When magma is erupted from a volcano, it moves down the slope of the volcano and surrounding topography as it cools. Lava can flow for distances of over 100km, and at speeds near 30 miles per hour. Depending on the chemistry, viscosity, and eruption style, lava flows can have very different and distinct appearances.

Pahoehoe is a smooth, ropy lava, common on the Hawaiian islands. 'A'a is a more chunky and thick flow, that is most commonly associated with composite volcano eruptions. 'A'a lava flows are also called clinkers, because of their rough and jagged appearance.