Chapter 42 - Volcanism on Venus

Abstract

Venus is almost as large as Earth and has both thermal and compositional potential for long-lasting volcanic activity, in contrast to the smaller terrestrial planets. Extrusive volcanic materials make up about 80% of the surface of the planet and its volcanic landforms range from small (several kilometers in diameter) to large (tens- to a few hundreds of kilometers across) volcanic constructs to volcanic plains thousands of kilometers in extent. The CO2 atmosphere of Venus is dense and its pressure at the surface is nearly 100 times that of Earth (or equivalent to pressures at about 1 km depth in the sea). The high pressure inhibits gas exsolution, magma disruption, and pyroclastic volcanism. The surface temperatures are high enough (~770 K) to potentially increase the time of cooling of magmatic bodies in near subsurface and lava flows on the surface. In contrast to the terrestrial plate tectonics, a hot spot style of volcanism, which is characteristic of the intraplate areas on Earth, prevails on Venus. In this style, heat is released by advection in isolated volcanoes distributed over the surface; the majority of the heat is currently lost by conduction through the lithosphere. Two major classes of volcanic features have been identified on Venus. (1) Features in which effusive characteristics are most important are volcanic plains, edifices, steep-sided domes, and channels. Among these landforms, plains are the most important and make up ~76% of the surface of Venus. (2) Coronae, arachnoids, and novae represent magmatic features that are dominated by tectonic structures with relatively minor effusive characteristics. Strongly deformed (densely lineated and ridged plains), mildly deformed (shield and regional plains), and non-deformed plains (smooth and lobate plains) compose three major types of volcanic plains on Venus. Regional plains represent the most extensive unit that makes up about 40% of the surface of the planet. Volcanic constructs and channels usually associate with a specific type of volcanic plains: small volcanoes and most of the steep-sided domes belong to shield plains, lava channels characterize regional and lobate plains, large shield volcanoes make a significant portion of lobate plains. The main volcanic plains on Venus display consistent relationships of relative ages among each other at the global scale: shield plains postdate strongly deformed units and predate regional plains; lobate plains overlay structures of regional plains. These stratigraphic relationships consistent are consistent at the global scale and allow division of the visible portion of the geologic history of Venus into three regimes of resurfacing during which specific types of endogenous activity dominated. (1) The majority of the tectonized terrains (e.g., densely lineated and ridged plains) define the first, tectonically dominated, regime. During this time, large regions of thickened crust (tesserae) were formed; a limited contraction and possible underthrusting along specific zones resulted in formation of ridge and mountain belts. The later phases of the ancient tectonic regime were manifested by the mutual development of groove belts and many coronae. All tectonized terrains of the first regime represent local- to regional topographic highs in the background topography. (2) During the second, volcanically dominated regime, the vast plains such as shield plains and regional plains were emplaced preferentially in regional lows. The density of craters on regional plains suggests that the first two regimes (tectonic and volcanic) operated during about the first one-third of the observable history. (3) Contemporaneous lobate plains and rift zones define the third, network rifting-volcanism regime. This regime dominated the last two-thirds of the observable geologic history and is linked to the later stages of evolution of the dome-shaped rises.