Silicic Lava Flows Research Articles

Mapping of the Culann–Tohil region of Io from Galileo imaging data

We have used Galileo spacecraft data to produce a geomorphologic map of the Culann–Tohil region of Io's antijovian hemisphere. This region includes a newly discovered shield volcano, Tsũi Goab Tholus and a neighboring bright flow field, Tsũi Goab Fluctus, the active Culann Patera and the enigmatic Tohil Mons-Radegast Patera–Tohil Patera complex. Analysis of Voyager global color and Galileo Solid-State Imaging (SSI) high-resolution, regional (50–330 m/pixel), and global color (1.4 km/pixel) images, along with available Galileo Near-Infrared Mapping Spectrometer (NIMS) data, suggests that 16 distinct geologic units can be defined and characterized in this region, including 5 types of diffuse deposits. Tsũi Goab Fluctus is the center of a low-temperature hotspot detected by NIMS late during the Galileo mission, and could represent the best case for active effusive sulfur volcanism detected by Galileo. The Culann volcanic center has produced a range of explosive and effusive deposits, including an outer yellowish ring of enhanced sulfur dioxide (SO 2), an inner red ring of SO 2 with short-chain sulfur (S 3–S 4) contaminants, and two irregular green diffuse deposits (one in Tohil Patera) apparently produced by the interaction of dark, silicate lava flows with sulfurous contaminants ballistically-emplaced from Culann's eruption plume(s). Fresh and red-mantled dark lava flows west of the Culann vent can be contrasted with unusual red–brown flows east of the vent. These red–brown flows have a distinct color that is suggestive of a compositional difference, although whether this is due to surface alteration or distinct lava compositions cannot be determined. The main massif of Tohil Mons is covered with ridges and grooves, defining a unit of tectonically disrupted crustal materials. Tohil Mons also contains a younger unit of mottled crustal materials that were displaced by mass wasting processes. Neighboring Radegast Patera contains a NIMS hotspot and a young lava lake of dark silicate flows, whereas the southwest portion of Tohil Patera contains white flow-like units, perhaps consisting of ‘ponds’ of effusively emplaced SO 2. From 0°–15° S the hummocky bright plains unit away from volcanic centers contains scarps, grooves, pits, graben, and channel-like features, some of which have been modified by erosion. Although the most active volcanic centers appear to be found in structural lows (as indicated by mapping of scarps), DEMs derived from stereo images show that, with the exception of Tohil Mons, there is less than 1 km of relief in the Culann–Tohil region. There is no discernable correlation between centers of active volcanism and topography.

The static shape of yield strength fluids slowly emplaced on slopes

A theoretical solution is presented for the static three‐dimensional shape of a fluid having a finite yield strength after it is extruded slowly from a small source on an inclined plane. The solution is compared to the shapes obtained in laboratory experiments in which slurries of kaolin in polyethylene glycol wax were ejected, slowly and in small incremental volumes, onto a slope. This study is aimed at understanding the shape of highly silicic lava flows, such as large rhyolite flows which occur on steep mountain sides or on gentle slopes, and the role of a finite yield strength in governing the slow spreading and the final static shape. The results indicate that it is possible to obtain an estimate of the yield strength of a fluid which is emplaced under such circumstances, using only horizontal aspect ratios and the slope of the underlying ground.

Recognition, Characterization and Implications of High-Grade Silicic Ignimbrite Facies from the Paleoproterozoic Bijli Rhyolites, Dongargarh Supergroup, Central India

A controversy regarding the distinction between the highly welded lava-like ignimbrites sometimes showing strongly rheomorphic characters, and the extensive silicic lava flow has been overwhelming in the recent literature. However, a rethinking, after Walker (1983), has brought into light the concept of ‘grade’ referring to the degree and extent of welding between the pyroclasts. Various parameters and characteristics were suggested for strengthening the idea of densely welded ignimbrites, which differentiate them from lava. Here, a comprehensive study on early Proterozoic acid magmatic rocks forming lower part of the Dongargarh Supergroup, central India, has been made to suggest extensive occurrence of high-grade welded rheomorphic tuffs. The possibility of their being welded ignimbrite rather than lava flow has been explored in the light of facies analysis as well as detailed microscopic evidences. Despite having overall monolithologic look various units bear distinction on account of their nature of welding, enrichment of phenocrysts and degree of stretching. The presence of vitroclastic texture, melt inclusions and radial fracturing of phenocrysts suggests pyroclastic nature of these deposits. Based on these characters four facies — A, B, C and D from bottom to the top respectively, have been identified from field studies around Salekasa. Facies-A and B represent clast-supported/matrix-supported welded pyroclastic flow deposits. Facies-C represents extremely welded thinly laminated rheomorphic tuffs while lava-like tuffs with an autobreccia carapace is represented by facies D. A complete gradation of facies A/B to D through C exists. High to extremely high-grade nature of welding in these deposits suggests a low column-height subaerial plinian to fissure eruption of a very high temperature silicic magma in a continental setting.

