Martian Volcanoes Research Articles

Stress and Flexural Modeling of the Martian Lithospheric Response to Alba Patera

The martian volcano Alba Patera is surrounded by a system of graben which appears to be the result of an extensional stress applied to the region after the construction of the volcano. These faults provide a map of the stress field that created them. We used this information, as well as the recent Goddard spherical harmonic gravity model (GMM-1) and a spherical harmonic representation of the Mars Digital Elevation Model, to constrain models of: (1) the lithospheric flexure under the load of the volcano, (2) the gravitational anomaly due to the volcano, and (3) the magnitude of the extensional stress in the region. We found that elastic lithospheric thicknesses between 10 and 50 km resulted in statistically acceptable fits to the gravity and topography data. We also found that a 1.5–2 kilobar extensional regional stress in conjunction with the axisymmetric stress field of the volcanic load produced a stress field that is consistent with the observed trends of the graben.

Ice hazes and clouds in the Martian atmosphere as derived from the Phobos/KRFM data

During the 1989 Phobos mission, the KRFM instrument observed the equatorial region of Mars in eight spectral bands from 315 to 550 nm. Previous analysis of the data mostly concentrated on extracting the optical properties of the dust particles in the atmosphere of Mars and the properties of the surface (Moroz et al., Planet. Space Sci. 41, 569, 1993). Some features of the KRFM photometric profiles, which appear to be of atmospheric origin, have however, remained unexplained. The present work considers three cases that exhibit such features and infers the effects to be caused by various types of water ice in the atmosphere. (1) For two sessions of observations, the intensity in UV near the limb was much higher than the mean of the morning limb profiles, but decreased later to the mean level as the line of sight moved further from the limb. Explaining these variations with a model of a homogeneous constant silicate haze does not seem possible. A model including a time evolving, near surface layer of silicate partieles covered with water ice provides a better fit to the data. The inferred column density of water vapour that condensed overnight on the dust particles is about 0.5 × 10 −5 gcm −2, and the sublimation rate of these ice mantles after sunrise is of the order of 10 −15 g μm −2 s −3. The ice haze contribution to the total optical depth decreases from about 0.05 at dawn to zero within about an hour. (2) A brightness increase of another type, observed in the afternoon UV profiles of the Valles Marineris is shown to be consistent with the presence of water ice clouds. The dependence on the properties of the clouds is discussed. The size distribution of the cloud particles seems of secondary importance. Their assumed shape, however, is critical. For spherical particles, the optical depth of the cloud is deduced to be 0.05 and the water content within the cloud to be 0.54 × 10 −5 g cm −2. For particles with slightly irregular shape and rough surface both of these estimates are three times higher. (3) The photometric profiles of the Tharsis Ridge where the KRFM traces passed over the Martian volcanoes exhibit an increase in brightness in all wavelengths, but stronger in shorter wavelengths. The spectral dependence of the measured brightening is analysed to extract the properties of the clouds around Arsia Mons and Pavonis Mons. The particles of these clouds are likely to be smaller than 1 μm, and the ice column density is estimated to be in the range of 2.5 × 10 −6–4.1 × 10 −6 g cm −2.

