#gravity current

One of the deadliest features of some volcanic eruptions is the pyroclastic flow, a current of hot gas and volcanic ash capable of moving hundreds of kilometers an hour and covering tens of kilometers. Since volcanic particles have a high static friction, it’s been something of a mystery how the flows can move so quickly. Using large-scale experiments (top), researchers are now digging into the details of these fast-moving flows.

What they found is that the two-phase flow results in a pressure gradient that tends to force gases downward. This creates a gas layer with very little friction near the bottom of the pyroclastic flow (bottom), essentially lubricating the entire flow with air. This helps explain why pyroclastic flows are so fast and long-lived despite their inherent friction and the roughness of the terrain over which they flow. (Image and research credit: G. Lube et al.; video credit: Nature; submitted by Kam-Yung Soh)

Kilauea continues to erupt without signs of abating. Aerial video, like this footage from Mick Kalber, shows the scope of the flow. Lava spurts like a hellish fountain from various fissures, then forms a gravity current that slowly flows downhill toward the ocean. Some of the angles give you an excellent view of the texture atop the flowing lava; it looks relatively rope-like now before solidification, indicating pahoehoe flow. Whether the flow will transition to the rougher appearance of a’a lava remains to be seen; as the lava cools and crystallizes, it may develop a yield strength. That would make it similar to fluids like your toothpaste, which only flow once a critical force is applied. Stay safe, Hawaiians! (Image and video credit: M. Kalber; via Colossal)