Emplacement and composition of steep‐sided domes on Venus

Abstract

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

The unique radar properties of silicic lava domes

“…Some cases were found where stereo data show portions of the central depressed areas of domes at similar elevations to the surroundings. Only two of the domes examined by Stofan et al [2000] showed evidence for elevated central vent structures that characterize many terrestrial silicic domes, including the Inyo domes.…”

“…Pavri et al [1992] found the majority of the 145 domes examined were in the first two categories: inverted bowl-shaped or flat-topped. However, a survey of 175 domes with diameters >19 km [Stofan et al, 2000], and analyses of stereo pairs for this study, indicate that the ''annular'' type profile (i.e., concave-up) is at least as common as the flat-topped or inverted bowl-shaped types. Some cases were found where stereo data show portions of the central depressed areas of domes at similar elevations to the surroundings.…”

“…[35] The morphology of Venusian steep-sided domes was examined by Pavri et al [1992], Fink et al [1993], and Stofan et al [2000]. Most domes are highly circular in plan view, and typically have steep margins (subject to radar layover in some instances).…”

“…The Magellan radar mapping mission to Venus revealed a variety of volcanic features, and the interpretation of the radar images inspired a number of studies utilizing radar remote sensing of terrestrial volcanoes and lava flows [e.g., Greeley and Martel, 1988;Ford et al, 1989;Gaddis et al, 1989Gaddis et al, , 1990Campbell and Campbell, 1992;Arvidson et al, 1993;Mouginis-Mark, 1995;Campbell and Shepard, 1996]. A remarkable finding from Magellan was a class of apparently volcanic landforms referred to as ''steep-sided'' or ''pancake'' domes Pavri et al, 1992;McKenzie et al, 1992;Fink et al, 1993;Bridges, 1997;Stofan et al, 2000]. The gross morphology of these features was reminiscent of silicic lava domes on Earth, and early workers suggested that they represented viscous lava flows, silicic in composition [McKenzie et al, 1992] or alternatively, less silicic lavas with enhanced gas bubble content [Pavri et al, 1992].…”

Volcanism and tectonism across the inner solar system: an overview

“…Typical volcanic landforms include lava flow fields, lava channels and steep-sided domes (Crumpler & Aubele 2000; Stofan et al 2000;Magee & Head 2001). Lava flow fields are composed of very long (up to 1000 km) digitated flows, both with radar bright (interpreted to be aa-type lavas) or dark (pahoehoe-type lavas) surfaces, which erupted from shield volcanoes and fissures (including those of coronae).…”