Calderas on Mars: characteristics, structure, and associated flank deformation

Abstract Calderas and flank structures of martian volcanoes yield insight into general questions of volcano structural evolution and the underlying magma chambers in an environment where erosion is minimal. We have documented, through detailed geological mapping, the structures, associated volcanological features, and the stratigraphical relationships between the flank structures and caldera events during the building of each martian edifice. Two fundamentally different types of calderas are identified on Mars (the Olympus type and the Arsia type) that may represent end member variations in the size and depth of magma chambers. Many of the flank structures adjacent to caldera rims are consistent with the predicted effects of magma chamber inflation as well as deflation that exert significant influences in the structural development of many volcanoes. Large-scale terracing and steepening of the upper flanks of the larger martian volcanoes may originate from magma chamber inflation and radial thrusting. Thus the endogenous component of volcano growth resulting from accumulated magma chamber growth may be significant. Many of the deepest calderas are associated with evidence for voluminous eruptions elsewhere on the flanks and along through-going fissures and appear to result largely from evacuation and deflation of magma chambers without extensive precursor inflation. Draining of the magma chamber in these cases may be aided by the lateral propagation of magma in the form of shallow dykes up to several hundred kilometres in length and the associated formation of linear fissures. Nested caldera sequences, related flank pits, large-scale slumping, terracing, and sector structure are frequently arranged in linear patterns and are part of through-going eruptive lines or fissures several hundred kilometres in length that characterize several martian shield volcanoes. Fissures this long are interpreted to be dykes propagated outward from shallow magma chambers that have followed a minimum regional stress orientation. Comparison of the observed shape and orientation of caldera structures with orientation and style of flank deformation, and with the predictions from theory, indicate that regional stresses have probably been an important influence on the caldera and flank structures of martian volcanoes. The minimum regional stress orientation may be controlled largely by regional slopes associated with the Tharsis region and Elysium regions, and, in the case of Tyrrhena Patera, pre-existing radial fractures associated with the Hellas basin.

Evidences of the regolith-atmosphere water exchange on Mars from the ISM (Phobos-2) infrared spectrometer observations

The infrared imaging spectrometer (ISM) on board Phobos-2 spacecraft provided the most important and valuable data on the atmospheric water content on Mars in the post-Viking period. Some of the results of this experiment are likely to be the evidence of the existence of the regolith-atmosphere water vapour exchange. Release of the water molecules, stored in the regolith in the adsorbed phase, to the atmosphere, induced by the noon temperature rise can cause up to a factor of 3 increase in the atmospheric water amount above the areas with high adsorbing capabilities such as clay regolith. Much less, about 30%, increase is expected above the terrains, covered with basalt grains. Local time variations of atmospheric water amount, observed by ISM, are within these limits. The dependence of water vapour desorption rate on the regolith properties can result in the spatial variations of atmospheric water content above clay and basalt regions. This is assumed to be the reason of a factor of 5±2 increase in the atmospheric water mixing ratio, observed by ISM above the flanks of Martian volcanoes as compared to the surrounding terrains.

Observations of water vapour anomaly above Tharsis volcanoes on Mars in the ISM (Phobos-2) experiment

Observations of Mars with the infrared imaging spectrometer ISM on board the Phobos-2 spacecraft provided more than 40,000 spectra in the wavelength range 0.7–3.2 μm with a spatial resolution of about 20 km. These spectra bear features of both atmospheric and surface origin. Measurements in the water vapour bands near 1.9 and 2.6 μm along the flanks of huge Martian volcanoes provided the data on the H 2O spatial variations above them. The retrieved mixing ratio was about 200 ppm above the volcanoes, which was about a factor of five higher than that found above the surrounding terrains. The origin of the observed water vapour anomaly is likely to be connected to the volcanoes and could be explained by the regolith-atmosphere water vapour exchange. On the basis of a new theoretical approach to the process of diffusion of adsorbed water from the regolith to the atmosphere, a model of the observed anomaly was proposed. It implies an intensive vapour desorption during the day hours from the volcanic regolith, which was assumed to be different in composition from that of surrounding highlands.

Observations, models, and mechanisms of failure of surface rocks surrounding planetary surface loads

Geophysical models of flexural stresses in an elastic lithosphere due to an axisymmetric surface load typically predict a transition with increased distance from the center of the load of radial thrust faults to strike‐slip faults to concentric normal faults. These model predictions are in conflict with the absence of annular zones of strike‐slip faults around prominent loads such as lunar maria, Martian volcanoes, and the Martian Tharsis rise. We suggest that this paradox arises from difficulties in relating failure criteria for brittle rocks to the stress models. Indications that model stresses are inappropriate for use in fault‐type prediction include (1) tensile principal stresses larger than realistic values of rock tensile strength, and/or (2) stress differences significantly larger than those allowed by rock‐strength criteria. Predictions of surface faulting that are consistent with observations can be obtained instead by using tensile and shear failure criteria, along with calculated stress differences and trajectories, with model stress states not greatly in excess of the maximum allowed by rock fracture criteria.

