane now represented by the Tyennan region. The volcanic belt is asymmetrical in section, with a main eastern zone of relatively massive lava-rich sequences, accompanied by abundant intrusions and a broader western zone of volcano-sedimentary sequences rich in volcaniclastic mass-flow sandstone and breccia. The rocks have been strongly deformed, regionally metamorphosed to lower greenschist facies, and hydrothermally altered in many areas.Stratigraphic and facies relationships within the highly complex volcanic belt are critically reviewed and reassessed. The main volcanic zone is considered to comprise two interfingering lava-rich associations: the quartz-feldspar-porphyritic Eastern sequence and the largely feldspar-porphyritic Central Volcanic Complex. This lava-rich zone interfingers with the western volcano-sedimentary sequences, herein referred to as: the Yolande River sequence, the Dundas Group, and the Mount Charter Group (new term).Recent volcanologic studies indicate that the lava-rich sequences, and indeed the bulk of the preserved Mount Read Volcanics, are submarine in character, despite the presence of abundant ignimbritelike pumiceous deposits.The early volcanism was predominantly rhyolitic-dacitic in composition but was followed by a period of active andesitic-basaltic volcanism during which the massive sulfide deposits at Hellyer and Que River, and some of those at Mount Lyell, were formed. This widespread andesitic-basaltic volcanism appears to coincide with a period of active extension and rifting along the belt, focused to some extent on the Henty fault system. The rifting episode is evidenced by abundant tholeiitic dikes along the Henty fault and throughout the northern Central Volcanic Complex and by tholeiitic pillow lavas within the Henty fault wedge.The andesitic-basaltic volcanism appears to have culminated in distinctive P 2 O 5 and light rare earth element (LREE)-enriched basalts at Hellyer, and similar rocks at Lynch Creek and Howards Plains near Queenstown may be correlates. The Hellyer basalts were buried by black shale, and the final volcanic phase was marked by a return to active felsic volcanism which produced widespread crystal- and pumice-rich mass-flow deposits of the Southwell Subgroup and Tyndall Group.

Geology of the Hazelton Volcanic Belt in British Columbia: Implications for the Early to Middle Jurassic evolution of Stikinia

Abstract: The Hazelton Group is an Early to Middle Jurassic volcanic and sedimentary succession that was deposited on the Stikine Terrane (Stikinia) of the Canadian Cordillera. The group is distributed across the entire width of Stikinia, a distance of about 300 km. Prior to post-Hazelton contractional deformation, the width across which volcanism had occurred was at least 450 km. A review of chronologic and stratigraphic information corroborates an earlier suggestion that the Hazelton Group formed as a pair of coeval, partly subaerial volcanic chains separated by a subsiding, mainly sedimentary marine basin (Hazelton Trough). The trough developed in response to extension of Stikinia during the latest Triassic and/or earliest Jurassic, and remained the locus of moderate extension during deposition of the Hazelton Group. Volcanic rocks of the group are diverse, comprising subaerially and subaqueously deposited lavas and volcaniclastic rocks of mafic to felsic composition. Calc-alkaline to tholeiitic characteristics, and depletion of high-field-strength elements relative to alkali and alkaline earth elements indicate a subduction-related origin. The degree of alkalinity (medium- to high-K) is fairly consistent throughout the group. The original width across which volcanism occurred is two to five times greater than widths of modern volcanic arcs. The anomalous width cannot be accounted for by time transgression of a single volcanic arc, because the two volcanic chains were coeval. Extremely low-angle subduction of a single oceanic plate is also rejected as an explanation because alkalinity does not differ significantly between the two volcanic chains. Genesis of the Hazelton Group above a single subduction zone is therefore considered unlikely. We propose a model in which a pair of oceanic plates were subducted beneath Stikinia from opposite sides, in a manner analagous to the tectonic regime of the Philippine archipelago. Subduction angles of the two downgoing plates are postulated to be 30° to a depth of 150 km, a configuration consistent with a postulated forearc width of about 175 km on each side. Stikinia is thereby considered to have been an 800-km-wide microplate on which two island arcs developed. The Hazelton Trough was a back-arc region common to both volcanic arcs. During deposition of the Hazelton Group, Stikinia was not part of the Intermontane Superterrane, an amalgam composed of more inboard terranes. Rather, it belonged to an oceanic microplate that accreted to the inboard terranes in late Early Jurassic to early Middle Jurassic time, during which Hazelton volcanism waned and ended.