Silicate Cooling Model Fits to Galileo NIMS Data of Volcanism on Io

The Near Infrared Mapping Spectrometer (NIMS) has obtained spectra of volcanoes on the surface of the jovian satellite Io. Fits to data using a silicate cooling model allow us to constrain lava eruption rates and eruption age. The thermal signatures of the hot spots are indicative of active and cooling silicate lava flows. For large, active hot spots maximum ages of flow surfaces detected by NIMS typically range from days to months, although in one case it is nearly 30 years. Mass eruption rates for the main hot spots are in the range 7 m 3 s −1 to 79 m 3 s −1, using an average flow thickness of 1 m. These mass eruption rates are orders of magnitude less than those implied for large thermal outbursts on Io: the eruptions observed during June 1996 are most likely of a different eruption style, on a much smaller scale. The observation analyzed here may be representative of the current background level of volcanic activity on the anti-jove hemisphere of Io. The global mass eruption rate from these “non-outburst” hot spots is calculated to be 43 km 3/year, equivalent to a global resurfacing rate of about 1 mm/year, about 10% of the minimum required global resurfacing volume. The total observed energy output from the 14 hot spots analyzed is 3.6×10 12 W. Normalized globally, these hot spots contribute approximately 10% of Io's radiometric heat flux.

Voluminous lava-like precursor to a major ash-flow tuff: low-column pyroclastic eruption of the Pagosa Peak Dacite, San Juan volcanic field, Colorado

The Pagosa Peak Dacite is an unusual pyroclastic deposit that immediately predated eruption of the enormous Fish Canyon Tuff (∼5000km3) from the La Garita caldera at 28Ma. The Pagosa Peak Dacite is thick (to 1km), voluminous (>200km3), and has a high aspect ratio (1:50) similar to those of silicic lava flows. It contains a high proportion (40–60%) of juvenile clasts (to 3–4m) emplaced as viscous magma that was less vesiculated than typical pumice. Accidental lithic fragments are absent above the basal 5–10% of the unit. Thick densely welded proximal deposits flowed rheomorphically due to gravitational spreading, despite the very high viscosity of the crystal-rich magma, resulting in a macroscopic appearance similar to flow-layered silicic lava. Although it is a separate depositional unit, the Pagosa Peak Dacite is indistinguishable from the overlying Fish Canyon Tuff in bulk-rock chemistry, phenocryst compositions, and 40Ar/39Ar age.The unusual characteristics of this deposit are interpreted as consequences of eruption by low-column pyroclastic fountaining and lateral transport as dense, poorly inflated pyroclastic flows. The inferred eruptive style may be in part related to synchronous disruption of the southern margin of the Fish Canyon magma chamber by block faulting. The Pagosa Peak eruptive sources are apparently buried in the southern La Garita caldera, where northerly extensions of observed syneruptive faults served as fissure vents. Cumulative vent cross-sections were large, leading to relatively low emission velocities for a given discharge rate. Many successive pyroclastic flows accumulated sufficiently rapidly to weld densely as a cooling unit up to 1000m thick and to retain heat adequately to permit rheomorphic flow. Explosive potential of the magma may have been reduced by degassing during ascent through fissure conduits, leading to fracture-dominated magma fragmentation at low vesicularity. Subsequent collapse of the 75×35km2 La Garita caldera and eruption of the Fish Canyon Tuff were probably triggered by destabilization of the chamber roof as magma was withdrawn during the Pagosa Peak eruption.

