Ravi Vallis is a Martian outflow channel that is widely interpreted as a product of one or more catastrophic outbursts of groundwater from adjacent Aromatum Chaos. However, solar system analogs are unknown for formation of large channels by outbursts from aquifers, and the Ravi Vallis region lacks the obvious sedimentological and mineralogical signatures expected of aqueous origins. Instead, the basic nature of this channel system is consistent with volcanic origins involving voluminous effusions of low-viscosity lavas. On bedrock slopes of only 0.2° 50-m-deep lava flows with viscosities of 1 Pa s are predicted to have been characterized by discharge rates of ∼3 × 10 7 m 3 /s for 25-km-wide flows, with associated mechanical incision rates of ∼4 m/day and thermal incision rates of ∼2 m/day. Formation of Aromatum Chaos and the preserved 215-km-long extent of Ravi Vallis is conservatively estimated on the basis of thermal considerations to have required eruption of a minimum of ∼64,000 km 3 of lava, but greater volumes would have been necessary to form the original (unknown) length of this channel system. A volcanic origin for Ravi Vallis is consistent with development of all Martian outflow channels by igneous processes, and with the broader premise that early development of immense volcanic channel systems is typical of all large rocky bodies.