State of stress, faulting, and eruption characteristics of large volcanoes on Mars

The formation of a large volcano loads the underlying lithospheric plate and can lead to lithospheric flexure and faulting. In turn, lithospheric deformation affects the stress field beneath and within the volcanic edifice and can influence magma transport. Modeling the interaction of these processes is crucial to an understanding of the history of eruption characteristics and tectonic deformation of large volcanoes. We develop models of time‐dependent stress and deformation for the Tharsis volcanoes on Mars. By means of a finite element code, we calculate stresses and displacements due to a volcano‐shaped load emplaced on an elastic plate overlying a viscoelastic mantle. Models variously incorporate growth of the volcanic load with time and a detachment between volcano and lithosphere. The models illustrate the manner in which time‐dependent stresses induced by lithospheric plate flexure beneath the volcanic load may affect eruption histories, and the derived stress fields can be related to tectonic features on and surrounding Martian volcanoes. As a result of flexure there are three regions where stresses become sufficiently large to cause failure by faulting, according to the Mohr‐Coulomb criterion: at the surface of the plate just outward of the volcano, near the base of the elastic lithosphere beneath the center of the volcano, and on the upper flanks of the volcano early in its growth history. Normal faulting is the dominant mode of failure predicted for the first region, consistent with circumferential graben observed around the Tharsis Montes and with the scarp at the base of Olympus Mons, interpreted as a large‐offset, listric normal fault. Normal faulting, mostly radially oriented, is predicted for the second region. Failure in the third region is predicted to consist of thrust faulting, circumferentially oriented on the upper and middle flanks and radially oriented on the lower flanks. In models simulating a growing volcano, this portion of the edifice is subsequently covered by later units which exhibit lower stresses and are not predicted to fail; this volume of early failure remains the most highly stressed area in the edifice. Concentric terraces, interpreted by some workers as thrust faults, on the upper flanks of Olympus Mons may correspond to the predicted circumferential thrust features, if the most recent increments of volcano growth were relatively large, or in the presence of local material property or stress field variations. For volcanoes detached from the plate, predicted failure in the edifice takes the form of radial normal faulting near the volcano base. The addition of a local extensional stress arising from the regional topographic slope yields a pattern of predicted faulting which closely matches that observed on the Tharsis Montes, including the development of radial rifts on the lower volcano flanks to the northeast and southwest and the asymmetric formation of circumferential flank graben. This stress state is also consistent with an interpretation of the aureole deposits of Olympus Mons as the result of gravity sliding along a basal detachment. Our models also suggest an explanation for the lack of strike‐slip features, predicted by previously published flexural models, around the Tharsis volcanoes. For a given load increment, the first mode of near‐surface failure for most of the area immediately outward of the load is circumferential normal faulting and graben formation. As the volcano grows and the flexural response to the increasing load proceeds, the predicted failure mode in a portion of this annular region surrounding the volcano changes to strike‐slip faulting. Because normal faulting has been predicted to have taken place earlier, however, it is likely that release of later stresses will occur by reactivation and growth of these normal faults and graben rather than by the formation of new strike‐slip faults.

