As soon as lava flows from a volcano, exposure to air and wind causes it to start to cool and harden. Rather than hardening evenly, the energy exchange tends to take place primarily at the surface. The cooling causes a crust to form on the outer edges of the lava flow, insulating the molten lava within. This hardened lava shell allows a lava flow to travel much further than it would otherwise, while cracks in the lava's crust can cause it to draw up short.