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At least 91 records · Page 5

Lava flows on Mars - Analysis of small surface features and comparisons with terrestrial analogs

High-resolution images of south and west Arsia Mons, eastern Tharsis, and Memnonia Fossae on Mars are analyzed. Lava flows with different types of features, such as pressure ridges, tumuli, festoon ridges, and ring ridges, are observed. The effects of these small features on the type of eruption, mode of flow emplacement, and characteristics of the terrain over which the lavas flow are examined. The development of terrestrial pressure ridges, festoon, and ring structures is discussed and they are compared to the observed Martian features. The interior viscosity of the features are calculated using the Fink and Fletcher (1978) and Fink (1980a) models; the interior viscosity was estimated as 100 MPa sec in flow margins, ponded areas, and across flow lobes. The data reveal that south of Arsia Mons, flows containing pressure ridges are emplaced as multiple flow units from sporadic low-effusion rate eruptions, and the flows at the other three areas are large sheet flows that represent basaltic flood-type volcanism.

Theilig, E.↗

Gish Bar Patera, Io: Geology and Volcanic Activity, 1996-2001

Since the two Voyagers passed by Jupiter in 1979, it has been known that volcanic activity is ubiquitous on the surface of Io. With over 400 volcanic centers, Io is even more volcanically active than the earth with massive flood basalt-style eruptions and komatitite lavas a common occurrence. Additionally, some volcanoes appear to be giant lava lakes, with violent activity churning the crust of the lake for periods of 20 years or more. Finally, sulfur is believed to play a large role in Io's volcanism, be it as a primary lava or as a secondary product of large, high-temperature eruptions. By studying one volcano in particular, Gish Bar Patera, one can observe many of these characteristics in one volcanic center.

Perry, Jason↗

Depth and Differentiation of the Orientale Melt Lake

Impact melt emplacement and evolution in lunar multi-ring basins is poorly understood since impact melt deposits in basins are generally buried by mare basalt fill and obscured by subsequent impact cratering. The relatively young Orientale basin, which is only partially flooded with mare basalt, opens a rare window into basin-scale impact melts. We describe the geology of impact melt-related facies in Orientale and suggest that the central depression of Orientale may represent a solidified impact melt lake that vertically subsided shortly after basin formation due to solidification and cooling. We use Lunar Orbiter Laser Altimeter (LOLA) data to measure the depth (approx. 1.75 km) and diameter (approx 350 km) of this central depression. If all the observed subsidence of the central depression is due to solidification and cooling, the melt lake should be approx 12.5-16 km deep, far more voluminous (approx 106 km3) than the largest known differentiated igneous intrusions on Earth. We investigate the possibility that the Orientale melt lake has differentiated and model 1) the bulk composition of the melt lake, 2) the operation of melt mixing in the melt lake, and 3) the chemical evolution of the resulting liquids on the An-Fo-Qz ternary in order to predict the lithologies that might be present in the solidified Orientale melt lake. Finally, we consider the possible significance of these lithologies.

Vaughan, W. M.↗

A remote-sensing and geologic investigation of the Crueger region of the moon

Earth-based and spacecraft photography were used along with spectral reflectance and radar data to study the origin, composition, and mode of emplacement of the geologic units in and around the Crueger crater. It is shown that the composition of the mare unit within the crater is similar to that of other intermediate TiO2 basalt deposits on the lunar near side. The results suggest that the Crueger crater is a normal pre-Orientale impact crater that was modified by Orientale ejecta and later flooded by mare basalt.

Hawke, B. R.↗

Lunar basin formation and highland stratigraphy

Multiring impact basins, formed after solidification of the lunar crust, account for most or all premare regional deposits and structures expressed in the lunar landscape and for major topographic and gravity variations. A fresh basin has two or more concentric mountain rings, a lineated ejecta blanket, and secondary impact craters. Crackled material on the floor may be impact melt. The ejecta blanket was emplaced at least partly as a ground-hugging flow and was probably hot. A suggested model of basin formation is that the center lifts up and the rings form by inward collapse during evisceration. The resulting basin is shallow and has a central uplift of the mantle. This results in a central gravity high and a ring low. Later flooding by mare basalt has since modified most near side basins. Highland deposits of plains, furrowed and pitted terrain, and various hills, domes, and craters that were interpreted before the Apollo missions as being volcanic can now be interpreted as being basin related.