Chronology, Eruption Duration, and Atmospheric Contribution of the Martian Volcano Apollinaris Patera

Geologic mapping, thermal inertia measurements, and an analysis of the color (visual wavelengths) of the martian volcano Apollinaris Patera indicate the existence of two different surface materials, comprising an early, easily eroded edifice, and a more recent, competent fan on the southern flank. A chronology of six major events that is consistent with the present morphology of the volcano has been identified. We propose that large scale explosive activity occurred during the formation of the main edifice and that the distinctive fan on the southern flank appears to have been formed by lavas of low eruptive rate similar to those that form compound pahoehoe flow fields on Earth. A basal escarpment typically 500 m in relief and morphologically similar to the one surrounding Olympus Mons was produced between the formation of the main edifice and the fan, indicating multistage eruptions over a protracted period of time. Contact relations between the volcanic units and the adjacent chaotic material indicate that formation of the chaotic material occurred over an extended period of time and may be related to the volcanic activity that formed Apollinaris Patera. Stereophotogrammetric measurements permit the volume of the volcano to be estimated at 10 5 km 3. From this volume measurement and an inferred eruption rate (1.5 × 10 -2 km 3 yr -1) we estimate the total eruption duration for the main edifice to be ∼10 7 yrs. Plausible estimates of the exsolved volatile content of the parent magma imply that greater than 10 15 kg of water vapor was released into the atmosphere as a consequence of this activity. This large amount of water vapor as well as other exsolved gases must have had a significant impact on local, and possibly global, climatic conditions.

Minor constituents in the Martian atmosphere from the ISM/Phobos experiment

Global Martian atmospheric results derived from the infrared-imaging spectrometer ISM flown aboard the Phobos 2 Soviet space-craft are presented. Over low altitude regions the expected CO mixing ratio of (8 ± 3) × 10 −4 is measured. Variations of the 2.35-μm feature are inconsistent with this value over the Great Martian Volcanoes. If the 2.35-μm band is entirely attributable to carbon monoxide, the CO mixing ratio is typically depleted by a factor of 3 over these high altitude areas. Orography should play a major role in the existence of this CO “hole.” If, however, these spectral variations at 2.35 μm are due to the surface composition, the fraction of the surface covered by the responsible mineral must smoothly decrease as the surface elevation decreases. This phenomenon implies a strong interaction between the surface and the atmosphere for the Great Martian Volcanoes. Diurnal behavior and latitudinal variations of water vapor are globally consistent with Viking measurements. During the Phobos observations, the water vapor amounts over the bright equatorial regions range around 11 pr-μm during the day. These amounts are slightly larger than those inferred from 1976 to 1979. The lack of global dust storms during 1988–1989 could explain the enhancement of H 2O in the atmosphere.

Evolution of the Olympus Mons Caldera, Mars

Synoptic images of the Martian volcano Olympus Mons are of a quality and quantity that are unique for mars and, somewhat surprisingly, are appreciably better than image data that exist for many volcanoes on Earth. Useful information about the evolution of shield volcanoes on Earth can thus be derived from the investigation of this extraterrestrial example. We have used shadow-length measurements and photoclinometrically derived profiles to supplement and refine the topographic map of the Olympus Mons caldera. As much as 2.5 km of collapse took place within the 80×65 km diameter caldera and the elevation of the caldera rim varies by almost 2.0 km (low around the oldest collapse events, high around the youngest). An eight-stage evolutionary sequence for the caldera of Olympus Mons is identified which shows that caldera subsidence was a longterm process rather than the near-instantaneous event that has been interpreted from comparable terrestrial examples. Tectonic features on the caldera floor indicate a transition from an extensional environment (graben formation) around the perimeter of the caldera to compression (ridge formation) towards the caldera center. This transition from a compressional to extensional environment is surprisingly sudden, occurs at a radial distance of ∼17 km from the caldera center, and is import because it can be used to infer that the magma chamber was relatively shallow (thought to be at a depth of <∼16 km beneath the caldera floor; Zuber and Mouginis-Mark 1990). Ample evidence is also found within the Olympus Mons caldera for solidified lava lakes more than 30 km in width, and for the localzed overturning and/or withdrawal of lava within these lakes.

