This is Pahoehoe lava flow from at Craters of the Moon National Park in Idaho. Pahoehoe lava is very hot and fluid. This lava was starting to cool as it flowed. Its surface began to thicken and was folded by the moving liquid underneath. As it continued cooling, it hardened into this formation.

Pahoehoe is Hawaiian for "smooth, unbroken lava". Much of the groundwork for volcanic science has been done with the volcanoes in Hawaii, so many of the geologic terms are Hawaiian.