VOLCANIC GEOLOGY OF HADRIACA-PATERA AND THE EASTERN HELLAS REGION OF MARS

Hadriaca Patera is a low-relief volcano in the southern highlands of Mars northeast of the Hellas basin. Layered, friable deposits composing the extensive channeled flanks of the volcano surround a well-defined, summit caldera containing late stage eruptive products. The morphologic characteristics of the channels suggest erosion by groundwater sapping and surface runoff. The erosional morphology of the volcano, the lack of lava flow features, and the friable nature of the flank materials indicate that Hadriaca Patera consists predominantly of pyroclastic deposits. Gravity-driven flow models demonstrate that the distribution of flank materials can be attributed to the emplacement of pyroclastic flows. Both magmatic and hydromagmatic eruption models are viable: For the magmatic case, the necessary mass eruption rates (10(7)-10(8) kg/s), ejection velocities (greater-than-or-equal-to approximately 400 m/s), and volatile contents (approximately 1.5-3.0 wt % H2O) are consistent with parameters derived for terrestrial Plinian eruptions; for the hydromagmatic case, the required energy conversion efficiencies are comparable to those of laboratory experiments, and the inferred permeability of the Martian crust allows large amounts of groundwater to be transported rapidly (at flow rates of 10(3)-10(4) m3/s) into the region. Models of cooling during emplacement indicate that welding of pyroclastic flows can occur at large distances (hundreds of kilometers) from a source vent on Mars; the layering within Hadriaca Patera could be attributed to welding of pyroclastic flows that would control its susceptibility to erosion. Morphologic similarities between Hadriaca and Tyrrhena paterae suggest a similar volcanic history, with an early pyroclastic-dominated, shield-building phase followed by effusive eruptions at their summit calderas and on the flank of Tyrrhena Patera. The formation of the extensive ridged plains of Hesperia Planum following the formation of the highland paterae supports the interpretation of a transition from explosive to effusive volcanism in the eastern Hellas region. A progressive depletion of volatiles on Mars is consistent with the morphologic properties of highland paterae and other central vent volcanoes. A predominance of hydrovolcanic eruptions in the development of Hadriaca and Tyrrhena paterae would imply that the transition in volcanic eruption style can be attributed to a volatile depletion of the crust, whereas magmatic eruptions at the paterae would be indicative of temporal changes in Martian magmas.