Volcanic Plains Research Articles

Lava heating and loading of ice sheets on early Mars: Predictions for meltwater generation, groundwater recharge, and resulting landforms

Recent modeling studies of the early Mars climate predict a predominantly cold climate, characterized by the formation of regional ice sheets across the highland areas of Mars. Formation of the predicted “icy highlands” ice sheets is coincident with a peak in the volcanic flux of Mars involving the emplacement of the Late Noachian – Early Hesperian ridged plains unit. We explore the relationship between the predicted early Mars “icy highlands” ice sheets, and the extensive early flood volcanism to gain insight into the surface conditions prevalent during the Late Noachian to Early Hesperian transition period. Using Hesperia Planum as a type area, we develop an ice sheet lava heating and loading model. We quantitatively assess the thermal and melting processes involved in the lava heating and loading process following the chronological sequence of lava emplacement. We test a broad range of parameters to thoroughly constrain the lava heating and loading process and outline predictions for the formation of resulting geological features. We apply the theoretical model to a study area within the Hesperia Planum region and assess the observed geology against predictions derived from the ice sheet lava heating and loading model. Due to the highly cratered nature of the Noachian highlands terrain onto which the volcanic plains were emplaced, we predict highly asymmetrical lava loading conditions. Crater interiors are predicted to accumulate greater thicknesses of lava over more rapid timescales, while in the intercrater plains, lava accumulation occurs over longer timescales and does not reach great thicknesses. We find that top-down melting due to conductive heat transfer from supraglacial lava flows is generally limited when the emplaced lava flows are less than ∼10m thick, but is very significant at lava flow thicknesses of ∼100m or greater. We find that bottom-up cryosphere and ice sheet melting is most likely to occur within crater interiors where lavas accumulate to a sufficient thickness to raise the ice-melting isotherm to the base of the superposed lavas. In these locations, if lava accumulation occurs rapidly, bottom-up melting of the ice sheet can continue, or begin, after lava accumulation has completed in a process we term “deferred melting”. Subsurface mass loss through melting of the buried ice sheets is predicted to cause substantial subsidence in the superposed lavas, leading to the formation of associated collapse features including fracture systems, depressions, surface faulting and folding, wrinkle-ridge formation, and chaos terrain. In addition, if meltwater generated from the lava heating and loading process becomes trapped at the lava flow margins due to the presence of impermeable confining units, large highly pressurized episodic flooding events could occur. Examination of the study area reveals geological features which are generally consistent with those predicted to form as a result of the ice sheet lava heating and loading process, suggesting the presence of surface snow and ice during the Late Noachian to Early Hesperian period.

Regional Genetic Structure and Environmental Variables Influence our Conservation Approach for Feather Heads (Ptilotus macrocephalus).

Continued alterations to the Australian environment compromise the long-term viability of many plant species. We investigate the population genetics of Ptilotus macrocephalus, a perennial herb that occurs in 2 nationally endangered communities on the Victorian Volcanic Plain Bioregion (VVP), Australia, to answer key questions regarding regional differentiation and to guide conservation strategies. We evaluate genetic structure and diversity within and among 17 P. macrocephalus populations from 3 regions of southeastern Australia using 17 microsatellite markers developed de novo. Genetic structure was present in P. macrocephalus between the 3 regions but not at the population level. Environmental factors, namely temperature and precipitation, significantly explained differentiation between the North region and the other 2 regions indicating isolation by environment. Within regions, genetic structure currently shows a high level of gene flow and genetic variation. Our results suggest that within-region gene flow does not reflect current habitat fragmentation in southeastern Australia whereas temperature and precipitation are likely to be responsible for the differentiation detected among regions. Climate change may severely impact P. macrocephalus on the VVP and test its evolutionary resilience. We suggest taking a proactive conservation approach to improve long-term viability by sourcing material for restoration to assist gene flow to the VVP region to promote an increased adaptive capacity.

