David A. Crown

Volcanic geology of Hadriaca Patera and the eastern Hellas region of Mars

Hadriaca Patera is a low-relief volcano in the southern highlands of Mars northeast of the Hellas basin. Layered, friable deposits composing the extensive channeled flanks of the volcano surround a well-defined, summit caldera containing late stage eruptive products. The morphologic characteristics of the channels suggest erosion by groundwater sapping and surface runoff. The erosional morphology of the volcano, the lack of lava flow features, and the friable nature of the flank materials indicate that Hadriaca Patera consists predominantly of pyroclastic deposits. Gravity-driven flow models demonstrate that the distribution of flank materials can be attributed to the emplacement of pyroclastic flows. Both magmatic and hydromagmatic eruption models are viable: For the magmatic case, the necessary mass eruption rates (107–108 kg/s), ejection velocities (≥ ∼400 m/s), and volatile contents (∼1.5–3.0 wt % H2O) are consistent with parameters derived for terrestrial Plinian eruptions; for the hydromagmatic case, the required energy conversion efficiencies are comparable to those of laboratory experiments, and the inferred permeability of the Martian crust allows large amounts of groundwater to be transported rapidly (at flow rates of 103–104 m3/s) into the region. Models of cooling during emplacement indicate that welding of pyroclastic flows can occur at large distances (hundreds of kilometers) from a source vent on Mars; the layering within Hadriaca Patera could be attributed to welding of pyroclastic flows that would control its susceptibility to erosion. Morphologic similarities between Hadriaca and Tyrrhena paterae suggest a similar volcanic history, with an early pyroclastic-dominated, shield-building phase followed by effusive eruptions at their summit calderas and on the flank of Tyrrhena Patera. The formation of the extensive ridged plains of Hesperia Planum following the formation of the highland paterae supports the interpretation of a transition from explosive to effusive volcanism in the eastern Hellas region. A progressive depletion of volatiles on Mars is consistent with the morphologic properties of highland paterae and other central vent volcanoes. A predominance of hydrovolcanic eruptions in the development of Hadriaca and Tyrrhena paterae would imply that the transition in volcanic eruption style can be attributed to a volatile depletion of the crust, whereas magmatic eruptions at the paterae would be indicative of temporal changes in Martian magmas.

Morphologic and topographic analyses of debris aprons in the eastern Hellas region, Mars

This paper presents new, detailed analyses of small-scale morphologic and topographic characteristics of martian debris aprons that support Viking-based interpretations of debris aprons as ice-rich flow features derived from local uplands. Fifty-four debris apron complexes in the eastern Hellas region of Mars were examined using Mars Global Surveyor data sets, including Mars Orbiter Camera images and Mars Orbiter Laser Altimeter topographic profiles. Consistent patterns in a suite of small-scale surface textures and geomorphic features observed throughout the population reflect a history of viscous flow and surface degradation through wind ablation and loss of contained ice. A wide variety of shapes seen in topographic profile reveal variations in distribution of contained ice and different stages of apron development and degradation. The results of this study provide new evidence consistent with multiple models of apron formation, including rock glacier, debris-covered glacier, and ice-rich landslide models. Typical eastern Hellas debris aprons formed from a series of large-scale events, emplacing debris that was enriched initially or later by ground ice, complemented by small-scale mass wasting of multiple styles and postemplacement flow of apron masses.

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.

