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At least 307 records · Page 17
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NASA NTRS1992

The Tharsis Montes, Mars - Comparison of volcanic and modified landforms

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.

Zimbelman, James R.↗
NASA NTRS1992

Simulated meteorite impacts and volcanic explosions: Ejecta analyses and planetary implications

Past cratering studies have focused primarily on crater morphology. However, important questions remain about the nature of crater deposits. Phenomena that need to be studied include the distribution of shock effects in crater deposits and crater walls; the origin of mono- and polymict breccia; differences between local and distal ejecta; deformation induced by explosive volcanism; and the production of unshocked, high-speed ejecta that could form the lunar and martian meteorites found on the Earth. To study these phenomena, one must characterize ejecta and crater wall materials from impacts produced under controlled conditions. New efforts at LLNL simulate impacts and volcanism and study resultant deformation. All experiments use the two-stage light-gas gun facility at LLNL to accelerate projectiles to velocities of 0.2 to 4.3 km/s, including shock pressures of 0.9 to 50 GPa. We use granite targets and novel experimental geometries to unravel cratering processes in crystalline rocks. We have thus far conducted three types of simulations: soft recovery of ejecta, 'frozen crater' experiments, and an 'artificial volcano. Our ejecta recovery experiments produced a useful separation of impactites. Material originally below the projectile remained trapped there, embedded in the soft metal of the flyer plate. In contrast, material directly adjacent to the projectile was jetted away from the impact, producing an ejecta cone that was trapped in the foam recovery fixture. We find that a significant component of crater ejecta shows no signs of strong shock; this material comes from the near-surface 'interference zone' surrounding the impact site. This phenomenon explains the existence of unshocked meteorites on the Earth of lunar and martian origin. Impact of a large bolide on neighboring planets will produce high-speed, weakly shocked ejecta, which may be trapped by the Earth's gravitational field. 'Frozen crater' experiments show that the interference zone is highly localized; indeed, disaggregation does not extend beyond approx. 1.5 crater radii. A cone-shaped region extending downward from the impact site is completely disaggregated, including powdered rock that escaped into the projectile tube. Petrographic analysis of crater ejecta and wall material will be presented. Finally, study of ejecta from 0.9- and 1.3-GPa simulations of volcanic explosions reveal a complete lack of shock metamorphism. The ejecta shows no evidence of PDF's, amorphization, high-pressure phases, or mosaicism. Instead, all deformation was brittle, with fractures irregular (not planar) and most intergranular. The extent of fracturing was remarkable, with the entire sample reduced to fragments of gravel size and smaller.

Gratz, A. J.↗
NASA NTRS1992

Debris avalanches and slumps on the margins of volcanic domes on Venus: Characteristics of deposits

Modified volcanic domes, referred to as collapsed margin domes, have diameters greater than those of terrestrial domes and were therefore thought to have no suitable terrestrial analogue. Comparison of the collapsed debris using the Magellan SAR images with volcanic debris avalanches on Earth has revealed morphological similarities. Some volcanic features identified on the seafloor from sonar images have diameters similar to those on Venus and also display scalloped margins, indicating modification by collapse. Examination of the SAR images of collapsed dome features reveals a number of distinct morphologies to the collapsed masses. Ten examples of collapsed margin domes displaying a range of differing morphologies and collapsed masses have been selected and examined.

Bulmer, M. H.↗
NASA NTRS1992

Mare volcanism in the Taurus-Littrow region

The products of mare volcanism at Taurus-Littrow occur in the form of crystalline basalts and volcanic glass beads. Both categories of samples define a compositionally diverse, but petrogenetically unrelated, suite of magmas derived by partial melting of a heterogenous, differentiated mantle beneath the region of the Apollo 17 landing site. This is a brief review of what is known and what is not known about mare volcanism at this location on the Moon.

Delano, J. W.↗
NASA NTRS1993

Stratospheric chlorine injection by volcanic eruptions - HCl scavenging and implications for ozone

Because the output of volatile chlorine during a major volcanic event can greatly exceed the annual anthropogenic emissions of chlorine to the atmosphere, the fate of volcanic chlorine must be known. Although numerous observations have shown that volcanoes do not significantly contribute to the stratospheric chlorine burden, no quantitative explanation has been published. Hydrogen chloride (HCl) scavenging processes during the early phases of a volcanic eruption are discussed. A plume dynamics and thermodynamics model is used to show that HCl removal in condensed supercooled water can reduce HCl vapor concentrations by up to four orders of magnitude, preventing substantial stratospheric chlorine injection.

