J. B. Murray

Recent and episodic volcanic and glacial activity on Mars revealed by the High Resolution Stereo Camera

The large-area coverage at a resolution of 10–20 metres per pixel in colour and three dimensions with the High Resolution Stereo Camera Experiment on the European Space Agency Mars Express Mission has made it possible to study the time-stratigraphic relationships of volcanic and glacial structures in unprecedented detail and give insight into the geological evolution of Mars. Here we show that calderas on five major volcanoes on Mars have undergone repeated activation and resurfacing during the last 20 per cent of martian history, with phases of activity as young as two million years, suggesting that the volcanoes are potentially still active today. Glacial deposits at the base of the Olympus Mons escarpment show evidence for repeated phases of activity as recently as about four million years ago. Morphological evidence is found that snow and ice deposition on the Olympus construct at elevations of more than 7,000 metres led to episodes of glacial activity at this height. Even now, water ice protected by an insulating layer of dust may be present at high altitudes on Olympus Mons.

Effusion rate trends at Etna and Krafla and their implications for eruptive mechanisms

Abstract Using effusion rates obtained from ground- and satellite-based data we build a data set of 381 effusion rate measurements during effusive activity at Etna and Krafla between 1980 and 1999. This allows us to construct detailed effusion rate curves for six fissure-fed eruptions at Etna and Krafla and four summit-fed eruptions at Etna. These define two trends: Type I and II. Type I trends have effusion rates that rise rapidly to an initial peak, before declining more slowly, resulting in an exponential decrease in eruption rate and declining growth in cumulative volume. Type II trends are characterised by steady effusion and eruption rates, and hence a linear increase in cumulative volume. The former is typical of fissure eruptions and can be explained by tapping of an enclosed, pressurised system. The latter are typical of persistent Etnean summit eruptions, plus one persistent effusive eruption at Stromboli (1985–1986) examined here, and can be explained by overflow of the time-averaged magma supply. We use our effusion rate data to assess the magma balance at Etna (1980–1995) and Krafla (1975–1984). Between 1980 and 1995, Etna was supplied at a time-averaged rate of 6.8±2.3 m 3 s −1 of which 13% was erupted. At Krafla 817±30×10 6 m 3 was erupted and intruded during 1975–1984, and the ratio of erupted to intruded volume was 0.3. At Etna there is evidence for intrusion of the unerupted magma within and beneath the edifice, as well as storage in the central magma column. At Krafla unerupted magma was intruded into a rift zone, but an increasing proportion of the supply was erupted from 1980 onwards, a result of the rift zone capacity being reached. Magma intruded prior to an eruptive event may also be entrained and/or pushed out during eruption to contribute to the initial high effusion rate phases of Type I events. The detail in our effusion rate curves was only possible using a thermal approach which estimates effusion rates using satellite data. We look forward to analysing satellite-derived effusion rate trends in real-time using data from current and soon-to-be-launched sensors.

Evidence from the Mars Express High Resolution Stereo Camera for a frozen sea close to Mars' equator

It is thought that the Cerberus Fossae fissures on Mars were the source of both lava and water floods two to ten million years ago. Evidence for the resulting lava plains has been identified in eastern Elysium, but seas and lakes from these fissures and previous water flooding events were presumed to have evaporated and sublimed away. Here we present High Resolution Stereo Camera images from the European Space Agency Mars Express spacecraft that indicate that such lakes may still exist. We infer that the evidence is consistent with a frozen body of water, with surface pack-ice, around 5° north latitude and 150° east longitude in southern Elysium. The frozen lake measures about 800 × 900 km in lateral extent and may be up to 45 metres deep—similar in size and depth to the North Sea. From crater counts, we determined its age to be 5 ± 2 million years old. If our interpretation is confirmed, this is a place that might preserve evidence of primitive life, if it has ever developed on Mars.

Surface movements of emplaced lava flows measured by synthetic aperture radar interferometry

Lava flows continue to move after they have been emplaced by flow mechanisms. This movement is largely vertical and can be detected using differential synthetic aperture radar (SAR) interferometry. There are three main components to this motion: (1) movement of surface scatterers, resulting in radar phase decorrelation, (2) measurable subsidence of the flow surface due to thermal contraction and clast repacking, and (3) time-dependent depression of the flow substrate. These effects act in proportion to the thickness of the lava flow and decay with time, although there is a time lag before the third component becomes significant. We explore these effects using SAR data from the ERS satellites over the Etna volcano, Sicily. Phase decorrelation on young, thick a’a lava flows persists for a few years and probably results from surface block rotations during flow contraction. Maximum measured subsidence rates of the 1991–1993 lava flow over a period of 70 days are about 0.7 mm day−1, but are potentially greater in areas of data decorrelation. These rates fall to <2.7×10−2 mm day−1 after about 20 years in flows about 50 m thick, sooner for thinner flows. Comparison with measured subsidence rates on Kilauean lava lakes suggests that thermal contraction only accounts for about one third of the observed subsidence. The remaining motion is thought to come from surface clast repacking during cooling and from creep mechanisms in the flow substrate. Measurements of postemplacement surface movement provide new constraints on the thermomechanical properties of lava flows and have cautionary implications for the interpretation of interferometric SAR data of volcanoes.

