What metaphor does Anderson use to depict the enormous anger that seemed to take over her rapist's body and why?

Anderson describes how she and her boyfriend were kissing on the bank of the creek. IT's hands started to wander and when Anderson protested, IT responded by flying into a rage. Anderson attempts to portray the danger and overwhelming nature of his rage by calling it an "avalanche of force" (54) that prevented her from doing anything to stop the attack.