Tabazadeh, A.↗
NASA NTRS1993

Relation of major volcanic center concentration on Venus to global tectonic patterns

Global analysis of Magellan image data indicates that a major concentration of volcanic centers covering about 40 percent of the surface of Venus occurs between the Beta, Atla, and Themis regions. Associated with this enhanced concentration are geological characteristics commonly interpreted as rifting and mantle upwelling. Interconnected low plains in an annulus around this concentration are characterized by crustal shortening and infrequent volcanic centers that may represent sites of mantle return flow and net downwelling. Together, these observations suggest the existence of relatively simple, large-scale patterns of mantle circulation similar to those associated with concentrations of intraplate volcanism on earth.

Crumpler, L. S.↗
NASA NTRS1993

Estimation of age of Dali-Ganis rifting and associated volcanic activity, Venus

This paper deals with the estimation of age for the Dali and Ganis Chasma rift zones and their associated volcanism based on photogeologic analysis of stratigraphic relations of rift-associated features with impact craters which have associated features indicative of their age. The features are radar-dark and parabolic, and they are believed to be mantles of debris derived from fallout of the craters' ejecta. They are thought to be among the youngest features on the Venusian surface, so their 'parent' craters must also be very young, evidently among the youngest 10 percent of Venus' crater population. Dali Chasma and Ganis Chasma are a part of a system of rift zones contained within eastern Aphrodite and Atla Regio which is a significant component of Venus tectonics. The rifts of this system are fracture belts which dissect typical Venusian plains with rare islands of tessera terrain. The rift zone system consists of several segments following each other (Diane, Dali, Ganis) and forming the major rift zone line, about 10,000 km long, which has junctions with several other rift zones, including Parga Chasma Rift. The junctions are usually locations of rift-associated volcanism in the form of volcanic edifices (Maat and Ozza Montes) or plain-forming flows flooding some areas within the rift zones and the adjacent plains.

Basilevsky, A. T.↗
NASA NTRS1993

SO2 on Venus: IUE, HST and ground-based measurements, and the active volcanism connection

Magellan images have shown that the volcanic features are widespread over the surface of Venus. The question of whether there is active volcanism is important for understanding both the atmospheric and the geological processes on Venus. The thick cloud cover of Venus precludes any direct observation of active volcanoes even if they exist. The only means of monitoring the active volcanism on Venus at present seems to be remote sensing from Earth. Continuous monitoring of SO2 is important to establish the long term trend of SO2 abundance and to understand the physical mechanism responsible for the change.

Na, C. Y.↗
NASA NTRS1993

Chronology and genesis of high-Ti mare volcanism: Melting of cogenetic depleted and enriched reservoirs

An understanding of the chronology and petrogenesis of volcanic rocks is paramount to unravelling the complexities of the composition and structure of the mantle of the Moon. High-Ti mare basalts represent the best-sampled suite of lunar volcanic rocks and likely represent melts of the uppermost mantle of the Moon. Compiled Nd and Sr isotopic data for high-Ti basalts, combined with weighted average ages determiend by various techniques, yield a complex melting history for the high-Ti mantle source. Melting occurred during three distinct episodes (3.84 Ga, 3.75-3.69 Ga, and 3.56 Ga) separated by hiatuses of 100 Ma or more. The first episode included the melting of a relatively trapped liquid (KREEPy)-rich source beneath the A-11 landing site. Later melting of this source produced magmas which were relatively trapped liquid-free at the A-11 site. Synchronous melting at the A-17 landing site produced magmas with variable proportions of this evolved trapped liquid component. The final phase of volcanism occurred at the A-11 site and involved the assimilation of an evolved 'neuKREEP' component. Continued melting of a similar source, which was nearly exhausted of its ilmenite component, beneath the A-12 landing site may have lead to production of magmas parental to the A-12 ilmenite basalts.

Snyder, Gregory A.↗
NASA NTRS1993

Whole rock major element chemistry of KREEP basalt clasts in lunar breccia 15205: Implications for the petrogenesis of volcanic KREEP basalts

KREEP basalts are a major component of soils and regolith at the Apollo 15 site. Their origin is controversial: both endogenous (volcanic) and exogenous (impact melt) processes have been proposed, but it is now generally agreed that KREEP basalts are volcanic rocks derived from the nearby Apennine Bench formation. Because most pristine KREEP basalts are found only as small clasts in polymict lunar breccias, reliable chemical data are scarce. The primary aim of this study is to characterize the range in chemical composition of pristine KREEP basalt, and to use these data to decipher the petrogenesis of these unique volcanic rocks.

Vetter, Scott K.↗
NASA NTRS1993

Terrestrial volcanism in space and time

A survey is presented of current volcanic activity around the world and of dated volcanism over the past 10,000 yrs. The patterns in the data are described. The hazard presented by volcanism is briefly examined.

