VOLCANISM ON MARS: NEW PERSPECTIVES FROM MOLA TOPOGRAPHY

Altimetric data from the Mars Global Surveyor MOLA instrument have considerably revised our view of the history and style of volcanism on Mars. In Tharsis, detailed topography has confirmed the role of effusive volcanism centered in Syria Planum in the Noachian as the major focus of early Tharsis activity, also involving later lateral movement and deformation, and ending with extrusion of over 100 small summit shield volcanoes. Similarities exist between the edifice building style of Syria Planum and Alba Patera. Later Tharsis activity is centered on the Tharsis Montes and altimetry data show the contrast in edifice development; early broad shields give way to flanking rift zones centered at neutral buoyancy zones within the edifices and extensive aprons of flows. The episodic nature of activity in Tharsis Montes is shown by sequential caldera morphology and the presence of radial graben extending thousands of km off the rise and interpreted to be due to dike emplacement during buffered magma supply conditions. Previously undetected small shields are scattered around Tharsis and concentrated in Ceraunius Fossae and Tempe Terra. The new MOLA data amply illustrate the relationships between volcanic plains and outflow channel formation. MOLA data also provide evidence for the presence of Hesperian ridged plains flooring the northern lowlands, extending considerably the importance of Early Hesperian volcanism and potential contributions of volatiles to the atmosphere. Documentation of the nature of Hesperian volcanic centers in Syrtis, Hesperia, and Malea Planum further show their distinctive nature and their contrast to later centers in Tharsis and Elysium. Activity at Elysium continued into the Late Amazonian and includes radial dike emplacement and release of groundwater to form mega-lahars in Utopia and lava-floodwater interactions in Elysium and Amazonia Planitiae. Abundant evidence exists for a wide range of interaction of ascending magma and the global cryosphere, including candidate subglacial volcanoes near the south pole.