Channels (especially large ones) are associated with 'a'a flows. Pahoehoe flows sometimes have small channels. Channels form because the margins of active flows experience much more cooling and friction against the ground; they eventually stagnate but meanwhile the central part continues to flow (e.g. Hulme 1974; Sparks et al. 1976).
There are many forms of channels, depending on the degree of distinctness between the levees and the moving centers. The front of an 'a'a flow often moves as a single body of lava and has no channel. The margins of 'a'a flows have a distinctive cauliflower-like shape when viewed from above, which is caused by small surges of lava during flow. As soon as the margins begin to stagnate, however, the velocity difference between the margins and the flow center becomes large enough so that the lava begins to shear to make up the difference. At this point the channel may be evident only by two shear planes parallel to the flow direction. Eventually the levees stop moving altogether and the central part of the flow forms a well-defined, distinct channel. In such a case all the relative motion between the stagnant levees and flowing channel interior is taken up in thin zones at the very edges of the channel, and this makes the channel more efficient. In fact, velocities within channels have been recorded in the range of 60-80 km/hour (remember, this is not the speed of the flow front). Overflows from the channel usually coat the levees and build them up higher than the level of flowing lava. This channel evolution progresses downflow as the flow advances, with the well-defined channel lagging behind the actual flow front (by ~1 km on the 1984 Mauna Loa flow; Lipman & Banks 1987). This means that an 'a'a flow can be quite different depending on where you look at it.