Geology of the Inyo Mountains Volcanic Complex: Implications for Jurassic paleogeography of the Sierran magmatic arc in eastern California

An ∼3.1-km-thick volcanic complex exposed in the southern Inyo Mountains, east-central California, records Jurassic subaerial depositional environments along the east flank of the Sierran arc. This complex, which we name the Inyo Mountains Volcanic Complex, is subdivided into lower, middle, and upper stratigraphic intervals. The 200–580-m-thick lower interval comprises predominantly epiclastic strata deposited on alluvial fans and adjacent river flood plains that were inclined northeast. Mafic lava flows and rare reworked tuff in this interval record the onset of Jurassic(?) volcanism in this part of the arc. The 300–700-m-thick middle interval is composed predominantly of intermediate to silicic lava flows and tuffs representing a major episode of volcanism ending at ca. 169 Ma that is contemporaneous with emplacement of numerous plutons in the region. The >2260-m-thick upper interval is composed of epiclastic strata with minor intercalations of volcanic rock. Most of this interval accumulated on low-gradient flood plains that hosted evaporative lakes and that were episodically invaded by alluvial fan complexes. Three new U-Pb age determinations constrain the lower half of the upper interval to have been deposited during the interval from ca. 169 Ma to 150 Ma. The uppermost part of the complex remains undated but probably accumulated prior to 140 Ma. The Inyo Mountains Volcanic Complex is part of a belt of volcanic complexes that are the easternmost preserved Jurassic complexes of the Sierran arc. These complexes share sufficient similarities to suggest that they represent a distinctive arc-flank depositional province significantly different from that represented by coeval volcanic complexes preserved in roof pendants farther west, closer to the magmatic axis of the arc. Similarities among arc-flank complexes include predominantly to exclusively subaerial settings, substantial (>30%) portions of epiclastic strata, and existence at times of north- to northeast-inclined paleoslopes. We infer on the basis of the varying types and amounts of volcanic rocks that whereas most complexes in the arc-flank province were rarely if ever proximal to major eruptive centers, complexes in two areas (White Mountains and eastern Mojave Desert) were at times located in or adjacent to such centers. These differences lead us to speculate that the east flank of the Jurassic arc consisted of eastward-projecting volcanic salients separated by arc recesses—typified by the Inyo Mountains area—in which epiclastic deposition was dominant.

Block size distributions on silicic lava flow surfaces: Implications for emplacement conditions

We determined block size distributions on the surfaces of Holocene silicic lava flows at the Inyo domes and the Medicine Lake volcano, and studied the development of blocks on the active Mount St. Helens and Mount Unzen lava domes to better understand the emplacement history of young viscous flows. We measured block chord lengths along perpendicular 25 m long transects within vent, jumbled, and ridged morphologic units. Vent regions generally contain the largest average block sizes and largest range of average blocks, whereas ridged areas tend to have the smallest average blocks. Observations at the active Mount St. Helens and Mount Unzen lava domes show that block size distributions reflect stress conditions during flow. High extrusion rates produce small primary blocks and lead to rapid fracturing of the flow surface, whereas low extrusion rates allow large slabs to form in the vent area and lead to less severe fragmentation. A dramatic increase in the size of blocks evident in active vent regions may indicate a significant decrease in eruption rate, and thus could signal the cessation of extrusion. However, if the extrusion rate is too high or the cooling rate too low, a rigid crust and accompanying blocks will not form on an eruptive time scale. Blocks may fracture through mechanical and thermal processes as they move downslope. Most silicic lava flows reach a steady state downslope, where the average block size at the surface remains in the 20–30 cm size range with increasing distance from the vent. Fines (blocks <12 cm) do not accumulate on the flow surface because they slip toward the flow interior through void spaces between surface blocks. We therefore expect long silicic lava flows to have blocky surfaces throughout their lengths, an important consideration for evaluation of planetary lava-flow emplacement.

