Venusian Channels and Valleys: Distribution and Volcanological Implications

Nearly complete image coverage of Venus by Magellan enabled us to map various venusian channel and valley landforms and to examine their associations with other geological units. Global mapping reveals a nonrandom distribution. The highest total concentration is in the equatorial regions, characterized by highlands, rift and fracture zones, and associated volcanic features. Many channels associated with flow deposits are similar to typical terrestrial lava drainage channels. They are observed to be associated with a wide range of volcanic edifices, such as coronae, shield volcanoes, and rift and fracture zones. One type of channel, similar morphologically to lunar sinuous rilles, is classified as a venusian sinuous rille. Based on the close associations of many venusian sinuous rilles with coronae, we hypothesize that mantle plume or blob volcanism has caused high effusion and sustained lava eruptions essential for sinuous rille formation. Mantle-derived, high-temperature, low-viscosity lava eruptions are responsible for the efficient erosional processes, particularly for thermal erosion that seems to be, at least partially, required for some sinuous rille formation. Many valley networks are observed in highlands and in association with coronae. Fracture systems and source zones for low-viscosity lavas, both key to network formation, were probably concentrated at highlands and coronae. Canali-type channels, which are morphologically unlike other known volcanic channels, are limited to certain plains regions. A wide range of low-viscosity lava types is possible for the channel-forming lava, depending on the formation mechanism. Their lengths exceed the lengths of other common volcanic channel types on Venus, implying a large volume of lava and long duration of the eruption. The close association of canali with plains regions implies that canali formation is probably related to the emplacement of plains. A hypothesized global resurfacing event late in Venusian history may be responsible for canali formation.