SMALL VOLCANOES ON VENUS: SHIELD FIELDS AND SHIELD PLAINS

Two different styles of concentrated small volcanoes have been identified on Venus: (1) shield fields, concentrations of small volcanoes within a limited, quasi-circular region, suggested to be comparable to terrestrial volcanic fields or Martian shield fields (melt areas of limited extent and low magma rates, delivered to the surface) and mapped stratigraphically throughout Venus geologic history; and (2) shield plains, small shield volcanoes distributed relatively uniformly over large regions, with associated eruptive material, suggested to be analogous to the Snake River Plains or terrestrial oceanic seamounts (volcanism associated with widespread melt sources during a restricted, specific time period). The shield plains identified in many Venus quadrangles show consistent stratigraphic relationships globally, and represent an unusual style of resurfacing with a major peak of small shield volcanic activity at the end of, or after, the formation of the oldest geologic unit on Venus and prior to the formation of the vast regional (ridged) plains.

New mapping of the shield plains in its originally defined location in Vellamo Planitia in the northern hemisphere of Venus, from 25°-50°N and 90°-150°E., has confirmed that the unit maintains consistent stratigraphic relationships across an extensive area, overlays ridges and fractures associated with older units, and shows a near absence of organized patterns or alignments of shields, indicating that each shield formed independently of surrounding shields with no apparent pervasive structural control.

The significance of the shield plains in Venus geologic history is still unclear. Are the shield plains a local/regional stratigraphic unit, as some researchers have proposed, or are they a global stratigraphic marker unit as other researchers have proposed. Could the shield plains, for example, represent part of proposed mechanisms for the cyclical resurfacing of Venus? More detailed mapping of regions on Venus will help add evidence, but in order to definitively understand the significance of the shield plains unit, some way to assign absolute stratigraphic time on Venus is required.