Geologic map of part of the Tyrrhena Patera region of Mars (MTM Quadrangle-20252)

Mars Transverse Mercator (MTM) -20252 quadrangle contains the volcano Tyrrhena Patera, a low, broad structure that has summit depressions and is dissected by prominent radial channels (fig. 1). Tyrrhena Patera is surrounded by cratered plains materials on the western edge of Hesperia Planum, ~2,000 km northeast of the center of the Hellas basin (Greeley and Guest, 1987; U. S. Geological Survey, 1991). On the basis of Mariner 9 data, Tyrrhena Patera was interpreted to be a shield volcano, composed of fluid basic or ultrabasic lavas (Peterson, 1978; Pike, 1978). Analysis of Viking Orbiter images led Greeley and Spudis (1981) to suggest that Tyrrhena Patera comprises pyroclastic deposits produced by explosive hydromagmatic eruption; Greeley and Crown (1990) support a pyroclastic origin for Tyrrhena Patera shield materials and assess several eruption styles for the volcano (see also Crown and Greeley, 1993).