Modified volcanic domes and associated debris aprons on Venus

The Magellan SAR images show that volcanic domes occur on the surface of Venus. Edifices with scalloped margins are similar to volcanic domes and fall within a spectrum ranging from unmodified to remnant forms. Over 320 domes have been located of which more than 80% have modified morphologies. Broadly, the modified domes can be described by five sub-categories which are related to three unmodified dome sub-categories. Many modified domes have deposits associated with them that possess characteristics indicative of their having been formed from mass movements. Evidence for slope failure is not seen on volcanic shields on Venus due to them having shallow flanks. Several steep-sided cones show some evidence of slope failure but on a much smaller scale than failures associated with domes. Slope failures on venusian domes appear to have been triggered by non-explosive and explosive events. Collapse on the edges of domes, either as a result of explosive events of oversteepening, is common on Earth but the collapses on Venus are on a scale that is more in common with major sector collapses of volcanoes on Earth. Four morphological sub-groups of debris deposit have been recognized. The deposits in groups one and two are analogous to terrestrial volcanic debris avalanche deposits and the deposits in group three are analogous to pyroclastic flows on Earth. The characteristics of the fourth group of deposits are similar to those resulting from deep-seated slides on Earth. During the early stages of the Magellan mission a volcanic edifice with scalloped margins whose origin was enigmatic was found to the north of Alpha Regio (18.PS, 5.5°E). As the mission progressed, other edifices with scalloped margins were identified. The radar characteristics and morphologies of many of these edifices are similar to volcanic domes and form a spectrum ranging from unmodified to remnant forms. Based on the characteristic margins, the edifices were termed scalloped margin domes, or SMDs (Guest el al. 1992a, b). Many of the domes are an order of magnitude larger than terrestrial domes. Debris aprons associated with modified domes possess a range of radar and morphologic characteristics indicative of mass movement deposits (Guest el al. 1991, 1992a, b; Bulmer el al. 1992, 1993). Large arcuate backscarps on the flanks of some domes indicate that large-scale collapses have occurred. There is an absence of debris aprons around some domes, which may be explained in some cases by burial by younger material. Elsewhere the deposits may form only a thin unconsolidated mantle that was penetrated by the radar or have the same radar backscatter as the surrounding plains. Using the Magellan dataset an extensive database was compiled from a global survey of volcanic domes on Venus (Bulmer 1994). It incorporates information on the morphological and morphometrical aspects of domes and large debris aprons associated with them. Over 300 modified domes were identified during the survey indicating that if SMDs are modified volcanic domes, then domes are more common than suggested in previous work (Head el al. 1992; Pavri et al. 1992). A greater number of modified domes were identified than unmodified domes. A range of morphological characteristics of modified domes was recognized from the survey but they can broadly be divided into five categories labelled MD1 to MD5. It is proposed that modified domes are derived from the different dome sub-categories D, to D% discussed in Guest el al. (1992a). As part of the global survey of modified domes, details of debris aprons associated with volcanic domes were compiled. Over 100 debris aprons were identified. The database includes information on the morphological and morphometric aspects of mass movement deposits. A descriptive classification using the headscarp-debris apron relationship, the surface texture and the plan view, was devised that allowed the deposits to be placed into one of four categories, labelled Gl to G4. These four different morphological groups exhibit distinctive radar characteristics and morphometrical trends. The data obtained from the survey were designed to build upon an extensive database of large landslides on From McGuire, W. J., Jones, A. P. & Neuberg, J. (eds) 1996, Volcano Instability on the Earth and Other Planets, Geological Society Special Publication No. 110, pp. 349-371. 350 M. H. BULMER & J. E. GUEST