Small Volcanic Vents and Vent Fields on Venus

Small (less than 20 km in diameter), shield-type volcanoes are the dominant volcanic edifices on Venus. They are interpreted to represent two fundamental types of vent distribution: (1) shield fields, clusters of small volcanoes distributed over a quasi-circular region of modal diameter from 100 to 150 km, some with associated volcanic material; and (2) shield plains, or shield terrain, a type of surface consisting of small shield volcanoes distributed nearly uniformly over regions that extend for thousands to millions of square kilometers. These two types of vent distribution may represent different styles of effusion and different temporal associations in Venus' geologic history, in turn associated with fundamentally different melt source depth and different mechanical properties of ascent pathways.

Shield fields have been considered to be a distinct type of volcanic center on Venus, arising from melt areas of limited extent, possibly deep magma sources, and low magma rates delivered to the surface; this is essentially the current interpretation of terrestrial volcanic fields with clustered monogenetic vents. Globally, shield fields occur that are stratigraphically older, stratigraphically younger, and apparently contemporaneous with, the surrounding plains. It is probable that the formation of shield fields has occurred locally throughout Venus' geologic history.

Shield plains may be analogous to terrestrial oceanic seamounts; volcanism associated with a widespread melt source and relatively shallow magma sources. The unit is interpreted as formed by the eruption of multiple small shields and associated flows over some finite geologic time interval; an interval that may have been geologically short or long. In its type locality in Akkruva Colles and in many other locations where it has been mapped, the unit occurs in a distinct and consistent stratigraphic position. Shield plains are evidence for a unique interval and mechanism of plains formation over many areas of the surface of Venus.