page_title: Alba Mons
section_title: Geologic history
hierarchical_section_title: Alba Mons / Geology / Geologic history
caption_attribution_description: English: Lava flows and channels on NW flank of Alba Mons (THEMIS)
context_page_description: Alba Mons is a volcano located in the northern Tharsis region of the planet Mars. It is the largest volcano on Mars in terms of area, with volcanic flow fields that extend for at least 1,350 km from its summit. Although the volcano has a span comparable to that of the United States, it reaches an elevation of only 6.8 km at its highest point. This is about one-third the height of Olympus Mons, the tallest volcano on the planet. The flanks of Alba Mons have very gentle slopes. The average slope along the volcano's northern flank is 0.5°, which is over five times lower than the slopes on the other large Tharsis volcanoes. In broad profile, Alba Mons resembles a vast but barely raised welt on the planet's surface. It is a unique volcanic structure with no counterpart on Earth or elsewhere on Mars.
  In addition to its great size and low relief, Alba Mons has a number of other distinguishing features. The central portion of the volcano is surrounded by an incomplete ring of faults and fractures, called Alba Fossae on the volcano's western flank and Tantalus Fossae on the eastern flank.
context_section_description: Alba's well preserved lava flows and faults provide an excellent photogeologic record of the volcano's evolution. Using crater counting and basic principles of stratigraphy, such as superposition and cross-cutting relationships, geologists have been able to reconstruct much of Alba's geologic and tectonic history. Most of the constructional volcanic activity at Alba is believed to have occurred within a relatively brief time interval (about 400 million years) of Mars history, spanning mostly the late Hesperian to very early Amazonian epochs. Faulting and graben formation in the region occurred in two early stages: one preceding and the other contemporaneous with the volcano's formation. Two late stages of graben formation occurred after volcanic activity had largely ended.
  Based on Viking Orbiter images, the volcanic materials related to the formation and evolution of the volcano have been grouped into the Alba Patera Formation, which consists of lower, middle, and upper members. Members low in the stratigraphic sequence are older than those lying above, in accordance with Steno's law of superposition.
  The oldest unit (lower member) corresponds to the broad lava apron surrounding the Alba Mons edifice. This unit is characterized by sets of low, flat-topped ridges that form a radial pattern extending for hundreds of kilometers to the west, north, and northeast of the main edifice. The ridges are interpreted to be lava flows, although the flow margins are now degraded and difficult to delineate. Broad lava flows with flat-topped ridges are characteristic features of lava flood provinces on Earth (e.g., Columbia River basalt) that were formed at high eruption rates. Thus, the earliest phase of volcanic activity at Alba Mons probably involved massive effusive eruptions of low viscosity lavas that formed the volcano's broad, flat apron. Lava flows of the apron unit straddle the early Hesperian-late Hesperian boundary, having erupted approximately 3700 to 3500 million years ago.
  The middle unit, which is early Amazonian in age, makes up the flanks of the main Alba edifice and records a time of more focused effusive activity consisting of long tube- and channel-fed flows. Volcanic spreading occurred in a northward direction forming the two flanking lobes. (See Olympus Mons and Tharsis for a discussion of volcanic spreading on Mars.) Faulting and graben formation at Alba and Tantalus Fossae occurred penecontemporaneous with the lava flows. Any early explosive activity on the volcano may have occurred during the culmination of this middle phase of activity, which ended about 3400 million years ago.
  The youngest unit, also early Amazonian, covers the summit plateau, dome, and caldera complex. This period of activity is characterized by relatively short-length sheet flows and construction of the summit dome and the large caldera. This phase ended with an eastward tilting of the summit dome, which may have initiated additional graben formation in Alba Fossae. The last volcanic features to form were the small shield and caldera at the summit. Much later, between about 1,000 and 500 million years ago, a final stage of faulting occurred that may have been related to dike emplacement and the formation of pit crater chains.
caption: Lava flows with central channels on NW flank of Alba Mons. Note that the lava flows are crosscut by faults and graben, indicating that the faults are younger than the flows (THEMIS VIS).
exif: {}