Emplacement and composition of steep‐sided domes on Venus

Steep-sided domes on Venus have surface characteristics that can provide information on their emplacement, including relatively smooth upper surfaces, radial and polygonal fracture patterns, and pits. These characteristics indicate that domes have surface crusts which are relatively unbroken, have mobile interiors after emplacement, and preserve fractures from only late in their history in response to endogenous growth or sagging of the dome surface. We have calculated the time necessary to form a 12-cm-thick crust for basalt and rhyolite under current terrestrial and Venusian ambient conditions. A 12-cm-thick crust will form in all cases in < 10 hours. Although Venusian lava flows should develop a brittle carapace during emplacement, only late-stage brittle fractures are preserved at steep-sided domes. We favor an emplacement model where early-formed surface crusts are entrained or continually annealed as they deform to accommodate dome growth. Entrainment and annealing of fractures are not mutually exclusive processes and thus may both be at work during steep-sided dome emplacement. Our results are most consistent with basaltic compositions, as rhyolitic lavas would quickly form thick crusts which would break into large blocks that would be difficult to entrain or anneal. However, if Venus has undergone large temperature excursions in the past (producing ambient conditions of 800-1000 K [e.g., Bullock and Grinspoon, 1996, 1998]), rhyolitic lavas would be unable to form crusts at high surface temperatures and could produce domes with surface characteristics consistent with those of Venusian steep-sided domes.

Relationships between pahoehoe surface units, topography, and lava tubes at Mauna Ulu, Kilauea Volcano, Hawaii

Lava flow field development at Mauna Ulu was analyzed by characterizing pahoehoe surface units and their distribution relative to pre-Mauna Ulu topography and the main lava tube system. Four pahoehoe surface units were identified in the field and described on the basis of color, surface texture, and morphology: broad, flat sheets (unit I), networks of interconnected glassy-surfaced toes (unit II), late stage breakout lobes of viscous toes (unit III), and irregular surfaces exhibiting meter-scale roughness, which typically occur as channels (unit IV). The distribution of these units was mapped on high-resolution aerial photographs using an automated supervised classification technique; Geographical Information Systems (GIS) analyses utilized digitized ∼6 m (20 foot) contour interval topography of the pre-Mauna Ulu surface and the mapped lava tube network to assess the influence of topography and lava tubes on the emplacement of surface flows. The four pahoehoe units represent variations in emplacement conditions, on the basis of the various flow regimes (sheet, toe, and channel) and surface textures (smooth/glassy and rough) displayed. These surface units show a limited correlation to pre-Mauna Ulu topography based on their mean underlying slopes. The higher flow rates indicated by the channelized surfaces of unit IV are spatially correlated with higher (22.2°) mean underlying slopes relative to those of the sheets of unit I (14.2°) and the toe networks representing units II (15.2°) and III (15.9°). The distribution of the four units does not appear to be directly related to their proximity to the largest scale of lava tubes, suggesting two possible scenarios: the main lava tubes do not significantly affect surface unit emplacement within the study area and/or these tubes do not preferentially emplace any of the four units identified in this study.

Volcanism on the Red Planet: Mars

Of all of the planets in the solar system, Mars is the most Earth-like in its geologic characteristics. Like Earth, it has been subjected to exogenic processes, such as impact cratering and erosion by wind and water, as well as endogenic processes, including tectonic deformation of the crust and volcanism. The effects of these processes are amply demonstrated by the great variety of surface features, including impact craters, landslides, former river channels, sand dunes, and the largest volcanoes in the solar system.

Downflow width behavior of Martian and terrestrial lava flows

Examination of the downflow width behavior of 59 terrestrial lava flows at Puu Oo (Hawaii) and Glass Mountain (California) and 86 Martian flows at Alba Patera, Tyrrhena Patera, Elysium, and Olympus Mons was completed using aerial photographs, topographic maps, previously published flow maps, and Viking Orbiter images. The examined lava flows exhibit diverse width behavior, from which information about flow processes and conditions was assessed. For Puu Oo flows, no significant correlation was found between the average width of a flow and flow length or average underlying slope. A significant, but weak relationship was found between average width and average flow thickness. In analyses of the downflow width behavior of individual flows, no consistent correlations were observed between width and thickness or underlying slope. When width was analyzed as a function of distance from the source for all flows, a variety of flow width behavioral trends were recognized and quantitatively classified. The most common behavior observed on Earth and Mars involved variations of width (sometimes significant) about a mean without a significant downflow narrowing or widening trend. The distributions of width behavior trends for the Alba Patera and Puu Oo flows examined were similar, with this type of “constant” behavior dominating. In contrast, Tyrrhena Patera flows showed a tendency to widen with distance downflow, and silicic flows at Glass Mountain were more likely to narrow. Flows were also subdivided by distance from the vent, and the width behavior of each division classified. Subdivision of flows resulted in significant changes in the classification of width behavior. While width behavior in the medial regions of flows was similar to that over entire flow lengths, proximal regions show more variability (possibly due to greater fluidity of lavas near the vent) and distal regions tend to uniformly narrow (possibly due to limited supply). In certain cases, classification and subdivision analysis can be used to link flow emplacement processes to the resulting morphology. In particular, width behavior can be correlated to the presence or absence of lateral levees.

