Alba Mons - Physical Description

Physical Description

Alba's size and low profile makes it a difficult structure to study visually, as much of the volcano's relief is indiscernible in orbital photographs. However, between 1997 and 2001, 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 1,500 km (930 mi) by 1,000 km (620 mi) across surrounded by 2) a vast, nearly level apron of lava flows that extends an additional 1,000 km (620 mi) 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 350 km (220 mi) 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 170 km (110 mi) by 100 km (62 mi) across at the center of the summit dome. A smaller, kidney-shaped caldera (about 65 km (40 mi) by 45 km (28 mi)) lies in the southern half of the larger one. Both calderas are relatively shallow, reaching a maximum depth of only 1.2 km (3,900 ft).

The larger caldera is bounded at the westernmost end by a steep, semicircular wall 500 m (1,600 ft) 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 50 km (31 mi) across. It is capped by a peculiar concentric circular feature 10 km (6.2 mi) in diameter (pictured left).

Calderas form by collapse following withdrawal and depletion of a magma chamber after an eruption. Caldera dimensions allow scientists to infer the geometry and depth of the magma chamber beneath the summit of the volcano. The shallowness of Alba’s calderas compared to those seen on Olympus Mons and most of the other Tharsis volcanoes implies that Alba’s magma reservoir was wider and shallower than that of its neighbors.