Melting processes and mantle sources of lavas on Mercury

The MESSENGER spacecraft provided geochemical data for surface rocks on Mercury. In this study, we use the major element composition of these lavas to constrain melting conditions and residual mantle sources on Mercury. We combine modelling and high-temperature (1320–1580 °C), low- to high-pressure (0.1 to 3 GPa) experiments on average compositions for the Northern Volcanic Plains (NVP) and the high-Mg region of the Intercrater Plains and Heavily Cratered Terrains (High-Mg IcP-HCT). Near-liquidus phase relations show that the S-free NVP and High-Mg IcP-HCT compositions are multiply saturated with forsterite and enstatite at 1450 °C – 1.3 GPa and 1570 °C – 1.7 GPa, respectively. For S-saturated melts (1.5–3 wt.% S), the multiple saturation point (MSP) is shifted to 1380 °C – 0.75 GPa for NVP and 1480 °C – 0.8 GPa for High-Mg IcP-HCT. To expand our experimental results to the range of surface compositions, we used and calibrated the pMELTS thermodynamic calculator and estimated phase equilibria of ∼5800 compositions from the Mercurian surface and determined the P–T conditions of liquid–forsterite–enstatite MSP (1300–1600 °C; 0.25–1.25 GPa). Surface basalts were produced by 10 to 50% partial melting of variably enriched lherzolitic mantle sources. The relatively low pressure of the olivine–enstatite–liquid MSP seems most consistent with decompression batch melting and melts being segregated from their residues near the base of Mercury's ancient lithosphere. The average melting degree is lower for the young NVP (0.27±0.04) than for the older IcP-HCT (0.46±0.02), indicating that melt productivity decreased with time. The mantle potential temperature required to form Mercurian lavas and the initial depth of melting also decreased from the older High-Mg IcP-HCT terrane (1650 °C and 360 km) to the younger lavas covering the NVP regions (1410 °C and 160 km). This evolution supports strong secular cooling of Mercury's mantle between 4.2 and 3.7 Ga and explains why very little magmatic activity occurred after 3.7 Ga.

The lunar Gruithuisen silicic extrusive domes: Topographic configuration, morphology, ages, and internal structure

The Gruithuisen domes, situated on the western portion of the Imbrium basin rim, form three tall mountains (NW, Gamma, Delta) totaling ∼780km3 in volume. The shapes of the domes are significantly different from that of mare-type domes elsewhere on the Moon. We use data from the Lunar Reconnaissance Orbiter (LRO) and Kaguya missions (LRO Lunar Orbiter Laser Altimeter, Lunar Reconnaissance Orbiter Camera, Diviner, and the Kaguya imager) to characterize the domes and assess models for their origin. The configuration of the domes (steep slopes, up to ∼18–20°) and their specific remote sensing characteristics (strong downturn in the UV, and results from the M3 and Diviner instruments) suggest that the domes formed by eruptions of highly viscous lava. The estimated surface volumes of the domes vary from ∼20km3 (NW dome) to ∼290km3 (Gamma dome) to ∼470km3 (Delta dome). The domes occur on the portion of the Imbrium basin rim that is overlain by ejecta from the post-Imbrium Iridum crater. In some areas, relatively high albedo smooth volcanic plains are seen within the Iridum ejecta near the Gruithuisen domes, and low albedo mare deposits surround and embay the domes and Iridum crater. Dating of different units and features by crater counts indicates that impact melts from the Iridum basin are ∼3.9Ga old, the domes Gamma and Delta are ∼3.8Ga, and the ages of the plains near the domes vary from ∼2.3 to ∼3.6Ga. A fresh impact crater exposes the internal structure of the Gamma dome. The most prominent features on the wall of the crater are rough, blocky layers that are typical of volcanic plains in the highlands and maria around the domes. The layers are interleaved with fine-grained materials of higher and lower albedo and the visible orientation of the layers changes over short (a few hundred meters) distances. These characteristics of the internal structure of the dome are consistent with eruptions of high viscosity lava (rough layers) that alternated with possible explosive activity (fine-grained materials). The spatial association of the Gruithuisen domes with the highland lava plains resembles the situation in which bimodal volcanism occur on Earth. The terrestrial association can be due to either fractional crystallization in basaltic magma reservoirs or remelting of high-silica crustal materials. In the first case, the evolved melts appear in later stages of volcanic activity and in the second case these melts are formed near the beginning of evolution of the magmatic systems. The age estimates of the Gruithuisen domes and the surrounding volcanic plains are more consistent with the crustal remelting scenario. However, remelting of primary anorthositic crust cannot readily produce the silica-rich melts and requires the presence of pre-existing granite-like materials. Formation of the domes by fractional crystallization avoids this difficulty but requires explanation of the older age of the domes relative to the volcanic plains in the surroundings. A third option is that the domes are unrelated genetically to the mare deposits.