Simkin, Tom↗
NASA NTRS1994

UV spectral irradiance measurements in New Zealand: Effects of Pinatubo volcanic aerosol

Since late 1989, regular UV spectral irradiance measurements have been made at Lauder, New Zealand (45 deg S, 170 deg E), whenever weather permits. Here, the instrumentation and measurement strategy are outlined, and early results are discussed. Following the eruption of Mt Pinatubo in June 1991, large amounts of volcanic aerosol were injected into the stratosphere and were subsequently transported to New Zealand's latitudes in the latter half of 1991. This provides an opportunity to investigate the effects of volcanic aerosols on UV irradiances measured at this clean-air site. Although changes in global (sum of diffuse plus direct) irradiances were below the detection threshold, there were significant changes in the partitioning of radiation between the direct beam and diffuse skylight. Decreases by nearly a factor of two in the direct/diffuse ratio were observed at longer wavelengths, and at smaller solar zenith angles (sza's). The aerosol optical depth due to volcanic aerosol over Lauder in December 1991 was 0.15 plus or minus 0.02 at 450 nm, with lower values at shorter wavelengths. Although effects were relatively small in the UVB region, an implication of the changes is that the contrast between shade and direct sun is reduced, so that shaded areas received relatively more radiation in the summer of 1991/92 in New Zealand.

Mckenzie, Richard L.↗
NASA NTRS1994

The volcanic signal in Goddard Institute for Space Studies three-dimensional model simulations

Transient calculations of the Goddard Institute for Space Studies general circulation model for the climatic signal of volcanic eruptions are analyzed. By compositing the output for two different volcanoes for scenario A and five different volcanoes for scenario B, the natural variability is suppressed and the volcanic signals are extracted. Significant global mean surface air temperature cooling and precipitation reduction are found for several years following the eruptions, with larger changes in the Northern Hemisphere (NH) than in the Southern Hemisphere. The global-average temperature response lasts for more than four years, but the precipitation response disappears after three years. The largest cooling in the model occurs in the NH summer of the year after spring eruptions. Significant zonal-average temperature reductions begin in the tropics immediately after the eruptions and extend to 45 deg S - 45 deg N in the year after the eruptions. In the second year, cooling is still seen from 30 deg S to 30 deg N. Because of the low variability in this model as compared to the real world, these signals may appear more significant here than they would by attempting to isolate them using real data. The results suggest that volcanoes can enhance the drought in the Sahel. No evidence was found that stratospheric aerosols from the low-atitude volcanic eruptions can trigger El Nino-Southern Oscillation (ENSO) events in this model.

Robock, Alan↗
NASA NTRS1995

The 5-6 December 1991 FIRE IFO II jet stream cirrus case study: Possible influences of Volcanic Aerosols

In presenting an overview of the cirus clouds comprehensively studied by ground-based and airborne sensors from Coffeyville, Kansas, during the 5-6 December 1992 Project First ISCCP Region Experiment (FIRE) Intensive Fields Observation (IFO) II case study period, evidence is provided that volcanic aerosols friom the June 1991 Pinatubo eruptions may have significantly influenced the formation and maintenance of the cirrus. Following the local appearance of a spur of stratospheric volcanic debris from the subtropics, a series of jet streaks subsequently conditioned the troposphere through tropopause foldings with sulfur-based particles that became effective cloud-forming nuclei in cirrus clouds. Aerosol and ozone measurements suggest a complicated history of stratospheric-tropospheric exchanges embedded within the upper-level flow, and cirrus cloud formation was noted to occur locally at the boundaries of stratospheric aerosol-enriched layers that became humidified through diffusion, precipitation, or advective processes. Apparent cirrus cloud alterations include abnormally high ice crystal concentrations (up to approximately 600/L), complex radial ice crystal types, and relatively large haze particles in cirrus uncinus cell heads at temperatures between -40 and -50 C. Implications for volcanic-cirrus cloud climate effects and usual (nonvolcanic aerosol) jet stream cirrus cloud formation are discussed.

Sassen, Kenneth↗
NASA NTRS1993

Paleomagnetism and tectonic interpretations of the Taos Plateau volcanic field, Rio Grande rift, New Mexico

The tectonic response of the Taos Plateau volcanic field in the southern San Luis basin to late stage extensional environment of the Rio Grande rift was investigate using paleomagnetic techniques. Sixty-two sites (533 samples) of Pliocene volcanic units were collected covering four major rock types with ages of 4.7 to 1.8 Ma. Twenty-two of these sites were from stratigraphic sections of the lower, middle and upper Servilleta Basalt collected in the Rio Grande gorge at two locations 19 km apart. Flows from the lower and middle members in the southern gorge record reversed polarities, while those in Garapata Canyon are normal with an excursion event in the middle of the sequence. The uppermost flows of the upper member at both sites display normal directions. Although these sections correlate chemically, they seem to represent different magnetic time periods during the Gilbert Reversed-Polarity Chron. The data suggest the Taos Plateau volcanic field, showing no rotation and some flattening in the south and east, has acted as a stable buttress and has been downwarped by overriding of the southeastern end of the plateau by the Picuris Mountains, which make up the northern corner of the counter-clockwise rotating Espanola block.

