mech3.gif. Diagram of an channel. The yellow planes are meant to illustrate how the central part of the channel moves faster than the sides. Torques (curved arrows) are set up by this velocity gradient, and continuously disrupt any surface crust that may form. For example, note that points A and B could rotate in different directions and pull the lava between them apart (adapted from Rowland & Walker 1990).

VolcanoWorld