the Mars Orbital Laser Altimeter (MOLA) instrument of the Mars Global Surveyor spacecraft took over 670 million precise elevation measurements across the planet. Using MOLA data, planetary scientists are able to study subtle details of the volcano's shape and topography that were invisible in images from earlier spacecraft such as Viking. The volcano consists of two, roughly concentric components: 1) an oval-shaped central body with approximate dimensions of by across surrounded by 2) a vast, nearly level apron of lava flows that extends an additional or so outward. The central body is the main topographic edifice of the volcano, marked by pronounced break in slope at the inner boundary of the apron. Extending east and west from the central edifice are two broad fan-shaped lobes (or shoulders), which give the volcano its elongation in the east-west direction. The central edifice has the steepest slopes on the volcano, although they are still only 1°. The crest and upper flanks of the edifice are cut by a partial ring of graben that are part of the Alba and Tantalus Fossae fracture system. Inside the ring of graben is an annulus of very low and in places reversed slopes that forms a plateau on top of which lies a central dome across capped by a nested caldera complex. Thus, the central edifice of Alba Mons resembles a partially collapsed shield volcano with a smaller, summit dome sitting on top (pictured right). The summit dome has a distinct tilt to the east. The caldera complex consists of a large caldera about by across at the center of the summit dome. A smaller, kidney-shaped caldera (about by ) lies in the southern half of the larger one. Both calderas are relatively shallow, reaching a maximum depth of only . The larger caldera is bounded at the westernmost end by a steep, semicircular wall tall. This wall disappears at the northern and southern sides of the caldera, where it is buried by volcanic flows originating from the younger, smaller caldera. The smaller caldera is outlined everywhere by a steep wall that varies in height over a range of a few hundred meters. The walls of both calderas are scalloped, suggesting multiple episodes of subsidence and/or mass wasting. Two small shields or domes, several hundred meters high, occur within and adjacent to the large caldera. The shield within the large caldera is about across. It is capped