Valley systems on Tyrrhena Patera, Mars: Earth-based radar measurements of slopes

Eight new topographic profiles across the Martian volcano Tyrrhena Patera have been obtained between latitudes 20.0° and 25.1°S from radar data collected by the JPL Goldstone Radar System in 1988. These profiles, which have a reproducible accuracy of better than 150 m, show the volcano to rise ∼1.5 km above the plains of Hesperia Planum to the east, and to have an average height-to-diameter ratio of ∼1:340. The maximum slopes of the flanks appear to be ∼3.0°. The slopes on the northern flanks of Tyrrhena Patera (0.2 to 3.0°) bear little correlation with the width (1.7 to 5.2 km) or depth (∼200 to 300 m) of valley systems found in that area. This suggests that erosion by gravity-driven flows was not responsible for valley formation and that other factors, such as spatial or temporal variations in the volume of ground water released by sapping, or strength differences in the materials comprising the surface units of the volcano, controlled the geometry and locations of the valleys.

The lobes of lava flows on Earth and Olympus Mons, Mars

The lobate distal margins of lava flows provide a useful source of morphological information on the rheology of the lava if the lobes are assumed to represent the arrest of free-flowing isothermal Bingham fluids on a slope. The widths of lobes are a more useful practical index than lobe thicknesses because they are about an order of magnitude larger and can be more accurately measured from aerial photographs and other remote images. Lobes do not suffer from the changes in morphology that channels undergo during the course of eruptions. A terrestrial data set of flow lobe and ancillary measurements from lavas throughout the range alkali olivine basalt to rhyolite shows some features that are predicted by the isothermal Bingham fluid model. These are correlation of width and thickness over more than two orders of magnitude and essentially no correlation of aspect ratio with slope. There is a positive correlation of lobe width with silica content of the lava. From a data set of measurements on lava flow lobes from the Martian volcano Olympus Mons the mean value of aspect ratio (0.07) was found to be significantly less than that for the terrestrial data set (0.19). Higher general levels of effusion rate on Olympus Mons are probably the factor responsible. After normalisation, lobe widths on Olympus Mons are found to be largely equivalent to those expected for terrestrial flows with andesitic/basaltic silica contents.

Martian atmosphere studies from the ISM experiment

Observations with the infrared spectrometer ISM on board the Phobos 2 spacecraft have provided spectra of Mars in the range 0.7–3.2 μm, with a spatial resolution of the order of 20 km. Spectra between 1.6 and 3.2 μm show evidence for scattering (probably by dust particles) and for minor gaseous constituents (H 2O and CO). The imaging capabilities of the ISM spectrometer allow the study of spatial variability of these atmospheric constituents. The observations of the Martian volcanoes whose vertical extension is higher than the atmospheric scale height give spectra corresponding to different altitudes of the Martian surface, which are used to study the vertical mixing ratio of H 2O and CO. In particular, CO is found to be depleted in the Tharsis region, compared with whole-disk measurements from ground-based observations with, possibly, a vertical depletion of the CO mixing ratio above 10 km. An abundance of H 2O corresponding to 4 precipitable-μm is found, with no evidence of vertical variation in the atmospheric layers sounded (0–27 km).

Topography of the Martian tropical regions with ISM

The measurement of the CO 2 column density on Mars, from the 2 μm band, in the Phobos/ISM spectra allows the determination of the altitude of the surface of Mars, with a horizontal spatial resolution of 20 km. The uncertainties involved in this method are described and compared with previous measurements of the Martian topography. Several topographic maps have been calculated, in the region of the high Martian volcanoes and Valles Marineris. Comparison with topographic maps derived from Mariner 9 and Viking measurements shows a good agreement. An average accuracy of about 400 m is obtained, which constitutes a significant improvement, compared with previous altimetry errors, of the order of 1 km. The method has a uniform accuracy over a full observation sequence (about 3000 km wide), and is complementary to the Viking photogrammetric measurements, more accurate on a 100 km scale.

