Emplacement and composition of steep-sided domes on Venus

Steep-sided domes on Venus have surface characteristics that can provide in formation on their emplacement, including relatively smooth upper surfaces, radial and polygonal fracture patterns, and pits. These characteristics in dicate that domes have surface crusts which are relatively unbroken, have m obile interiors after emplacement, and preserve fractures from only late in their history in response to endogenous growth or sagging of the dome surf ace. We have calculated the time necessary to form a 12-cm-thick crust for basalt and rhyolite under current terrestrial and Venusian ambient conditio ns. A 12-cm-thick crust will form in all cases in < 10 hours. Although Venu sian lava flows should develop a brittle carapace during emplacement, only late-stage brittle fractures are preserved at steep-sided domes. We favor a n emplacement model where early-formed surface crusts are entrained or cont inually annealed as they deform to accommodate dome growth. Entrainment and annealing of fractures are not mutually exclusive processes and thus may b oth be at work during steep-sided dome emplacement. Our results are most co nsistent 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 excur sions in the past (producing ambient conditions of 800-1000 K [e.g., Bulloc k and Grinspoon, 1996, 1998]), rhyolitic lavas would be unable to form crus ts at high surface temperatures and could produce domes with surface charac teristics consistent with those of Venusian steep-sided domes.