This volcano is comparable to Olympus Mons in horizontal extent but not in vertical relief; recent topographic maps show a total vertical relief of less than 7 km (4.2 mi) for Alba Patera. Just as the basal scarp is unique to Olympus Mons, Alba Patera is surrounded by a distinctive set of enormous fractures having a predominantly north-south orientation. The fractures have apparently been diverted around the central part of the Alba Patera complex. The low relief of Alba Patera may be due to the collapse of a structure comparable to the Olympus Mons shield but there is no direct evidence that such a collapse actually occurred. An alternative explanation for the low relief is that Alba Patera may include a significant portion of pyroclastic materials (ash) from its early history, caused by eruptions that are more explosive in nature than those typically associated with the effusion of lava flows. Alba Patera is older than the shield volcanoes in the Tharsis region and may represent a change in the style of centralized volcanic eruptions on Mars. The arrow indicates the lava flows shown in slide #10. (40 N, 110 W; Viking Orbiter frames 783A11 to 16, high-pass spatially filtered orthographic version.)

Alba Patera

A color image of the Alba Patera region of Mars; north toward top. The scene shows a central circular depression surrounded by splays of fractures, named Alba Fossae (west of Alba Patera) and Tantalus Fossae (east of Alba Patera). A patera (Latin for shallow dish or saucer) is a volcano of broad areal extent with little vertical relief; a fossa is a linear depression. This image is a composite of Viking medium-resolution images in black and white and low- resolution images in color. The image extends from latitude 30 degrees N. to 50 degrees N. and from longitude 95 degrees to 125 degrees; Lambert projection. Alba Patera has a 100-km-diameter caldera at its center surrounded by a fracture ring. In total, the approximately 1,200- km-diameter Alba Patera far exceeds any other known volcano in areal extent; it covers eight times the area of Olympus Mons (the highest volcano in the Solar System) but reaches only about 6 km in height. The patera lies directly north of the Tharsis bulge, which encompasses the most intensely and most recently active volcanic region of the planet. The fossae of the Alba area are fault-bound graben that can be traced south through the Tharsis bulge and therefore likely formed by upwarping of the Tharsis bulge as well as the coeval upwelling of Alba Pateria magma. (Original Caption Released with Image)