Rheology of lava flows on Mercury: An analog experimental study

We experimentally determined the rheological evolution of three basaltic analog compositions appropriate to Mercury's surface, during cooling, and crystallization. Investigated compositions are an enstatite basalt, and two magnesian basalts representing the compositional end-members of the northern volcanic plains with 0.19 wt % (NVP) and 6.26 wt % Na2O (NVP-Na). The viscosity-strain rate dependence of lava was quantified using concentric cylinder viscometry. We measured the viscosities of the crystal-free liquids from 1600°C down to the first detection of crystals. Liquidus temperatures of the three compositions studied are around 1360°C, and all three compositions are more viscous than Hawaiian basalt at the same temperature. The onset of pseudoplastic behavior was observed at crystal fractions ~0.05 to 0.10, which is consistent with previous studies on mafic lavas. We show that all lavas develop detectable yield strengths at crystal fractions around 0.20, beyond which the two-phase suspensions are better described as Herschel-Bulkley fluids. By analogy with the viscosity-strain rate conditions at which the pahoehoe to `a`a transition occurs in Kilauea basalt, this transition is predicted to occur at ~1260 ± 10°C for the enstatite basalt, at ~1285 ± 20°C for the NVP, and at ~1240 ± 40°C for the NVP-Na lavas. Our results indicate that Mercury lavas are broadly similar to terrestrial ones, which suggests that the extensive smooth lava plains of Mercury could be due to large effusion rates (flood basalts) and not to unusually fluid lavas.

The origin of boninites on Mercury: An experimental study of the northern volcanic plains lavas

Phase equilibrium experiments were conducted on a synthetic rock composition matching that of the northern volcanic plains of Mercury as measured by the MErcury Surface, Space ENvironment, GEochemistry and Ranging spacecraft (MESSENGER). The northern volcanic plains are smooth plains of suspected volcanic origin that cover more than 6% of the surface area of Mercury. The northern volcanic plains are less cratered than their surroundings and reported to be the product of flood volcanism, making them a prime candidate for experimental study. The bulk composition of the northern volcanic plains is that of an alkali-rich boninite and represents the first silica-enriched crustal terrane identified on an extraterrestrial planet from orbital data. Phase equilibrium experiments were conducted over the pressure range of the mercurian mantle (0.5–5GPa) at very low oxygen fugacity (∼ΔIW0 to −7) using a piston-cylinder apparatus (P 0.5–1.7GPa) and a Walker-style multi-anvil device (P⩾2.5GPa). Our results indicate the origin of the northern volcanic plains lavas (boninites) are best explained by high degrees of partial melting of an olivine-dominant, pyroxene- and plagioclase-bearing mantle source at low pressure (⩽1.4GPa) and does not require hydrous melting to achieve the silica-enriched melt composition. The formation mechanism for boninites on Mercury contrasts substantially with terrestrial boninites, which typically occur in oxidized and hydrous arc environments associated with subduction zones. Instead, mercurian boninites form at exceptionally low oxygen fugacity and do not require melting of hydrated source materials. The NVP lavas represent a novel mechanism by which planetary bodies can form silica-enriched secondary crusts without the aid of water.