Active Volcanism on Io as Seen by Galileo SSI

Active volcanism on Io has been monitored during the nominal Galileo satellite tour from mid 1996 through late 1997. The Solid State Imaging (SSI) experiment was able to observe many manifestations of this active volcanism, including (1) changes in the color and albedo of the surface, (2) active airborne plumes, and (3) glowing vents seen in eclipse.About 30 large-scale (tens of kilometers) surface changes are obvious from comparison of the SSI images to those acquired by Voyager in 1979. These include new pyroclastic deposits of several colors, bright and dark flows, and caldera-floor materials. There have also been significant surface changes on Io during the Galileo mission itself, such as a new 400-km-diameter dark pyroclastic deposit around Pillan Patera. While these surface changes are impressive, the number of large-scale changes observed in the four months between the Voyager 1 and Voyager 2 flybys in 1979 suggested that over 17 years the cumulative changes would have been much more impressive. There are two reasons why this was not actually the case. First, it appears that the most widespread plume deposits are ephemeral and seem to disappear within a few years. Second, it appears that a large fraction of the volcanic activity is confined to repeated resurfacing of dark calderas and flow fields that cover only a few percent of Io's surface.The plume monitoring has revealed 10 active plumes, comparable to the 9 plumes observed by Voyager. One of these plumes was visible only in the first orbit and three became active in the later orbits. Only the Prometheus plume has been consistently active and easy to detect. Observations of the Pele plume have been particularly intriguing since it was detected only once by SSI, despite repeated attempts, but has been detected several times by the Hubble Space Telescope at 255 nm. Pele's plume is much taller (460 km) than during Voyager 1 (300 km) and much fainter at visible wavelengths. Prometheus-type plumes (50–150 km high, long-lived, associated with high-temperature hot spots) may result from silicate lava flows or shallow intrusions interacting with near-surface SO2.A major and surprising result is that ∼30 of Io's volcanic vents glow in the dark at the short wavelengths of SSI. These are probably due to thermal emission from surfaces hotter than 700 K (with most hotter than 1000 K), well above the temperature of pure sulfur volcanism. Active silicate volcanism appears ubiquitous. There are also widespread diffuse glows seen in eclipse, related to the interaction of energetic particles with the atmosphere. These diffuse glows are closely associated with the most active volcanic vents, supporting suggestions that Io's atmopshere is dominated by volcanic outgassing.Globally, volcanic centers are rather evenly distributed. However, 14 of the 15 active plumes seen by Voyager and/or Galileo are within 30° of the equator, and there are concentrations of glows seen in eclipse at both the sub- and antijovian points. These patterns might be related to asthenospheric tidal heating or tidal stresses. Io will continue to be observed during the Galileo Europa Mission, which will climax with two close flybys of Io in late 1999.

Io's Volcanism: Thermo-Physical Models of Silicate Lava Compared with Observations of Thermal Emission

Analyses of thermal infrared outbursts from the jovian satellite Io indicate that at least some of these volcanic events are due to silicate lava. Analysis of the January 9, 1990 outburst indicates that this was an active eruption consisting of a large lava flow (with mass eruption rate of order 105m3sec−1) and a sustained area at silicate liquidus temperatures. This is interpreted as a series of fire fountains along a rift zone. A possible alternative scenario is that of an overflowing lava lake with extensive fire fountaining. The January 9, 1990 event is unique as multispectral observations with respect to time were obtained. In this paper, a model is presented for the thermal energy lost by active and cooling silicate lava flows and lakes on Io. The model thermal emission is compared with Earth-based observations and Voyager IRIS data. The model (a) provides an explanation of the thermal anomalies on Io's surface; (b) provides constraints on flow behavior and extent and infers some flow parameters; and (c) determines flow geometry and change in flow size with time, and the temperature of each part of the flow or lava lake surface as a function of its age. Models of heat output from active lava flows or inactive but recently emplaced lava flows or overturning lava lakes alone are unable to reproduce the observations. If the January 9, 1990 event is the emplacement of a lava flow, the equivalent of 27 such events per year would yield a volume of material sufficient, if uniformly distributed, to resurface all of Io at a rate of 1 cm/year.

In situ formation of welded tuff-like textures in the carapace of a voluminous silicic lava flow, Owyhee County, SW Idaho

The Badlands rhyolite, on the Owyhee Plateau of southwestern Idaho, can be demonstrated to be a large lava flow on the basis of its geometry of large and small flow lobes, its well-exposed near-vent features, and its response to pre-existing topography. However, samples of the dense upper vitrophyre of the unit reveal a range of annealed fragmental textures, including material which closely resembles the compressed, welded glass shards which are characteristic of ignimbrites. Formation of these tuff-like textures involved processes probably common to emplacement of most silicic lava flow units. Decompression upon extrusion causes inflation of pumice at the surface of the lava flow; some of this pumice is subsequently comminuted, producing loose bubble-wall shards, bits of pumice, chips of dense glass, and fragments of phenocrysts. This debris sifts down around loose blocks and into open fractures deeper in the flow, where it can be reheated, compressed, and annealed to varying degrees. The end result is a dense vitrophyre layer (beneath the true upper, non-welded carapace breccia) which can be extremely texturally heterogeneous, with areas of flow-foliated lava occurring very near lava which in many aspects looks like welded ignimbrite, complete with flattened pumices. Identical textures in other silicic units have been cited by previous workers as evidence that those units erupted as pyroclastic flows which then underwent sufficient rheomorphism to create a flow-foliated rock which otherwise appears to be lava. The textures described herein indicate that lava flows can come to mimic rheomorphic ignimbrites, at least at scales ranging from thin sections to outcrops. Voluminous silicic units with scattered fragmental textures, but with otherwise lava-like features, are probably true effusive lava flows.

