Geology
Ascraeus Mons was built by many thousands of fluid basaltic lava flows. Except for its great size, it resembles terrestrial shield volcanoes like those that form the Hawaiian Islands. The flanks of Ascraeus Mons are covered with narrow, lobate lava flows and lava channels. Many of the lava flows have levees along their margins. Levees are parallel ridges formed at the edges of lava flows. The cooler, outer margins of the flow solidify, leaving a central trough of molten, flowing lava. Partially collapsed lava tubes are visible as chains of pit craters.
By examining the morphologies of lava flow structures on Ascraeus Mons, geologists are able to calculate the rheological properties of the lava and estimate the rate at which it poured out during eruption (effusion rate). Results show that the lava was highly fluid (low viscosity) with low yield strength, resembling Hawaiian and Icelandic basaltic lavas. Average effusion rates are about 185 m3/sec. These rates are comparable to those seen in Hawaii and Iceland. Earth-based radar studies show that Ascraeus Mons has a higher radar echo strength than other volcanic structures on the planet. This could indicate that the lava flows on the flanks of Ascraeus Mons consist of rough ʻAʻā-type flows, a conclusion supported by photogeologic analysis of lava flow morhpologies.
The flanks of Ascraeus Mons have a rumpled appearance caused by numerous low, rounded terrace-like structures arranged concentrically around the summit of the volcano. The terraces are spaced 30 to 50 km apart, have lengths up to 100 km, radial widths of 30 km, and heights of about 3 km. Individual terraces are not continuous around the volcano, but instead consist of arcuate segments that overlap with each other, forming an imbricate pattern. They are interpreted to be the surface expression of thrust faults that formed due to compression along the volcano's flanks. Flank terraces are also common on Olympus Mons and the other Tharsis shield volcanoes. The source of the compressive stresses is still debated. The flank terraces may be due to compressional failure of the volcano, flexing of the underlying lithosphere due to the volcano's massive weight, cycles of magma chamber inflation and deflation, or shallow gravitational slumping.
Fissures, or flank vents, at the southwestern and northeastern edges of the volcano are the sources of the lava aprons that spread out across the surrounding plains. The fissures seem to have formed by the merger of numerous, narrow rille-like depressions. In places, the depressions form sinuous channels with islands and other features suggestive of erosion by a fluid. Whether the channels were formed predominately by water or lava is still a topic of debate.
The caldera complex consists of a central caldera surrounded by four coalesced calderas. The central caldera measures about 24 km across and 3.4 km deep and is the youngest of the collapse structures. Crater counting indicates that the central caldera is about 100 million years (Myr) old. The surrounding calderas have ages of about 200, 400, and 800 Myr old, or earlier. A small, partly preserved depression southeast of the main caldera may be as old as 3.8 billion years (Gyr). If the dates are valid, then Ascraeus Mons may have been active through most of Mars' history.
An area of peculiar, fan-shaped deposits (FSD) lies on the volcano's western flank. The FSD consists of a zone of knobby terrain outlined by a semicircular zone of concentric ridges. Similar deposits are also found at the northwestern edges of Pavonis Mons and Arsia Mons, the other two of the Tharsis Montes. The FSD at Ascraeus Mons is the smallest of the three, covering an area of 14,000 km2 and extending outward from the volcano's base for about 100 km. The origin of these deposits has been debated for decades. However, recent geologic evidence suggests that FSDs are deposits left by glaciers, which covered portions of the volcanoes during a recent period of high obliquity. During periods of high obliquity (axial tilt) the polar regions receive higher levels of sunlight. More water from the poles enters the atmosphere and condenses as ice or snowfall in the cooler equatorial regions. Mars changes its obliquity from about 15° to 35° in cycles of 120,000 years.
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