Channelized Dacite Flow

A crystallized dacite flow in northern Chile. Dacite is extrusive and the volcanic equivalent of granodiorite. 

If lava has the right viscosity, it can travel across a landscape via channels. The lava either forms the channels itself or uses a preexisting one. Along the same vein, lava tubes are essentially channels that reside underground and also allow lava to move quickly. Tubes form one of two ways. A lava channel can form an arc above it that chills and crystallizes, or an insulated pahoehoe flow can have lava still running through it while outer layers freeze. Lava tubes, by their nature, are buried. However, skylights form when the lava tube collapses in a specific area and allow one to see the flow inside the tube. Tubes can collapse completely and become channels, drain out, or get blocked up.

In terms of Mars, channels are more readily found and recognized. Lava tubes are more obscure and might not be identified unless they have some inflation on the surface or have a skylight.