Mount St. Helens and Santiaguito lava domes: The effect of short-term eruption rate on surface texture and degassing processes

In order to assess the effect of eruption rate on the surface morphology and degassing mechanisms of silicic lava flows, we studied surface characteristics and obtained water content and hydrogen isotopic values of samples from flows at the Mount St. Helens and Santiaguito lava domes. We compared the surface textures and inferred degassing processes to short-term extrusion rates and found that when domes are small and eruption rates are high, lava will not completely degas in transit to the surface, allowing additional volatile loss through surface vesiculation which results in the formation of a scoriaceous carapace. When domes exceed a critical size and/or their cooled crusts reach a critical strength, emergence of new magma is impeded, short-term eruption rates decline, and more thorough degassing can take place leading to smooth-textured flows lacking scoria development. At Mount St. Helens, this transition occurred during the dome's third year, when it grew from 31.8 to 53.2 × 106 m3. Santiaguito attained a comparable state after 2–3 years of growth, and for most of its 70-year history has produced non-vesicular lava. Degassing patterns that combine closed, open, and kinetic processes can be distinguished using isotope data obtained from samples whose positions on a flow and emplacement histories are well-constrained. Evidence for these patterns is most clearly preserved in lavas erupted during early, rapid stages of dome growth. Petrologists and volcanologists seeking to infer magma chamber conditions from the volatile contents of extruded lavas thus need to sample flows early in their emplacement while paying attention to surface texture, position relative to the vent and flow front, and time of emergence.

Volcanic Eruptions on Io: Heat Flow, Resurfacing, and Lava Composition

We model an infrared outburst on Io as being due to a large, erupting lava flow which increased its area at a rate of 1.5 × 10 5 m 2 sec -1 and cooled from 1225 to 555 K over the 2.583-hr period of observation. The inferred effusion rate of 3 × 10 5 m 3 sec -1 for this eruption is very high, but is not unprecedented on the Earth and is similar to the high eruption rates suggested for early lunar volcanism. Eruptions occur ∼6% of the time on Io. These eruptions provide ample resurfacing to explain Io's lack of impact craters. We suggest that the large total radiometric heat flow, 10 14W, and the size and temperature distribution of the thermal anomalies (McEwen et al. 1992, Bull. Am. Astron. Soc. 24, 935; Veeder et al. 1994, J. Geophys. Res. 99, 17,095-17,162) can be accounted for by a series of silicate lava flows in various stages of cooling. We propose that the whole suite of Io's currently observed thermal anomalies was produced by multiple, high-eruptive-rate silicate flows within the past century.

Criteria for recognition of constructional silicic lava flow surfaces

AbstractSurface‐exposure dating (SED) methods typically rely on the measurement of a geochemical parameter that systematically changes with time. A pivotal task in the calibration of many of these techniques is to demonstrate that lava flow surfaces sampled for dating have not experienced erosion. Although criteria for identification of constructional basaltic lava flow surfaces have been published, no such criteria presently exist for the recognition of constructional silicic flows. Here we present several criteria for identifying constructional silicic lava flow features in the field. First, crease structures are fractures with easily identified, curved, striated walls that are commonly observed on recent and active silicic lava flows. Crease structures form during extrusion, and are resistant to mechanical disintegration because they expose dense material from the flow interior. Second, some crease structures break apart during formation, leaving a deposit of striated blocks on the flow surface. Crease structure blocks are striated on only one side, whereas blocks from internal columnar joints exposed through erosion are striated on two or more sides. Only the striated side of the crease structure block is definitively constructional. Finally, many silicic flow surfaces exhibit expanded or breadcrusted textures. These features consist of a dense, fractured rind, 1 –2cm thick, enclosing highly vesicular material. Breadcrust flow textures appear similar to breadcrust bombs produced during volcanic explosions, so it is imperative to demonstrate that they are part of the lava flow surface. These criteria should enable investigators to positively identify constructional silicic lava flow surfaces when calibrating an SED method.