Pitted terrains on (1) Ceres and implications for shallow subsurface volatile distribution

Abstract Prior to the arrival of the Dawn spacecraft at Ceres, the dwarf planet was anticipated to be ice‐rich. Searches for morphological features related to ice have been ongoing during Dawns mission at Ceres. Here we report the identification of pitted terrains associated with fresh Cerean impact craters. The Cerean pitted terrains exhibit strong morphological similarities to pitted materials previously identified on Mars (where ice is implicated in pit development) and Vesta (where the presence of ice is debated). We employ numerical models to investigate the formation of pitted materials on Ceres and discuss the relative importance of water ice and other volatiles in pit development there. We conclude that water ice likely plays an important role in pit development on Ceres. Similar pitted terrains may be common in the asteroid belt and may be of interest to future missions motivated by both astrobiology and in situ resource utilization.

Volcanism on Mars

Mars displays a wide variety of volcanic landforms, ranging from enormous Olympus Mons, the largest volcano in the solar system, to small cones and hills. The ancient cratered highlands include many paterae (complex, possibly explosively produced craters), which are among the oldest volcanic features on the planet. Volcanic flows form vast plains in many areas of the planet, particularly in the equatorial and northern midlatitudes. Large shield volcanoes dominate the Tharsis and Elysium volcanic provinces, along with many domical or conic volcanic hills. Pyroclastics likely comprise a significant portion of some of the highland paterae, and possibly also the vast volume of the enigmatic Medusae Fossae Formation. Volcanic activity may have continued until quite recently in Martian history, particularly at plains-forming flow fields that have very few superposed impact craters. Most Martian meteorites are basaltic in composition.

Correlations between topography and intraflow width behavior in Martian and terrestrial lava flows

Local correlations between topography and width behavior within lava flows at Puu Oo, Mount Etna, Glass Mountain, Cerro Bayo, Alba Patera, Tyrrhena Patera, Elysium Mons, and Olympus Mons were investigated. For each flow, width and slope data were both referenced via downflow distance as a sequence of points; the data were then divided into collections of adjacent three-point features and two-point segments. Four discrete types of analyses were conducted: (1) Three-point analysis examined positional correlations between width and slope features, (2) two-point analysis did the same for flow segments, (3) mean slope analysis included segment slope comparisons, and (4) sudden width behavior analysis measured abruptness of width changes. The distribution of types of correlations compared to random combinations of features and segments does not suggest a significant correlation between flow widths and local underlying slopes and indicates that for these flows at least, other factors have more influence on changes in width than changes in underlying topography. Mean slopes underlying narrowing, widening, and constant flow width segments were calculated. An inverse correlation between slope and width was found only at Mount Etna, where slopes underlying narrowing segments were greater than those underlying widening in 62% of the examined flows. For the majority of flows at Mount Etna, Puu Oo, and Olympus Mons, slopes were actually greatest under constant width segments; this may imply a topographically dependent resistance to width changes. The rate of change of width was also examined. Sudden width changes are relatively common at Puu Oo, Mount Etna, Elysium Mons, and Tyrrhena Patera and relatively rare at Glass Mountain, Cerro Bayo, Olympus Mons, and Alba Patera. After correction for mapping scale, Puu Oo, Mount Etna, Olympus Mons, and Alba Patera appear to fall on the same trend; Glass Mount exhibits unusually small amounts of sudden width behavior, and Tyrrhena Patera exhibits a relatively large number of sudden width behavior occurrences.