Why do lava flows stop advancing?
The following is this week’s edition of “Volcano Watch” from the USGS Hawaiian Volcano Observatory:
Much scientific study has been done to determine what types of eruptions produce long lava flows. In the 1970s, a very simple idea seemed to guide future work—the length of a lava flow is limited by either the supply of lava or by how well the lava is thermally insulated during its transport to the flow front.
Supply-limited lava flows are those that stop advancing when the vent closes down or eruptive activity moves to another location, cutting off the lava supply. This is a fairly simply idea that’s easy to visualize—the lava flow lasts as long as its vent erupts. The best examples of supply-limited lava flows in Hawaiʻi are from Mauna Loa. Lava flows that advanced on Hilo in 1855, 1881, and 1984, eventually stalled above the town as their respective eruptions waned and the supply of lava to the flow stopped.
But what about vents that keep erupting, like Puʻu ʻŌʻō? Are they capable of producing lava flows that get longer and longer? It depends on how well the lava is thermally insulated as it’s transported through the flow’s tube system.
Solidified lava is an excellent thermal insulator. As lava forms a surface crust, the flow progressively insulates its internal plumbing and eventually forms a lava tube system. That tube system transports lava from the vent to its leading edge, and, as long as the lava doesn’t lose too much heat, the flow will continue to advance.
Over the past three decades, lava flows erupted at Puʻu ʻŌʻō have formed some robust tube systems that have delivered lava to ocean entries for months—or up to a few years—before changes at the vent caused the tube to be abandoned. If those flows hadn’t encountered a coastline, the lava might have continued to advance for a much greater distance.
The Kazumura lava tube system is a particularly good example of how far lava can travel. The tube is within the ʻAilāʻau lava flow field that erupted from vents near the summit of Kīlauea 550–600 years ago and sent flows down the northeast flank of the volcano and into the sea. Kaloli Point was built by an ocean entry during this eruption, which is estimated to have lasted more than 50 years. The Kazumura lava tube system is more than 65 km (40 miles) long, and the part of it not filled with lava is now listed as the longest lava tube cave in the world.
Within this context, why did the June 27th lava flow slow as it approached Pāhoa? HVO scientists noted that the lava flow began to slow considerably as the summit of Kīlauea began to deflate on Thursday, September 18. Such deflations have caused decreases in eruptive output at Puʻu ʻŌʻō in the past, and that could be the case with the current flow as well.
Another possibility for the stalled flow front is that the tube system feeding the June 27th lava flow could be approaching its limits in terms of efficiently insulating the lava moving through it. Based on HVO observations during our September 24 overflight, the lava tube system seems fairly robust from the vent at Puʻu ʻŌʻō to just before the point where lava first flowed into a crack. The exact condition of the lava tube within the crack is unknown because we can’t see into it, but it seems to also be robust.
However, the nature of the tube system from the point at which the lava flow exits the ground crack system near Kaohe Homesteads and heads northeast toward Pāhoa is not yet clear. Although lava has traveled beneath this section of the flow to the flow front since early September, a lava tube system is not yet evident.
The June 27th lava flow is now longer than any other flow formed during the ongoing eruption of Puʻu ʻŌʻō. While the evidence favors a decrease in eruptive output as the cause of the current slow-down, careful study is still needed. If and how a lava tube develops within the distal portion of the June 27th lava flow is important to understanding just how far it, and other pāhoehoe flows like it, is capable of advancing.
It’s important to note that although the flow front has slowed, the June 27th lava flow remains active. Puna residents are encouraged to stay informed about the flow’s status and progress through daily updates posted on HVO and Hawaiʻi County Civil Defense websites (http://hvo.wr.usgs.gov and http://www.hawaiicounty.gov/active-alerts/).