Howard, K. A.↗

The moon - Sources of the crustal magnetic anomalies

Evidence from low altitude Apollo 16 subsatellite magnetometer measurements of the lunar near side is presented showing that basin and crater ejecta are major sources of lunar magnetic anomalies. It is found that anomalies increase in amplitude and complexity with a decrease in altitude, indicating localized, near surface sources, but it is also found that anomalies are less intense and numerous over maria than over highlands. Also, few anomalies are found to be associated with young craters, eliminating direct shock magnetization as a possible cause, but numerous anomalies are present over formations not flooded by mare basalts. The largest observed anomaly is well correlated with the location of a conspicuous deposit, believed to be crater ejecta, with a mean magnetization level of 5.2 + or - 2.4 x 10 to the -2 electromagnetic units/g. It is also concluded that the magnetization of the lunar crust must have taken place over a period of over one billion years.

Hood, L. L.↗

A preliminary assessment of the fluid erosional processes that shaped the Martian outflow channels

Catastrophic floods may be the mechanism which created Martian outflow channels and the distinctive assemblage of channel landforms, including regional and local anastomosing patterns, expanding and contracting reaches correlated with flow constrictions, streamlined uplands, inner channels with recessional headcuts, pendant forms on the downcurrent sides of flow obstacles, and scour marks around the obstacles. A terrestrial landscape which contains analogs to all the Martian outflow features is the Channeled Scabland of eastern Washington, formed by catastrophic flooding across differentially resistant basalt, loess and sedimentary rock. Both hydrodynamic theory and the properties of the boundary undergoing deformation need to be considered in studying the forms that result from catastrophic flooding.

Baker, V. R.↗

New Insights from Legacy Seismic Data regarding Basalt Elevations and Variability on the Hanford Site

Migration of groundwater contaminants in the Gable Gap area of the Hanford Site in southeastern Washington State is strongly influenced by the distribution and permeability of basalts that lie beneath an unconfined aquifer. Locally, folding and faulting of the Columbia River Basalt associated with the Yakima fold and thrust belt followed by erosion due to the Lake Missoula floods resulted in a complex basalt surface that represents either an impermeable lower boundary to the unconfined aquifer system or localized regions of increased permeability that potentially promote communication between the unconfined aquifer system and deeper, confined aquifer systems. Paleo-channels carved into the basalt by floodwaters are thought to provide preferential flow paths for groundwater contaminants. In 2011, a seismic landstreamer campaign was carried out to image the basalt surface and produced pre-stack depth migrated p-wave reflection images. The reflection images identified two large troughs that may represent paleo-channels and several areas of possible faulting. Here, the streamer data are re-analyzed using refraction travel-time and Rayleigh wave dispersion analyses to obtain images of compressional and shear wave velocities within the suprabasalt sediment sections and the upper basalt surface. The combined interpretation of reflection and seismic velocity images shows complexity in the basalt velocity and elevation, which varies by 50 m or more within the study area. These results, along with other ongoing geophysical investigations, will be used to inform the site geologic model and potentially guide placement of future boreholes needed to quantify vertical flow between the confined and unconfined aquifers.

58 GEOSCIENCES↗

Charting the southern seas - The evolution of the lunar Mare Australe

Mare Australe has been subjected to at least four major episodes of basalt eruption ranging in age from early Imbrian to Eratosthenian. The basalts were emplaced largely in flood eruptions from at least 197 vents located on post-basin impact crater floors. The youngest basalts occur in an annulus near the outer edge of the basin. The fill thickness apparently reflects a multiring structure for the post-impact morphology of the Australe basin; the thin basaltic fill was not sufficient a load to produce tectonic rilles, but mare ridges are present and exhibit a prominent north-south alignment.

Whitford-Stark, J. L.↗

Basaltic Magmatism: The Dominant Factor in the Petrologic and Tectonic Evolution of the Earth

