Morphology and emplacement of a long channeled lava flow near Ascraeus Mons Volcano, Mars

Channeled lava flows, hundreds of kilometers long, are common on the lower flanks of the Tharsis Montes on Mars. Our analysis of a 690- km- long lava flow along the southwest perimeter of Ascraeus Mons shows that it was emplaced on low local slopes (< 0.3 degrees), with a deep channel ( similar to 20 m), and at high effusion rates ( 19,000-29,000 m(3)/ s) calculated from the Graetz number. These parameters are similar to conditions needed to yield rapidly emplaced terrestrial flows > 100 km in length, but the maximum effusion rates necessary on Earth are essentially the minimum for Martian flows. On the basis of our calculated effusion rates, the eruption duration was 3 to 7 Earth months, assuming a constant effusion rate and continuous eruption. The morphology of the Ascraeus Mons flow shows similarities to terrestrial and simulated channeled flows. Downstream changes in morphology resemble those observed in the 1907 flow, Mauna Loa Volcano, Hawaii and channeled polyethylene glycol ( PEG) flows. Braided sections of the channel in the Ascraeus Mons flow contain islands which are hundreds of meters across and resemble features observed in the 1907 and 1984 flows on Mauna Loa Volcano. Crosscutting relationships suggest islands in the proximal section were shaped by thermal and mechanical erosion, whereas islands in the medial section are inferred to be material rafted by surges of lava through the channel. Overall, understanding the morphology of long lava flows on Mars is essential to the interpretation of their emplacement and constraining eruption conditions in the saddle regions of the Tharsis volcanoes.