We’re not sure if this channel-inside-a-channel was carved by flowing water or lava. Flowing water erodes channels, and flowing lava both erodes and melts surrounding rock to form channels.
It’s not clear whether a huge surge of water or lava first formed the wide channel and then subsided into a trickle to form this narrow, inner channel, or if a trickle formed the inner channel and a subsequent surge formed the wider one. Detailed analysis of the shape could reveal… [More at link]