Silicate bodies such as the Moon, Mars, probably Mercury, and possibly Venus, appear to have evolved in three main stages: a first (felsic) differentiation, a late heavy bombardment, and a second (basaltic) differentiation. It has been proposed that the Earth underwent a similar sequence. This paper argues that the second differentiation, basaltic magmatism, has dominated the petrologic and tectonic evolution of the Earth for four billion years. A global andesitic crust, formed during and after accretion of the planet, was disrupted by major impacts that triggered mantle upwelling and sea-floor spreading about 4 billion years ago. The oceanic crust collectively has since been formed by basaltic volcanism, from spreading centers and mantle plumes. However, the continental crust has also been greatly affected. Basaltic underplating has promoted anatexis and diapiric intrusion of granitoids in granite-greenstone terrains, as well as providing heat for regional metamorphism. Basaltic intrusions, such as the Nipissing diabase of the Sudbury area, have added to the thickness of continental crust. Satellite magnetic surveys suggest that there are more such basaltic intrusions than previously realized; examples include the Bangui anomaly of central Africa and the Kentucky anomaly. Basaltic overplating from mafic dike swarms has repeatedly flooded continents; had it not been for erosion, they would be covered with basalt as Venus is today. The tectonic effects of basaltic volcanism on continents have only recently been realized. The World Stress Map project has discovered that continents are under horizontal compressive stress, caused by push from mid-ocean ridges, i.e., by basaltic volcanism. The stress fields are generally uniform over large intraplate areas, and could contribute to intraplate tectonism. Seafloor spreading has demonstrably been effective for at least 200 million years, and ridge push thus a contributor to tectonic activity for that long. Collectively, the petrologic and tectonic evolution of the Earth has been dominated for about 4 billion years by the 'second differentiation,' i.e., by basaltic magmatism.

Lowman, Paul D., Jr.↗

Origin of multi-ring basin ridge systems - An upper limit to elastic deformation based on a finite-element model

A finite-element model of a 500-km diameter lunar basin flooded with 10 km of basalt was used to represent the central portion of a multi-ring basin undergoing elastic deformation as a result of basin subsidence. Gravitational forces due to the weight of the basalt were set as the initial forces in the model; elastic constants computed from seismic velocities of lunar samples provided strength specifications. In addition, the crust was constrained by permitting no displacements at a depth of 60 km. Because of the strength of the lithosphere, surface subsidence amounted to only 20 m. This magnitude of elastic deformation does not equal that found on the lunar surface, but the orientation of principal stress and the position of circumferential compressional stress are compatible with the evolution of inner-ring and radial ridge systems by gravitational-induced downdropping.

Maxwell, T. A.↗

Quantitative Morphologic Analysis of Boulder Shape and Surface Texture to Infer Environmental History: A Case Study of Rock Breakdown at the Ephrata Fan, Channeled Scabland, Washington

Boulder morphology reflects both lithology and climate and is dictated by the combined effects of erosion, transport, and weathering. At present, morphologic information at the boulder scale is underutilized as a recorder of environmental processes, partly because of the lack of a systematic quantitative parameter set for reporting and comparing data sets. We develop such a parameter set, incorporating a range of measures of boulder form and surface texture. We use standard shape metrics measured in the field and fractal and morphometric classification methods borrowed from landscape analysis and applied to laser-scanned molds. The parameter set was pilot tested on three populations of basalt boulders with distinct breakdown histories in the Channeled Scabland, Washington: (1) basalt outcrop talus; (2) flood-transported boulders recently excavated from a quarry; and (3) flood-transported boulders, extensively weathered in situ on the Ephrata Fan surface. Size and shape data were found to distinguish between flood-transported and untransported boulders. Size and edge angles (approximately 120 degrees) of flood-transported boulders suggest removal by preferential fracturing along preexisting columnar joints, and curvature data indicate rounding relative to outcrop boulders. Surface textural data show that boulders which have been exposed at the surface are significantly rougher than those buried by fan sediments. Past signatures diagnostic of flood transport still persist on surface boulders, despite ongoing overprinting by processes in the present breakdown environment through roughening and fracturing in situ. Further use of this quantitative boulder parameter set at other terrestrial and planetary sites will aid in cataloging and understanding morphologic signatures of environmental processes.

Ehlmann, Bethany L.↗

Basaltic Fissure Types on Earth: Suitable Analogs to Evaluate the Origins of Volcanic Terrains on the Moon and Mars?