Rembrandt impact basin: Distinguishing between volcanic and impact-produced plains on Mercury

The surface of Mercury has been heavily modified by impact cratering and volcanic plains since the initial formation of its lithosphere and crust. As in the case of the Moon, the origin of early plains deposits has been difficult to determine because ponded impact basin ejecta and impact melt deposits can mimic the topography and texture of extrusive volcanic plains. In order to understand better the role that impact basins play in resurfacing on Mercury, we used MESSENGER Mercury Dual Imaging System (MDIS) data to map the distribution of plains deposits within and around the relatively young Rembrandt impact basin (∼720km in diameter). There are two different Rembrandt plains units: (1) high-reflectance interior and exterior plains, and (2) low-reflectance exterior plains. The morphology, crater size–frequency distribution, and N(20) crater density values were analyzed for each deposit. Observations of the two high-reflectance plains deposits are consistent with previous studies of volcanically produced smooth plains on Mercury. The low-reflectance exterior plains are older than the other high-reflectance plains and comparable in age to the Rembrandt basin impact event. On the basis of age, areal distribution, and albedo relationships with basin materials, the low-reflectance plains are interpreted as basin impact melt deposits. The large areal extent of the low-reflectance plains suggests that basin impact melt deposits may have played an important role in resurfacing Mercury, especially during the early geologic history of the planet when impact rates were higher.

Orbital multispectral mapping of Mercury with the MESSENGER Mercury Dual Imaging System: Evidence for the origins of plains units and low-reflectance material

A principal data product from MESSENGER’s primary orbital mission at Mercury is a global multispectral map in eight visible to near-infrared colors, at an average pixel scale of 1km, acquired by the Mercury Dual Imaging System. The constituent images have been calibrated, photometrically corrected to a standard geometry, and map projected. Global analysis reveals no spectral units not seen during MESSENGER’s Mercury flybys and supports previous conclusions that most spectral variation is related to changes in spectral slope and reflectance between spectral end-member high-reflectance red plains (HRP) and low-reflectance material (LRM). Comparison of color properties of plains units mapped on the basis of morphology shows that the two largest unambiguously volcanic smooth plains deposits (the interior plains of Caloris and the northern plains) are close to HRP end members and have average color properties distinct from those of most other smooth plains and intercrater plains. In contrast, smaller deposits of smooth plains are nearly indistinguishable from intercrater plains on the basis of their range of color properties, consistent with the interpretation that intercrater plains are older equivalents of smooth plains. LRM having nearly the same reflectance is exposed in crater and basin ejecta of all ages, suggesting impact excavation from depth of material that is intrinsically dark or darkens very rapidly, rather than gradual darkening of exposed material purely by space weathering. A global search reveals no definitive absorptions attributable to Fe2+-containing silicates or to sulfides over regions 20km or more in horizontal extent, consistent with results from MESSENGER’s Mercury Atmospheric and Surface Composition Spectrometer. The only absorption-like feature identified is broad upward curvature of the spectrum centered near 600nm wavelength. The feature is strongest in freshly exposed LRM and weak or absent in older exposures of LRM. We modeled spectra of LRM as intimate mixtures of HRP with candidate low-reflectance phases having a similar 600-nm spectral feature, under the assumption that the grain size is 1μm or larger. Sulfides measured to date in the laboratory and coarse-grained iron are both too bright to produce LRM from HRP. Ilmenite is sufficiently dark but would require Ti abundances too high to be consistent with MESSENGER X-Ray Spectrometer measurements. Three phases or mixtures of phases that could be responsible for the low reflectance of LRM are consistent with our analyses. Graphite, in amounts consistent with upper limits from the Gamma-Ray Spectrometer, may be consistent with geochemical models of Mercury’s differentiation calling for a graphite-enriched primary flotation crust from an early magma ocean and impact mixing of that early crust before or during the late heavy bombardment (LHB) into material underlying the volcanic plains. The grain size of preexisting iron or iron sulfide could have been altered to a mix of nanophase and microphase grains by shock during those impacts, lowering reflectance. Alternatively, iron-bearing phases and carbon in a late-accreting carbonaceous veneer may have been stirred into the lower crust or upper mantle. Decoupling of variations in color from abundances of major elements probably results from the very low content and variation of Fe2+ in crustal silicates, such that reflectance is controlled instead by one or more minor opaque phases and the extent of space weathering.

