Lava flows like these Hawaii’an ones are endlessly mesmerizing. This type of flow is gravity-driven; rather than being pushed by explosive pressure, the lava flows under its own weight and that of the lava upstream. In fact, fluid dynamicists refer to this kind of flow as a gravity current, a term also applied to avalanches, turbidity currents, and cold drafts that sneak under your door in the wintertime. How quickly these viscous flows spread depends on factors like the density and viscosity of the lava and on the volume of lava being released at the vent. As the lava cools, its viscosity increases rapidly, and an outer crust can solidify while molten lava continues to flow beneath. Be sure to check out the full video below for even more gorgeous views of lava. (Image/video credit: J. Tarsen, source; via J. Hertzberg)
A lake being completely drained by a lava tube. A lava tube is a channel formed by flowing lava which moves beneath the hardened surface of a lava flow. Tubes can be actively draining lava from a volcano during an eruption, or it can be a long, cave-like channel because the rock has cooled.