Volcano spreading controlled by dipping substrata

Most volcanoes grow on slopes, and some tend to fail catastrophically on the downslope side. Many volcanoes also deform by volcano spreading, which may lead to failure. We look at the effect of dipping substrata on the potential for spreading and collapse with analogue models. The dip is found to strongly control the spreading style, rate, and direction. A distinct change from purely radial spreading occurs even with small (<1°) substrata tilt. Structures on the cone and surrounding area are modified according to the underlying dip direction. Spreading becomes concentrated on downslope sectors, where movement is predominantly in the dip direction. The degree of structural realignment is a function of the slope angle. Our models are applied to previously known and new examples of volcano spreading. The effect of dipping substrata in confining spreading to sectors increases the potential for deep-seated sector collapses. This finding provides a mechanism for failure on the downslope side. The incorporation of substrata can create very large volume collapses.

Magma movements in Etna volcano associated with the major 1991–1993 lava eruption: evidence from gravity and deformation

The 1991–1993 eruption was probably the largest on Mt. Etna for 300 years. Since then the volcano has entered an unusually quiescent period. A comprehensive record of gravity and ground deformation changes presented here bracket this eruption and give valuable insight into magma movements before, during and after the eruption. The gravity and deformation changes observed before the eruption (1990–1991) record the intrusion of magma into the summit feeder and the SSE-trending fracture system which had recently been active in 1978, 1979, 1983 and 1989, creating the feeder dyke for the 1991–1993 eruption. In the summit region gravity changes between 1992 and 1993 (spanning the end of the eruption) reflect the withdrawal of magma from the conduit followed more recently (1993–1994) by the re-filling of magma in the conduit up to pre-eruption levels. In contrast, in the vicinity of the fracture zone, gravity has remained at the 1991–1992 level, indicating that no withdrawal has occurred here. Rather, magma has solidified in the fracture system and sealed it such that the 1993–1994 increase in magma level in the conduit was not accompanied by further intrusion into the flanks. Mass calculations suggest that a volume of at least 107 m3 of magma has solidified within the southeastern flank of the volcano.

Lava flow volume and morphology from digitised contour maps: a case study at Mount Etna, Sicily

Abstract The volume and morphology of a lava flow-field can be measured by mapping and comparing the topography before and after lava emplacement. Contour maps are a widely available source of topographic data, containing a record of the geomorphological changes at volcanoes due to lava emplacement. This paper explores the use of 1:25,000 scale digitised contour maps for mapping two lava flows of diverse eruption history and morphology at Mount Etna, Sicily. We find that subtracting DEMs created from these maps gives a good representation of volume and shape for thicker lava flows (>10 m mean thickness) in the case of the 1983 lava flow. However, thinner (

A sagging-spreading continuum of large volcano structure

Gravitational deformation strongly infl uences the structure and eruptive behavior of large volcanoes. Using scaled analog models, we characterize a range of structural architectures produced by volcano sagging and volcano spreading. These arise from the interplay of variable basement rigidity and volcano-basement (de-)coupling. From comparison to volcanoes on Earth (La Reunion and Hawaii) and Mars (Elysium and Olympus Montes), the models highlight a structural continuum in which large volcanoes throughout the Solar System lie.

The indication of Martian gully formation processes by slope–area analysis

Abstract The formation process of recent gullies on Mars is currently under debate. This study aims to discriminate between the proposed formation processes – pure water flow, debris flow and dry mass wasting – through the application of geomorphological indices commonly used in terrestrial geomorphology. High-resolution digital elevation models (DEMs) of Earth and Mars were used to evaluate the drainage characteristics of small slope sections. Data from Earth were used to validate the hillslope, debris-flow and alluvial process domains previously found for large fluvial catchments on Earth, and these domains were applied to gullied and ungullied slopes on Mars. In accordance with other studies, our results indicate that debris flow is one of the main processes forming the Martian gullies that were being examined. The source of the water is predominantly distributed surface melting, not an underground aquifer. Evidence is also presented indicating that other processes may have shaped Martian crater slopes, such as ice-assisted creep and solifluction, in agreement with the proposed recent Martian glacial and periglacial climate. Our results suggest that, within impact craters, different processes are acting on differently oriented slopes, but further work is needed to investigate the potential link between these observations and changes in Martian climate.

Persistent summit subsidence at Volcán de Colima, México, 1982–1999: strong evidence against Mogi deflation

This paper re-examines recent ground-deformation measurements at Volcan de Colima, Mexico, to test the hypothesis that the observed movements are in response to pressure changes within a sub-volcanic magma chamber, as suggested for other volcanoes by [Mogi (1958) Earthq. Res. Inst. 36, 99-134]. Measurements of vertical ground deformation across the summit dome complex of Volcan de Colima from a precise levelling network between 1982 and 1999, together with vertical and horizontal displacements derived from dual-frequency GPS measurements in 1994 and 1997, show continuous subsidence. The deformation pattern derived from the levelling shows that subsidence increases towards the summit. The closest stations to the summit (1.1 km distant) show a mean subsidence rate of 5 mm per year compared to the reference station at 2.3 km distance, which may be subsiding itself. Vertical displacements of individual stations since 1982 show that the subsidence has been fairly continuous. The GPS stations, which are distributed more widely and include some close to the edge of the active dome, confirm summit subsidence. The largest measured value, a decrease of 280 mm, or 93 mm subsidence per year, was obtained at the edge of the dome. Horizontal displacements measured during 1994–1997 also show the largest values at the summit, but these are much smaller than the vertical displacements, with the maximum rate of 23 mm per year recorded close to the dome. It is conclusively shown that these measured movements cannot be due to deflation of a buried Mogi source, as vertical and horizontal displacements for some stations are in contrary directions to those predicted by the model, and there is no consistent pattern to the horizontal movements. We attribute the measured deformation to downslope creep, settling and compaction of the edifice, gravitational spreading, or a combination of these processes.