Brown, Laurie L.↗
NASA NTRS1993

Observations of the loss of stratospheric NO2 following volcanic eruptions

Observations of stratospheric column amounts of nitrogen dioxide (NO2), nitric oxide (NO) and nitric acid (HNO3) have been made following major eruptions of the El Chichon and Mt. Pintatubo volcanoes. Midlatitude abundances of NO2 and NO were reduced by as much as 70% in the months following the appearance of the volcanic aerosols as compared to volcanically quite periods. There are heterogeneous reactions which could occur on the volcanic aerosols to convert NO2 into HNO3 but no commensurate increase in HNO3 column amounts was observed at the times of NO2 decrease.

Coffey, M. T.↗
NASA NTRS1996

The 5-6 December 1991 FIRE IFO 2 Jet Stream Cirrus Case Study: Possible Influences of Volcanic Aerosols

In presenting an overview of the cirrus clouds comprehensively studied by ground based and airborne sensors from Coffeyville, Kansas, during the 5-6 December 1992 First ISCCP Regional Experiment (FIRE) intensive field observation (IFO) case study period, evidence is provided that volcanic aerosols from the June 1991 Pinatubo eruptions may have significantly influenced the formation and maintenance of the cirrus. Following the local appearance of a spur of stratospheric volcanic debris from the subtropics, a series of jet streaks subsequently conditioned the troposphere through tropopause foldings with sulfur based particles that became effective cloud forming nuclei in cirrus clouds. Aerosol and ozone measurements suggest a complicated history of stratospheric-tropospheric exchanges embedded with the upper level flow, and cirrus cloud formation was noted to occur locally at the boundaries of stratospheric aerosol enriched layers that became humidified through diffusion, precipitation, or advective processes. Apparent cirrus cloud alterations include abnormally high ice crystal concentrations (up to approximately 600 L(exp. 1)), complex radial ice crystal types, and relatively large haze particles in cirrus uncinus cell heads at temperatures between -40 and -50 degrees C. Implications for volcanic-cirrus cloud climate effects and unusual (nonvolcanic) aerosol jet stream cirrus cloud formation are discussed.

Sassen, Kenneth↗
NASA NTRS1997

Large Volcanic Rises on Venus

Large volcanic rises on Venus have been interpreted as hotspots, or the surface manifestation of mantle upwelling, on the basis of their broad topographic rises, abundant volcanism, and large positive gravity anomalies. Hotspots offer an important opportunity to study the behavior of the lithosphere in response to mantle forces. In addition to the four previously known hotspots, Atla, Bell, Beta, and western Eistla Regiones, five new probable hotspots, Dione, central Eistla, eastern Eistla, Imdr, and Themis, have been identified in the Magellan radar, gravity and topography data. These nine regions exhibit a wider range of volcano-tectonic characteristics than previously recognized for venusian hotspots, and have been classified as rift-dominated (Atla, Beta), coronae-dominated (central and eastern Eistla, Themis), or volcano-dominated (Bell, Dione, western Eistla, Imdr). The apparent depths of compensation for these regions ranges from 65 to 260 km. New estimates of the elastic thickness, using the 90 deg and order spherical harmonic field, are 15-40 km at Bell Regio, and 25 km at western Eistla Regio. Phillips et al. find a value of 30 km at Atla Regio. Numerous models of lithospheric and mantle behavior have been proposed to interpret the gravity and topography signature of the hotspots, with most studies focusing on Atla or Beta Regiones. Convective models with Earth-like parameters result in estimates of the thickness of the thermal lithosphere of approximately 100 km. Models of stagnant lid convection or thermal thinning infer the thickness of the thermal lithosphere to be 300 km or more. Without additional constraints, any of the model fits are equally valid. The thinner thermal lithosphere estimates are most consistent with the volcanic and tectonic characteristics of the hotspots. Estimates of the thermal gradient based on estimates of the elastic thickness also support a relatively thin lithosphere (Phillips et al.). The advantage of larger estimates of the thermal lithospheric thickness is that they provide an explanation for the apparently modest levels of geologic activity on Venus over the last half billion years.

Smrekar, Suzanne E.↗