The history of tectonism on Venus: A stratigraphic analysis

The surface of Venus displays several tectonized terrains in which the morphologic characteristics of the original materials are almost completely erased by superposed tectonic structures whose large dimensions (»100km) suggest formation related to mantle convection. The characteristics of these tectonized terrains are in contrast to volcanic units in which tectonic structures are less significant or absent and thus do not obscure the volcanic character of the units. We describe the temporal distribution of tectonized terrains, their stratigraphic relationships with volcanic units, and how these outline the major episodes in the geological evolution of Venus. Five major tectonized units make up ~20% of the planet: (1) tessera (t, 7.3%), (2) densely lineated plains (pdl, 1.6%), (3) ridged plains/ridge belts (pr/rb, 2.4%), (4) groove belts (gb, 8.1%), and (5) rift zones (rz, 5.0%). Clear relationships of relative age are often seen among the tectonic and volcanic units at the global scale and define three contrasting regimes of volcanic and tectonic resurfacing. The majority of tectonized terrains (t through gb) are the products of tectonic resurfacing and are embayed by the vast volcanic plains and, thus, are older. There are no units with either mildly- or non-tectonized surfaces that interleave the tectonic terrains, which would be expected if the tectonic resurfacing operated only during specific repetitive phases in discrete regions. These tectonized terrains (t through gb) thus define a tectonically dominated regime of resurfacing that occurred at a global-scale near the beginning of the observable geological history of Venus. This ancient tectonic regime began with formation of tessera and was followed by formation of pdl and pr/rb. Groove belts formed near the end of this regime. Branches of groove belts compose the tectonic components of many coronae, suggesting that these features are genetically related (e.g., mutual development of mantle diapirs and zones of extension) and that coronae may have punctuated the final stages of the ancient tectonic regime. This regime was followed by emplacement of the vast volcanic plains, such as shield and regional plains, the surfaces of which are extensively deformed by the global network of wrinkle ridges. Emplacement of the plains defines the second, volcanically dominated regime, representing a time when surface tectonic deformation related to the mantle convection waned. Rift zones are the stratigraphically youngest manifestations of regional-scale tectonic deformation on Venus. Rifts are spatially and temporarily associated with the youngest lava flows and often cut the crest areas of large, but isolated, dome-shaped rises. Structures of rift zones always cut the surface of the vast plains, which means that rifts are separated in time from the ancient tectonic regime, post-date the regional plains, and represent a new phase of tectonism that was contemporaneous with the late volcanism of lobate plains. Rift zones and lobate plains define the third, network rifting-volcanism regime, of resurfacing that was related to late stages of evolution of the dome-shaped rises.

Geochemical terranes of Mercury’s northern hemisphere as revealed by MESSENGER neutron measurements

The first map of variations in the abundances of thermal-neutron-absorbing elements across Mercury’s surface has been derived from measurements made with the anti-coincidence shield on MESSENGER’s Gamma-Ray Spectrometer (GRS). The results, which are limited to Mercury’s northern hemisphere, permit the identification of four major geochemical terranes at the 1000-km horizontal scale. The chemical properties of these regions are characterized from knowledge of neutron production physics coupled with elemental abundance measurements acquired by MESSENGER’s X-Ray Spectrometer (XRS) and GRS. The results indicate that the smooth plains interior to the Caloris basin have an elemental composition that is distinct from those of other volcanic plains units, suggesting that the parental magmas were partial melts from a chemically distinct portion of Mercury’s mantle. Mercury’s high-magnesium region, first recognized from XRS measurements, also contains high concentrations of unidentified neutron-absorbing elements. At latitudes north of ∼65°N, there is a region of high neutron absorption that corresponds closely to areas known to be enhanced in the moderately volatile lithophile elements Na, K, and Cl, and which has distinctly low Mg/Si ratios. The boundaries of this terrane differ from those of the northern volcanic plains, which constitute the largest geological unit in this region.

