Tyrrhena Patera is a large, low-relief volcano located in the southern cratered highlands of Mars on a proposed ring fracture of the Hellas basin. The volcano is ccomposed of four or possibly five principal units, the oldest and most areally extensive of whicch are suggested to be ash on the basis of their morphology andd erosional characteristics. Lava flows are observed in a flank flow unit extending from the central caldera to the southwest and apparently cap the summit region. Models derived from the study if terrestrial pyroclastic flows have been applied to the deposits at Tyrrhena patera and indicate that the distribution and lengths of the observed deposits are cnsistent with an origin by the emplacement of gravity- driven ash flows generated by hydromagmatic or magmatic explosive eruptions. The hydromagmaticc case requires considerable quantities of water (~7.5¿ 1016 kg); however, at volumertricc water flow rates determined by other investigators (~105 m3/s) and calculated from the regional slopes in the Hesperia Planum region (~106 m3/s), the amounts necessary could be supplied relatively in geologic time. Tyrrhena Patera represents the earliest central vent volcanism identified on Mars and may reflect a transition from the flood-style eruptions which dominated early Martian history. A hydromagmatic origin for Tyrrhena Patera is in agreement with suggested climatic changes on Mars and may explain the absence of this style of volcanism in later eras. Alternatively, a magmatic origin for Tyrrhena Patera impliles an evolution in the types of magmas produced on Mars from Hesperian to Amazonian time, during whicch the prominent shields in the Tharsis region formed. ¿ American Geophysical Union 1990