The Sandy Face of Mars

t he number of sand, as Archimedes put it, has served as a time-honored metaphor for an inconceivably large quantity of almost anything. But an extreme among extremes is to consider grains of on scale of a planet, and a particularly sandy planet at that. The huge dune fields of Mars unique among extraterrestrial surfaces photographed by planetary spacecraft unlike either ice that blankets many outer-planet moons or bare, solid rock and hardened lava flows typical of Earth's moon, Mercury, Venus and even parts of Mars itself. The Martian dunes also demonstrate sculpting ability of that ruddy world's winds, a phenomenon otherwise seen only on parts of Earth. True, these dunes fail to match dramatic splendor of huge Martian volcano called Olympus Mons (the size of Arizona) or vast canyon network known as Valles Marineris (as long as United States is wide). Yet any proper list of Martian spectaculars must include planet's awesome sand Best known from photos taken in 1976 and 1977 by two Mars-orbiting Viking spacecraft, dunes sweep across planet's barren terrain in varying patterns (SN: 10/30/76, p.276; 11/20/76, p.330). But just how much do they actually hold? Nicholas Lancaster and Ronald Greeley of Arizona State University in Tempe calculate total volume of dunes in north polar region, where they seem most ubiquitous, at 1,158 cubic kilometers (km3). If shaped into a ball, this would form a sphere 13.028 kilometers in diameter (a little more than 8 miles). That may seem tiny by astronomical standards, but it would exceed volume of Deimos, one of Mars' two irregularly shaped moons, for which Thomas C. Duxbury of Jet Propulsion Laboratory in Pasadena, Calif., calculates a volume of about 1,052 km3. Phobos, Mars' larger moon, amounts to about 5,680 km3. Furthermore, Lancaster and Greeley assert in July 10 JOURNAL OF GEOPHYSICAL RESEARCH that their dune accounting should be regarded as a minimum, because they cannot be certain of depth of Martian seas. Nor does their estimate include flat stretches of separating dunes. Dunes dominate about half total area of north polar seas, most of them rippling surface in a band between 760 and 83?N latitude. Around part of that band, from 110? to 260?W longitude, lies what Lancaster and Greeley call the largest continuous area of dunes on Mars, covering about 470,000 km2 -an area size of Nebraska and Colorado combined. According to researchers, four major seas, totaling 680,000 km2-a region slightly smaller than Texas nearly encircle north polar ice cap. Arrayed like threads in a tapestry and scores of kilometers long, Mars' transverse dunes angle across direction of dominant winds, with widely varied distances separating successive ridges or crests. Individual dunes, called barchans, appear elsewhere. Winds have sculpted these crescent-shaped features at intervals ranging from less than 200 meters to as much as a full kilometer apart. Lancaster and Greeley say that if dunes of Mars resemble those of Earth as much as Viking photos suggest, then heights of many Martian dunes can be estimated by assuming an approximately Earth-like ratio between their heights and distances between them. This means that most dunes near Martian north pole rise between 10 and 25 meters, they say. On Earth, scientists note, heightto-spacing ratios of dunes shaped by wind approximate those of features on bottom of sea, shaped by deep ocean currents. This suggests that difference between density of an atmosphere and that of an ocean has little effect on dunes' overall shapes, they conclude. Most desert dune patterns on Earth are very regular, and vary spatially in a systematic fashion, Lancaster and Greeley say. Thus, there seem to be similar relationships between dune height, spacing, and equivalent sediment thickness on Earth and Mars. The origin of Martian dunes remains unknown. Viking photos show dnsLoephotos: sho barha C ~