Factors controlling the lengths of channel-fed lava flows

Factors which control lava flow length are still not fully understood. The assumption that flow length as mainly influenced by viscosity was contested by Walker (1973) who proposed that the length of a lava flow was dependent on the mean effusion rate, and by Malin (1980) who concluded that flow length was dependent on erupted volume. Our reanalysis of Malin's data shows that, if short duration and tube-fed flows are eliminated, Malin's Hawaiian flow data are consistent with Walker's assertion. However, the length of a flow can vary, for a given effusion rate, by a factor of 7, and by up to 10 for a given volume. Factors other than effusion rate and volume are therefore clearly important in controlling the lengths of lava flows. We establish the relative importance of the other factors by performing a multivariate analysis of data for recent Hawaiian lava flows. In addition to generating empirical equations relating flow length to other variables, we have developed a non-isothermal Bingham flow model. This computes the channel and levee width of a flow and hence permits the advance rates of flows and their maximum cooling-limited lengths for different gradients and effusion rates to be calculated. Changing rheological properties are taken into account using the ratio of yield strength to viscosity; available field measurements show that this varies systematically from the vent to the front of a lava flow. The model gives reasonable agreement with data from the 1983–1986 Pu'u ‘O’o eruptions and the 1984 eruption of Mauna Loa. The method has also been applied to andesitic and rhyolitic lava flows. It predicts that, while the more silicic lava flows advance at generally slower rates than basaltic flows, their maximum flow lengths, for a given effusion rate, will be greater than for basaltic lava flows.

Mapping the distribution of vesicular textures on silicic lavas using the thermal infrared multispectral scanner

The Thermal Infrared Multispectral Scanner (TIMS) is a potentially powerful tool for mapping detailed chemical variations in silicic lava flows which in turn could expand knowledge of pre‐eruption compositional gradients and mixing processes in silicic magma bodies. However, thermal infrared image data may be greatly influenced by the range of vesicular textures which occur on silicic flows. To investigate the effect of vesicularity on TIMS imagery independent of chemical variations, we studied Little Glass Mountain at the Medicine Lake Volcano of northern California, a large rhyolitic flow of uniform composition but textural heterogeneity. The imagery was recalibrated so that the digital number values for a lake in the scene matched a calculated ideal spectrum for water. TIMS spectra for the lava show useful differences in coarsely and finely vesicular pumice data, particularly in TIMS bands 3 and 4. Images generated by ratioing these bands accurately map out those areas known from field studies to be coarsely vesicular pumice. These texture‐related emissivity variations are probably due to the larger vesicles being relatively deeper and separated by smaller septa leaving less smooth glass available to give the characteristic emission of the lava. In studies of inaccessible lava flows (as on Mars) areas of coarsely vesicular pumice must be identified and avoided before chemical variations can be interpreted. Remotely determined distributions of vesicular and glassy textures can also be related to the volatile contents and potential hazards associated with the emplacement of silicic lava flows on Earth.

Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition

The ≈8-km-thick sequence of plateau lavas (North Shore Volcanic Group) in northeastern Minnesota that was erupted during the formation of the 1.1 Ga Midcontinent Rift System includes an unusually large proportion (10–25%) of felsic flows (rhyolites and icelandites). Individual flows are now recognized with estimated volumes up to 600 km 3 and lateral extents up to 40 km; some were probably originally much broader. Many are anomalously large compared to most silicic lava flows. Primary structures and textures of these essentially unmetamorphosed rocks indicate eruptive style for most of these large flows. In particular, the features of the basal meter or so and uppermost several meters show that some flows were emplaced as fluid lavas, and others as strongly rheomorphic tuffs. It may be essential in interpreting the origin of felsic flows elsewhere to examine petrographically the lowermost few decimeters and the top, where the only evidence (poorly to strongly welded shards) may exist of the pyroclastic origin of a large, otherwise lava-like flow. Interior textures of these crystalline flows provide evidence of high-temperature eruption, especially in the form of abundant paramorphs of magmatic tridymite in the groundmass, commonly arranged in a pilotaxitic flow structure. Based on structures, textures, field relations, and geochemistry, We conclude that (a) all the icelandites and some rhyolites were emplaced as large, moderately to very fluid lava flows that spread widely while crystallizing tridymite; and that (b) some large rhyolites, which are only sparsely phyric, were emplaced as pyroclastic flows subsequently remobilized as rheoignimbrites. The mode of emplacement of one of the largest aphyric flows remains enigmatic. Vent areas or characteristics are unknown. The low apparent viscosity of the erupted magmas is probably attributable to compositional factors (F content, high Fe 2+) as well as to high temperatures and eruption rates. Estimated temperatures of about 1000–1100 C are associated with the low water content and perhaps great depth of origin of the magmas at the time of massive influx of hot matic melts from the mantle during continental rifting.

