lava flow

AA - type lava flow 
advancing over pahoehoe,
Kilauea Volcano, Hawaii

Photo Credit: USGS 
Used with permission

Pahoehoe-type lava flow

Photo Credit: USGS
Used with permission

Lava flows are streams of molten rock material extruded onto the surface of the earth. The physical characteristics of lava flow depends on the chemistry of the lava, slope, duration of flow and the discharge from the vent. An AA-type lava flow (a Hawaiian term pronounced "ah ah") has a rough, rubbley surface and flows across the surface in a roller-like motion.

Pahoehoe (pronounced "pah hoy hoy") has a smooth glassy, ropy appearance and advances the across the surface in a series of overlapping lobes.