Origin and evolution of valleys on Martian volcanoes

Morphological analyses of six Martian volcanoes, Ceraunius Tholus, Hecates Tholus, Alba Patera, Hadriaca Patera, Apollinaris Patera, and Tyrrhena Patera, indicate that fluvial processes were the dominant influence in the initiation and subsequent development of many dissecting valleys. Lava processes and possibly volcanic density flows were also important as valley‐forming processes. Fluvial valleys are especially well developed on Alba Patera, Ceraunius Tholus, and Hecates Tholus. These valleys are inset into the surrounding landscape. They formed in regions of subdued lava flow morphology, contain tributaries, and tend to widen slightly in the downstream direction. Lava channels on Alba Patera are located on the crest of lava flows and have a discontinuous, irregular surface morphology, and distributary patterns. These channels sometimes narrow toward their termini. Possible volcanic density flow channels are located on the northern flank of Ceraunius Tholus. Valleys dissecting Apollinaris Patera, Hadriaca Patera, and Tyrrhena Patera appear to have a complex evolution, probably a mixed fluvial and lava origin. They are inset into a subdued (possibly mantled) surface, lack tributaries, and either have fairly constant widths or widen slightly downvalley. Valleys surrounding the caldera of Apollinaris appear to have formed by fluvial and possibly by volcanic density flow processes, while those on the Apollinaris fan structure may have a mixed lava and fluvial origin. Valleys on Tyrrhena have broad flat floors and theater heads, which have been extensively enlarged, probably by sapping. Hadriaca valleys have a trough‐shaped form like those on the fan structure on Apollinaris but are more enlarged and have theater heads. Fluvial valleys on the Martian volcanoes generally followed a two‐stage evolutionary sequence similar to that of the Hawaiian volcano valleys, including (1) initial formation by surface runoff and (2) subsequent enlargement by groundwater sapping. Because basalt is highly permeable, valleys on Hawaii Island formed only in regions where lava flows were mantled by less permeable volcanic ash. Similarly, fluvial valleys on the Martian volcanoes, particularly those on Alba Patera, formed in regions where lava flow morphology is subdued, probably by ash mantling. Thus the presence or absence of this mantling may explain why fluvial valleys are present on some Martian volcanoes but not on others of the same age. The formation of fluvial or fluvial‐modified valleys on the older volcanoes Ceraunius, Hecates, Tyrrhena, Apollinaris, and Hadriaca implies that surface and near‐surface water activity was widespread during late Noachian to early Hesperian (period of heavy bombardment). However, the presence of anomalously young fluvial valleys on Alba Patera indicates that fluvial valley activity did not entirely cease near the end of late heavy bombardment but rather persisted in isolated regions until mid to late Amazonian.

Flank tectonics of Martian volcanoes

On the flanks of Olympus Mons is a series of terraces, concentrically distributed around the caldera. Their morphology and location suggest that they could be thrust faults caused by compressional failure of the cone. In an attempt to understand the mechanism of faulting and the possible influences of the interior structure of Olympus Mons, we have constructed a numerical model for elastic stresses within a Martian volcano. In the absence of internal pressurization, the middle slopes of the cone are subjected to compressional stress, appropriate to the formation of thrust faults. These stresses for Olympus Mons are ∼250 MPa. If a vacant magma chamber is contained within the cone, the region of maximum compressional stress is extended toward the base of the cone. If the magma chamber is pressurized, extensional stresses occur at the summit and on the upper slopes of the cone. For a filled but unpressurized magma chamber, the observed positions of the faults agree well with the calculated region of high compressional stress. Three other volcanoes on Mars, Ascraeus Mons, Arsia Mons, and Pavonis Mons, possess similar terraces. Extending our analysis to other Martian volcanoes, we find that only these three and Olympus Mons have flank stresses that exceed the compressional failure strength of basalt, lending support to the view that the terraces on all four are thrust faults.

Emplacement of lava flow fields: Application of terrestrial studies to Alba Patera, Mars

Morphological data are at present our major source of information for extraterrestrial lavas. Effusion conditions must therefore be inferred from the final shapes of flow fields, generally using terrestrial lavas as analogues and so presupposing similar emplacement regimes on Earth and other planets. Studies of terrestrial lavas suggest that the overall development of flow fields is systematic and that a general, normalized relation can be established linking the final dimensions of a flow field (specifically, average thickness and the ratio of maximum width to maximum length) to underlying slope and eruption duration, independent of explicit knowledge of discharge rate, gravitational acceleration, lava density, and rheology. This relation is applied to lavas on the Martian volcano Alba Patera, on which two distinct planimetric types of lava flow fields are identified, and eruption durations, average discharge rates, and average velocities are obtained. Imposing the constraint of a terrestrial emplacement regime, the model yields internally consistent results for subliquidus lavas and suggests that, at least for basaltic‐basaltic andesitic compositions, the essential conditions of eruption may have been similar to those currently observed on Earth.