Basaltic eruptive fissures of the Great Rift and surroundings on the eastern Snake River Plain of Idaho, USA, and selected volcanic features in Hawai`i, Iceland and northern Africa were surveyed for their relevancy as planetary analogs. Evaluated during field investigations and in satellite imagery for structures, physiography, and geologic setting, fissures were categorized into four broad types: (1) simple, monogenetic fissures with obvious volcanic constructs or deposits, (2) monogenetic fissures now obscured by low shields or relatively large cones, (3) polygenetic volcanic rift zones with multiple vents and deposits, and (4) compound regional fissure systems or dike swarms that comprise major rift zones or large volcanic terrains. Using this classification as an initial base, we surveyed imagery of volcanic features for likely fissure vents in two major geologic settings on the Moon: floor-fractured craters (FFCs) and mare and cryptomare provinces. Two major regions on Mars, the volcanic plains around Alba Mons and the greater Tharsis region, were also surveyed for fissure types and volcanic associations of fissure-like features. The planetary surveys suggest that the proposed classification provides a suitable analog starting point to interpret structures associated with fissure systems on the Moon and Mars. With few exceptions, our survey indicates that each of the studied terrains exhibits a dominant fissure type. Type 1 fissures, most with pyroclastic deposits, prevail in lunar FFCs and mare-like regions; whereas type 2 fissures are ubiquitous in the Tharsis region of Mars and a few exist on the Moon as low shields. Type 3 volcanic rift zones are not common on either the Moon or Mars, although they might become evident in future work on chemically evolved terrains. Type 4 fissures are inferred in mare terrains, often represented as the extensions of major linear rille networks or rimae, with possibly complex dike swarms that were buried beneath voluminous mare basalt lava flows. Likewise, numerous flood lavas on Mars are possibly associated with now-obscured or difficult to define type 4 fissure systems.

Scott S. Hughes↗

Spectral reflectance studies of the Humorum Basin region

A portion of the mare-bounding (MB) ring of Humorum Basin is composed of pure anorthosite while other parts of the ring are composed of noritic anorthosite. An episode of mare volcanism emplaced basaltic units in the region northwest of the MB ring after the Humorum impact event. Subsequently, large impacts emplaced a veneer of highlands material atop the basalt flows. Some mare material could have been mixed with this highlands debris either by local mixing by secondary craters or by vertical mixing. Spectra for most other highlands units in the region indicate a noritic anorthosite lithology. Spectra of mare basalts in Mare Humorum and nearby mare flooded craters show relatively deep absorption bands due to the presence of abundant high-Ca pyroxene. An analysis of spectra for a small number of craters in the highlands west of the outer ring of Humorum reveals the presence of high-Ca pyroxene. This suggests the possible presence of an extensive gabbroic province.

Peterson, C. A.↗

Late high-titanium basalts of the western maria - Geology of the Flamsteed region of Oceanus Procellarum

The evolution, geology, geochemistry and topology of the Flamsteed region of Oceanus Procellarum are examined, considering remote sensing data including multispectral images and spectral reflectance measurements along with crater degradation studies and radar backscatter. Seven spectrally distinct basaltic units ranging in age from 2.5 plus or minus 0.5 to 3.5 plus or minus 0.5 billion years have been identified. The earliest units of the mapped area are composed of highlands material and include partially flooded impact craters, and the oldest surface exposed mare basalts are very low Ti basalts of the Telemann formation.

Pieters, C. M.↗

Thickness of western mare basalts

An isopach map of the basalt thickness in the western mare basins is constructed from measurements of the exposed external rim height of partially buried craters. The data, although numerically sparse, is sufficiently distributed to yield gross thickness variations. The average basalt thickness in Oceanus Procellarum and adjacent regions is 400 m with local lenses in excess of 1500 m in the circular maria. The total volume of basalt in the western maria is estimated to be in the range of 1.5 x 10 to the 6th power cu km. The chief distinction between the eastern and western maria appears to be one of basalt volumes erupted to the surface. Maximum volumes of basalt are deposited west of the central highlands and flood subjacent terrain to a greater extent than on the east. The surface structures of the western maria reflect the probability of a greater degree of isostatic response to a larger surface loading by the greater accumulation of mare basalt.

Dehon, R. A.↗

The Spokane flood controversy

An enormous plexus of proglacial channels that eroded into the loess and basalt of the Columbia Plateau, eastern Washington is studied. This channeled scabland contained erosional and depositional features that were unique among fluvial phenomena. Documentation of the field relationships of the region explains the landforms as the product of a relatively brief, but enormous flood, then so-called the Spokane flood.

Baker, V. R.↗

The Spokane flood controversy and the Martian outflow channels

The controversy over Bretz's hypothesis concerning the cataclysmic Spokane flood is discussed. Attention is directed to similarities between the Channeled Scabland of Washington and some Martian land features considered to be catastrophic flood channels. Characteristics of the enormous plexus of proglacial stream channels eroded into the loess and basalt of the Columbia Plateau in eastern Washington are described. The controversiality of the suggestion that a catastrophic flood is responsible for the Martian features is considered with respect to the Spokane flood controversy.

Baker, V. R.↗