Textural constraints on effusive silicic volcanism: Beyond the permeable foam model

Transitions between explosive and effusive phases of silicic volcanic eruptions have been related either to stratification of volatiles in the source magma body or to the loss of volatiles through the permeable host rock of the conduit. One way to distinguish between these two models is to map and analyze the vesicular and glassy textures found in silicic lava flows. In this paper we present textural observations and isotopic evidence from active and Recent silicic lava flows which show that at least some vesiculation occurs during surface advance of extrusions, after magma has reached the earth's surface. This view is in contrast to the widely promoted “permeable foam” model, which states that all volatiles escape during ascent of the magma, and that all dense glassy material in lava flows forms from the collapse of pumiceous lava, i.e., that silicic lavas emerge as highly inflated foam flows. Such interpretations which claim that silicic lavas are completely degassed upon extrusion, and that all degassing must take place on the time scale of the eruption, neglect several important pieces of evidence, including the presence of obsidian in extremely small domes, and of vesicular zones in the interiors of silicic flows; the copious loss of volatiles through eruption plumes between eruptive phases; and direct observations of surface vesiculation during growth of the Mount St. Helens lava dome. The permeable foam model also implies the unlikely requirement that explosive‐to‐effusive transitions be associated with an increase in eruption rate. We present a more comprehensive model for the emplacement of silicic extrusions that allows for early gas loss during ascent, as well as late‐stage vesiculation. We then discuss how the redistribution of volatiles during surface flow can increase explosive hazards from silicic lavas days, weeks, or months after the lava emerges from the vent.

Crease structures: Indicators of emplacement rates and surface stress regimes of lava flows

Research Article| May 01, 1992 Crease structures: Indicators of emplacement rates and surface stress regimes of lava flows STEVEN W. ANDERSON; STEVEN W. ANDERSON 1Geology Department, Arizona State University, Tempe, Arizona 85287 Search for other works by this author on: GSW Google Scholar JONATHAN H. FINK JONATHAN H. FINK 1Geology Department, Arizona State University, Tempe, Arizona 85287 Search for other works by this author on: GSW Google Scholar Author and Article Information STEVEN W. ANDERSON 1Geology Department, Arizona State University, Tempe, Arizona 85287 JONATHAN H. FINK 1Geology Department, Arizona State University, Tempe, Arizona 85287 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1992) 104 (5): 615–625. https://doi.org/10.1130/0016-7606(1992)104<0615:CSIOER>2.3.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 W. ANDERSON, JONATHAN H. FINK; Crease structures: Indicators of emplacement rates and surface stress regimes of lava flows. GSA Bulletin 1992;; 104 (5): 615–625. doi: https://doi.org/10.1130/0016-7606(1992)104<0615:CSIOER>2.3.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 Crease structures are features commonly found on lava flow surfaces and consist of a fracture with curved walls that extend outward from a linear valley. These crease structures are found on flows of nearly all compositions and crystallinities. We have mapped the distributions of crease structures on many flows in the western United States and found that (1) axial length is not dependent upon composition and crystallinity; (2) adjacent crease structures are generally aligned in an en echelon pattern; (3) crease structures located adjacent to flow margins are generally perpendicular to these margins; and (4) at Mount St. Helens, Washington, large lobe-bisecting crease structures are found on lobes situated on slopes of less than 20 degrees. A primary surface feature found on many crease structures is striations. Striations are sets of long stripes on the walls of the crease structure that extend approximately parallel to the axis of the central valley, and they appear to be analogous to those found on the faces of cyclically fractured basalt columns.Observations of developing crease structures on 6 of the nearly 20 Mount St. Helens dome lobes show that they form throughout the extrusion of flows situated on slopes of less than 20 degrees, but only at the very beginning and/or end of extrusion of flows on steeper slopes. These observations imply that crease structures form when the lava flow is forced to spread laterally, either as the flow advances over a flat area, or as the down-slope movement stagnates near the end of extrusion (causing the rate of spreading to exceed the downslope rate of flow). This lateral spreading of lava results in the concentration of tensile stress along a line oriented perpendicular to the direction of spreading. The cooled crust of the extrusion is therefore torn apart about this line of tensile stress concentration, forming a central valley that exposes hot, ductile material from the flow interior to the atmosphere. The presence of striations on many crease-structure walls implies that the emplacement mechanism is similar to that suggested for columnar basalts, where each striation is produced by incremental fracturing.The stress responsible for crease structure formation is due to both cooling and lateral spreading of lava. Calculations show that stress due to spreading is three orders of magnitude higher than that due to cooling. Fracture advance times were calculated using a simple one-dimensional conductive cooling model adjusted for the special geometry of the crease structure. The model closely approximates measured crease-structure emplacement times at Mount St. Helens and gives reasonable estimates for crease structures found on older silicic lava flows. We then used this model to calculate emplacement times of entire domes that consist of a single crease structure. By dividing the dome volume by the formation time, we are able to calculate extrusion rates for these older domes. These values, which include the first flow-rate estimates reported for rhyolitic lavas (0.03-106 m3/s), similar to average flow rates of 0.7-40 m3/s calculated for Mount St. Helens dacite lobes from detailed topographic maps. 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.