Radio reflection imaging of asteroid and comet interiors II: Results and recommendations

We modeled orbital surface-penetrating radar of an asteroid and comet using two-dimensional finite-difference wavefield migration, in order to assess key target properties and experiment parameters required to fully image internal structure. Migration places radar echoes in their correct positions in a complex subsurface and is a complementary tool to travel-time tomography. The target shape was scaled from 433 Eros to 0.5-km mean diameter for an asteroid and 10km for a comet. The interiors were populated with a power-law distribution of spherical blocks. We used an image structural similarity index to compare the internal surfaces reconstructed under different assumptions to a “best” image using optimum parameters. We found that successful internal imaging of the asteroid was not sensitive to whether the block interstices were regolith or void. Frequency dependence between 5 and 15MHz was also minor. Internal interfaces could also be imaged if the attenuation was higher than that inferred within volcanic plains on Mars, but not as high as measured in a strongly fractured volcanic tuff on Earth. The overall imaging quality for the comet was statistically similar to the asteroid, but there was less variability due to smaller internal contrasts. A key finding is that imaging was vastly improved by using a second spacecraft as a radar receiver. A subsatellite with a different orbit will eventually provide a range of different illumination geometries over each part of the target. Finally, the results depend strongly on the specified internal velocity distribution, representing partial progress in complementary tomographic velocity estimation. The modeled impedance contrasts within the asteroid are larger than those typically encountered in exploration seismology and very much larger than in medical imaging, and so the velocity used to migrate the reflections must be close to the actual distribution. This again emphasizes the need for joint traveltime tomography and wavefield migration for asteroid imaging, which is optimized using two orbiters.

Kilometer‐scale topographic roughness of Mercury: Correlation with geologic features and units

AbstractWe present maps of the topographic roughness of the northern circumpolar area of Mercury at kilometer scales. The maps are derived from range profiles obtained by the Mercury Laser Altimeter (MLA) instrument onboard the MErcury Surface, Space ENvironment, Geochemistry, and Ranging (MESSENGER) mission. As measures of roughness, we used the interquartile range of profile curvature at three baselines: 0.7 km, 2.8 km, and 11 km. The maps provide a synoptic overview of variations of typical topographic textures. They show a dichotomy between the smooth northern plains and rougher, more heavily cratered terrains. Analysis of the scale dependence of roughness indicates that the regolith on Mercury is thicker than on the Moon by approximately a factor of three. Roughness contrasts within northern volcanic plains of Mercury indicate a younger unit inside Goethe basin and inside another unnamed stealth basin. These new data permit interplanetary comparisons of topographic roughness.

Stratigraphy of the Caloris basin, Mercury: Implications for volcanic history and basin impact melt

Caloris basin, Mercury’s youngest large impact basin, is filled by volcanic plains that are spectrally distinct from surrounding material. Post-plains impact craters of a variety of sizes populate the basin interior, and the spectra of the material they have excavated enable the thickness of the volcanic fill to be estimated and reveal the nature of the subsurface. The thickness of the interior volcanic plains is consistently at least 2.5km, reaching 3.5km in places, with thinner fill toward the edge of the basin. No systematic variations in fill thickness are observed with long-wavelength topography or azimuth. The lack of correlation between plains thickness and variations in elevation at large horizontal scales within the basin indicates that plains emplacement must have predated most, if not all, of the changes in long-wavelength topography that affected the basin. There are no embayed or unambiguously buried (ghost) craters with diameters greater than 10km in the Caloris interior plains. The absence of such ghost craters indicates that one or more of the following scenarios must hold: the plains are sufficiently thick to have buried all evidence of craters that formed between the Caloris impact event and the emplacement of the plains; the plains were emplaced soon after basin formation; or the complex tectonic deformation of the basin interior has disguised wrinkle-ridge rings localized by buried craters. That low-reflectance material (LRM) was exposed by every impact that penetrated through the surface volcanic plains provides a means to explore near-surface stratigraphy. If all occurrences of LRM are derived from a single layer, the subsurface LRM deposit is at least 7.5–8.5km thick and its top likely once made up the Caloris basin floor. The Caloris-forming impact would have generated a layer of impact melt 3–15km thick; such a layer could account for the entire thickness of LRM. This material would have been derived from a combination of lower crust and upper mantle.

