Weathered alkaline lavas, Iceland. The volcanic rocks in the vicinity of Hellnar on the Snaefellsnes peninsular are brittle alkaline lavas, benmoreite and mugearite. Freeze-thaw weathering happens when water enters fissures and cracks in rock. When this water freezes it expands and enlarges the crack. When the ice thaws the wider crack allows more water to penetrate and the cycle continues, breaking up the rock into flakes.