A faster-flowing glacier. Normally, glaciers flow at 3–300 myr⁻¹, whereas a surging glacier may reach 4–12 kmyr⁻¹. Some glaciers always flow at surging speed; others surge periodically, suddenly accelerating to 10–100 times their normal velocity. Surging glaciers are closely crevassed, with compressional flow at the leading edge and extending flow at the rear. The net result is a deepening of the long profile of the glacier. Kamb et al. (1985) Science 227 and Kamb (1987) J. Geophys. Res. 92 suggest that surging is initiated by a switch from a tunnel system to a distributed system that facilitated sliding. The surge will continue as long as high water pressure sustains rapid sliding.