From volcanic plains to glaciated peaks: Burial, uplift and exhumation history of southern East Greenland after opening of the NE Atlantic

In southern East Greenland (68–70°N), voluminous flood basalts erupted onto a largely horizontal lava plain near sea level at the Paleocene–Eocene transition when sea-floor spreading started in the NE Atlantic. Based on synthesis of geological observations, stratigraphic landform analysis and apatite fission-track analysis data in 90 rock samples, we show how three regional phases of uplift and exhumation subsequently shaped the present-day margin and controlled the discontinuous history of the Greenland ice sheet. A late Eocene phase of uplift led to formation of a regional erosion surface near sea level (the Upper Planation Surface, UPS). Uplift of the UPS in the late Miocene led to formation of the Lower Planation Surface (LPS) by incision below the uplifted UPS, and a Pliocene phase led to incision of valleys and fjords below the uplifted LPS, leaving mountain peaks reaching 3.7km above sea level. Local uplift affected the Kangerlussuaq area (~68°N) during early Eocene emplacement of the Kangerlussuaq Intrusion and during late Oligocene block movements, that may be related to the detachment of the Jan Mayen microcontinent from Greenland, while middle Miocene thermal activity, coeval with lava eruptions, heated rocks along a prominent fault within the early Cretaceous to Paleocene Kangerlussuaq Basin. The three regional uplift phases are synchronous with phases in West Greenland, overlap in time with similar events in North America and Europe and also correlate with changes in plate motion. The much higher elevation of East Greenland compared to West Greenland suggests support in the east from the Iceland plume. These observations indicate a connection between mantle convection, changes in plate motion and vertical movements along passive continental margins.

China's Jade Rabbit Moon Rover Sends Back First Photos

The first robot rover to land on the Moon in nearly 40 years, China's Jade Rabbit, has begun sending back photos, with shots of its lunar lander. Jade Rabbit rolled down a ramp lowered by the lander and on to the volcanic plain known as Sinus Iridum at 04:35 Beijing time on Saturday (20:35 GMT). It moved to a spot a few metres away, its historic short journey recorded by the lander. On Sunday evening the two machines began photographing each other. A Chinese flag is clearly visible on the Jade Rabbit as it stands deployed on the Moon's surface.

Mortality of native grasses after a summer fire in natural temperate grassland suggests ecosystem instability

SummaryThe exclusion of regular fire and the introduction of livestock grazing have altered native grassland composition on Victoria's volcanic plains, commonly resulting in spear‐grass and wallaby‐grass pastures replacing Kangaroo Grass grasslands. The effect of reintroducing fire to these pastures is currently unknown, although it may be an important part of restoring this ecosystem. We measured the changes in basal area of the dominant grasses in a mixed Spear‐grass/Wallaby‐grass pastures after a summer wildfire, which we assume burnt a relatively homogenous grass sward. We found a 90–95% reduction in the basal area of live spear‐grass tussocks in burnt plots compared with unburned controls, due to the mortality of tussocks. This suggests that caution and structured experimentation should be applied when using fire to manage spear‐grass‐dominated grasslands.

Formation of Hrad Vallis (Mars) by low viscosity lava flows

Hrad Vallis is a Martian outflow channel previously interpreted as a product of aqueous outbursts from the subsurface, possibly involving mudflows associated with lahar-like events. However, an alternative volcanic hypothesis for the development of the system is worthy of consideration on the basis of (1) the nature of landforms preserved along component channels and adjacent uplands and (2) similarities between the basic properties of this system and large volcanic channels of the inner solar system. Hrad Vallis commences on the distal flanks of the Elysium Mons shield volcano, terminates within extensive volcanic plains, is associated with landforms typical of large volcanic channels, and shows evidence for having been a conduit for large volumes of lava. The properties of this system are consistent with incision by low viscosity lava. Crude thermal estimates suggest that this system could have formed through effusion of as little as ~10,900km3 of magma to the surface, or ~6% of the volume of the terrestrial Columbia River Basalt Group. Incision rates of up to several meters per day are estimated for mechanical and thermal processes involving lava flows with depths of 5–20m and dynamic viscosities of ~1Pas. Flow of lava within the Hrad Vallis system is predicted to have been fully turbulent and characterized by discharges as great as ~865,000m3/s. Predicted flow conditions are consistent with those previously determined for Athabasca Valles, which also formed as a result of the expulsion of flows from structures associated with Elysium Mons.