Fault propagation folds induced by gravitational failure and slumping of the central Costa Rica Volcanic Range: Implications for large terrestrial and Martian volcanic edifices

Long sublinear ridges and related scarps located at the base of large volcanic structures are frequently interpreted as normal faults associated with extensional regional stress. In contrast, the ridges bordering the Central Costa Rica volcanic range (CCRVR) are the topographic expression of hanging wall asymmetric angular anticlines overlying low‐angle thrust faults at the base of the range. These faults formed by gravitational failure and slumping of the flanks of the range due to the weight of the volcanic edifices and were perhaps triggered by the intrusion of magma over the past 20,000 years. The thrust fault underlying Alajuela ridge rises from a depth of ∼700 m at a dip of ∼15° over a distance of a few kilometers. It ramps up at ∼600 m depth on a ∼40° slope and terminates in the axis of the forward syncline, ∼400 m below the surface. The hanging wall of this fault has been thrusted about 200 m away from the volcanic range axis. Thrusting on the lower flanks of the CCRVR is compensated by extension on the range axis with formation of summit grabens. Shallow seismicity in the volcanic range suggests that thrusting is still active. We postulate that at Kilauea volcano, Hawaii, the gravity‐ and magma‐induced slumping of the southern flank on low‐angle thrust faults generates hanging wall anticlines similar to those observed in Costa Rica. These anticlines are hypothesized to occur along the base of the volcano, where the thrust faults ramp up toward the sea bottom. Ridges and scarps between 2000 and 5000 m below sea level are interpreted as the topographic expression of these folds. We further suggest that the scarps of the CCRVR and Kilauea are valid scaled terrestrial analogs of the perimeter scarp of the Martian volcano Olympus Mons. We suggest that the crust below Olympus Mons has failed under the load of the volcano, triggering the radial slumping of the flanks of the volcano on basal thrusts. The thrusting would have, in turn, formed the anticlinal ridges and scarps that surround the edifice. The thrusting seems to be preferential toward the northwest and southeast, and a northeast‐southwest rift zone seems to have developed, perpendicular to the regional topographic gradient. The thrust faults may extend all the way to the base of the Martian crust (about 40 km), and they may have been active until almost the end of the volcanic activity. We suggest that gravitational failure and slumping of the flanks of volcanoes is a process common to most large volcanic edifices. In the CCRVR and at Kilauea this slumping of the flanks is a slow intermittent process, but it could evolve to rapid massive avalanching leading to catastrophic eruptions. Thus monitoring of uplift and displacement of the folds related to the slump tectonics could become an additional effective method for mitigating volcanic hazards.

Polygenic eruptions on Alba Patera, Mars

A combination of photogeologic mapping, analysis of Viking Orbiter thermal inertia data, and numerical modelling of eruption conditions has permitted us to construct a new model for the evolution of the martian volcano Alba Patera. Numerous digitate channel networks on the flanks of the volcano are interpreted to be carved by sapping due to the release of non-juvenile water from unconsolidated flank deposits. Using the thermal inertia measurements, we estimate the particle size of these deposits to be 3–10 µm, which, together with theoretical modelling of the disperison of explosively derived volcanic materials, leads us to conclude that the flank deposits on Alba Patera are low-relief pyroclastic flows. The recognition of numerous late-stage summit and sub-terminal lava flows thus makes Alba Patera a unique martian volcano that is transitional between the older pyroclastic-dominated highland paterae and the more recent effusive central-vent volcanoes such as the Tharsis Montes.