Magmatic inclusions in silicic and intermediate volcanic rocks

Fine‐grained ellipsoidal inclusions from a few millimeters to over l m in size are present in many intermediate to silicic lava flows and domes. Only recently has it become widely accepted that such inclusions are chilled blobs of magma. Their magmatic origin is manifested by vesicularity and high groundmass porosity, by ellipsoidal shapes, by mingling at contacts with the host, and by textural evidence for groundmass crystallization in an undercooled state. Groundmass textures reflect the degree of undercooling, which is a function of thermal contrast (ΔT) between inclusions and host magmas before mingling. Compositions of inclusions are invariably more mafic than their hosts and indicate that the magmatic systems were compositionally zoned. Many inclusions were formed from hybrid magmas. Four examples are given in order to document effects of decreasing ΔC (compositional contrast) and ΔT, as hosts for mafic andesitic to andesitic inclusions vary from high‐silica rhyolite (HSR) to silicic andesite. In the HSR example from the Coso volcanic field, California, inclusions track the evolution of zonation in a long‐lived magmatic system. At the other extreme, andesitic inclusions in silicic andesite from Crater Lake, Oregon, demonstrate zonation of a system of low ΔT and ΔC and show effects of in situ differentiation by gas filter‐pressing of residual liquid. Inclusions form when a relatively small proportion of mafic magma comes into contact with a larger proportion of silicic magma. Several mechanisms by which the magmas mix have been proposed: buoyant rise of vesiculated mafic magma or convective stirring at the interface between mafic and overlying silicic magma, forcible injection of mafic magma, diking of fractured silicic magma, and mixing during ascent in a conduit. No single mechanism explains all field observations. Many inclusions in plutons are analogous to magmatic inclusions in volcanic rocks but are likely to have suffered greater chemical and textural change during comparatively slow postentrapment cooling. Magmatic inclusions are evidence of open system behavior of the differentiated parts of magma bodies, and the inclusions themselves may behave as open systems where ΔT is low.

Cooling and deformation of sulfur flows

An investigation is made to determine whether discrimination of sulfur from silicate lava flows on Io might be possible on the basis of surface morphology resulting from the unusual rheology of liquid sulfur. The cooling of a sulfur flow by upward radiation and downward conduction is modeled in order to determine how the rheological profile evolves with time, and what surface deformation results. In contrast to silicate lavas, which become increasingly viscous on cooling, liquid sulfur has a viscosity minimum at a temperature of about 160°C. Flows extruded below 160°C develop crusts at their upper and lower surfaces like silicate lavas, whereas flows with higher eruption temperatures develop a series of alternating fluid and viscous interior zones. In both cases a surface folding instability results which can produce ridges with spacings on the order of centimeters. The density of sulfur increases upon cooling so that gravity instabilities can develop at the upper flow surface, leading to crustal overturning and resurfacing. Tensile stresses due to cooling may cause fracturing of the surface which can also promote upwelling of hot sulfur. Local turbulence destroys the vertical zonation of viscosity formed in slower moving areas. These processes may cause a sulfur flow to have surface ridges interrupted by cracks, zones of diapiric upwelling, and turbulence. None of these surface structures are large enough to be identified on Voyager images, but they will affect surface coloration, which has been the principal criterion used to distinguish sulfur from silicate flows on Io by several previous investigators.