Insights into the subsurface structure of the Caloris basin, Mercury, from assessments of mechanical layering and changes in long‐wavelength topography

The volcanic plains that fill the Caloris basin, the largest recognized impact basin on Mercury, are deformed by many graben and wrinkle ridges, among which the multitude of radial graben of Pantheon Fossae allow us to resolve variations in the depth extent of associated faulting. Displacement profiles and displacement‐to‐length scaling both indicate that faults near the basin center are confined to a ~ 4‐km‐thick mechanical layer, whereas faults far from the center penetrate more deeply. The fault scaling also indicates that the graben formed in mechanically strong material, which we identify with dry basalt‐like plains. These plains were also affected by changes in long‐wavelength topography, including undulations with wavelengths of up to 1300 km and amplitudes of 2.5 to 3 km. Geographic correlation of the depth extent of faulting with topographic variations allows a first‐order interpretation of the subsurface structure and mechanical stratigraphy in the basin. Further, crosscutting and superposition relationships among plains, faults, craters, and topography indicate that development of long‐wavelength topographic variations followed plains emplacement, faulting, and much of the cratering within the Caloris basin. As several examples of these topographic undulations are also found outside the basin, our results on the scale, structural style, and relative timing of the topographic changes have regional applicability and may be the surface expression of global‐scale interior processes on Mercury.

Transient hydrological conditions implied by chloride mass balance in southeast Australian rivers

A robust correlation between electrical conductivity (EC) values and Cl concentrations in river water from southeast Australia allows detailed Cl fluxes to be calculated from continuous EC and river discharge records. Many Victorian rivers export significantly more Cl than is delivered to their catchments by rainfall. Cl* is defined as the mass of Cl exported in the rivers relative to that input by rainfall over a multi-year period (Cl*=100% indicates that the river exports the same mass of Cl as is input by rainfall). There is a systematic relationship between catchment type and Cl*. Rivers draining cleared plains have Cl* values between 50 and 750%, rivers draining volcanic plains have Cl* values of 770–1600%, whereas rivers with large forested upland catchments have Cl* values of 50–110%. These values are minima as they do not account for Cl exported by groundwater from the catchments. The calculations are based on long-term (up to 22year) records that span drought and high rainfall periods. The magnitude of Cl* is far higher than can be explained by errors in the calculations or variability in rainfall and runoff, and Cl/Br ratios preclude halite dissolution as a source of Cl. The excess Cl reflects hydrological changes in the catchments. Land clearing on the cleared plains has caused the rise of regional water tables which results in the export of Cl from saline groundwater via increased baseflow to the river systems. Drainage systems on the volcanic plains are re-establishing following impoundment by recent (<4.5Ma) lava flows; Cl which accumulated in shallow groundwater around saline lakes and marshes developed on these volcanic plains is now being exported via the rivers. The upland catchments have undergone less landscape change and may be in chemical balance. The methodology outlined here provides a straightforward assessment of whether catchments are in chemical balance that may in turn indicate whether they are undergoing hydrological changes.

An assemblage of lava flow features on Mercury

AbstractIn contrast to other terrestrial planets, Mercury does not possess a great variety of volcanic features, its history of volcanism instead largely manifest by expansive smooth plains. However, a set of landforms at high northern latitudes on Mercury resembles surface flow features documented on Earth, the Moon, Mars, and Venus. The most striking of such landforms are broad channels that host streamlined islands and that cut through the surrounding intercrater plains. Together with narrower, more sinuous channels, coalesced depressions, evidence for local flooding of intercrater plains by lavas, and a first‐order analysis of lava flow rates, the broad channels define an assemblage of flow features formed by the overland flow of, and erosion by, voluminous, high‐temperature, low‐viscosity lavas. This interpretation is consistent with compositional data suggesting that substantial portions of Mercury's crust are composed of magnesian, iron‐poor lithologies. Moreover, the proximity of this partially flooded assemblage to extensive volcanic plains raises the possibility that the formation of these flow features may preface total inundation of an area by lavas emplaced in a flood mode and that they escaped complete burial only due to a waning magmatic supply. Finally, that these broad channels on Mercury are volcanic in nature yet resemble outflow channels on Mars, which are commonly attributed to catastrophic water floods, implies that aqueous activity is not a prerequisite for the formation of such distinctive landforms on any planetary body.