Volcanic flows
Complex channel, Outflow channel
The most intricate kind of lava channel. It has streamlined islands and braided, branching reaches, down which fluid lava may have flowed like a flood.
Kallistos Vallis was first called an outflow channel because it resembles the outflow channels of Mars (Baker et al. 1992, 1997). It starts in a collapsed source area. It then winds a long way, branching at times and forming streamlined islands. It ends in a branching delta, where extensive, lobate lava flows lie beyond the channel banks (see the plate).
Complex channels are 100–1000 km long and more than 1 km wide.
Their complexity can show in two ways. Branching or braided reaches around streamlined islands may alternate with simple reaches. Or lobate lava flows may line their “floodplain” beyond the banks.
On Earth, similar forms are found in outflow channels from ice-dammed floods. They also occur in several carbonatite or basaltic volcanic channels (Ol Doinyo Lengai, Hualālai).
The complex channels of Venus are certainly volcanic. Leverington (2007) argues that this supports a volcanic origin for the outflow channels of Mars, too. The debate is not closed. Most researchers think that the Martian channels were cut by water floods, though lava may also have flowed in them at times.
Other complex channels on Mars also have forms like those on Venus. One example is an intricate channel system that starts sideways from a fissure, unlike sinuous rilles. Such channels are found on lava fields east of Olympus Mons that are barely 100 million years old. There, too, it is undecided whether water, lava or both shaped them (Hargitai et al. 2018).
For Venus, the main questions are these. What was the composition of the lava that flowed in these channels? How fast did it flow (at what discharge), and for how long? And why in exactly these areas?
