December 6, 2007
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Can Hawaiian lava flows be diverted?
Whenever lava flows threaten communities, a commonly asked question is "Can lava flows be diverted if populated areas are threatened?" Answers have been debated in Hawai`i for decades, and the current eruption has raised the question again.
Engineered lava flow diversion in Hawai`i has a short history, possibly starting with some unsuccessful wall-building in the Kukuau area of Hilo to control the advance of a Mauna Loa pahoehoe flow in 1881 and in Puna to control Kilauea lava flows in 1955 and 1960. Aerial bombing was tried on Mauna Loa flows advancing on Hilo in 1935 and in 1942 without success. Water quenching was tried unsuccessfully in 1989 as a pahoehoe lava flow advanced on the Hawai`i Volcanoes National Park Waha`ula Visitor Center.
The technology of lava diversion is pretty simple and capitalizes on observations of the way lava flows behave. Strategically placed barriers on sloping ground could, theoretically, deflect lava flows from highly developed areas into less developed areas. Water-quenching could, theoretically, freeze an advancing lava flow front and deflect the molten portion behind it onto another path. Explosives could, theoretically, be used to breach established lava tubes, channels, or vent walls to deflect lava onto different paths and rob the threatening flow of its supply.
There have been some successful attempts at lava diversion elsewhere in the world. The most well-known and successful water-quenching effort was the campaign to save Vestmannaeyjar harbor at Heimaey Island, Iceland. A thick, blocky, slow-moving lava flow from an eruption of Eldfell volcano in January 1973 threatened to close off the harbor, a nationally important resource.
When the eruption ended more than five months later, the harbor entrance had been preserved. The effort required the pumping of about 6 million cubic meters (1.5 billion gallons) of seawater, which solidified an estimated 4 million cubic meters of molten lava (1 billion gallons) (of the 250 million cubic meters of lava and ash produced during the eruption) at a cost of $1.5 million. The effort succeeded primarily because the lava flow was slow-moving, allowed time to act, advanced along the coastline (where seawater was readily available), and lasted only five months.
Two other important lava diversions were attempted at Mount Etna on the island of Sicily. The earliest documented lava diversion was in 1669, when a channel feeding a lava flow headed toward the city of Catania was artificially breached by a group of Catania townsfolk. The effort was technically successful, but the celebration was short-lived, because the diverted flow headed for the town of Paterno, whose people were not pleased. The Paterno folk then prevented the Catania folk from maintaining their artificial breach, which sealed up. The flow continued toward Catania and produced substantial destruction.
After a number of attempts in intervening years, the most recent success came in May 1992. Lava flows from the 1991 eruption of Mount Etna were threatening the town of Zafferana. Barriers and explosives were used in several attempts to save the town, but none successfully halted or deflected the flow. Success was finally achieved by an explosive-induced breach of a channel, which diverted about two-thirds of the supply into an artificial trench. Dumping blocks into the channel stopped up the remaining channel flowage, achieving 100 per cent diversion.
Can lava diversion be used successfully in Hawai`i? The answer is pretty clear: only under optimum circumstances. Quenching requires so much water that it may be practical only near the coast. Successful barriers and/or explosive-induced breaching would be practical only on flows that advance slowly enough to allow time for their planning and execution. The 1669 experience at Etna showed that any diversion requires a suitable place to put the lava.
Diversion during a short eruption has better chances at succeeding than during a long eruption. The 1973 Eldfell and 1992 Etna diversions may not have succeeded had their respective eruptions continued. The 1669 Etna diversion may have been judged successful had the eruption stopped before the flow reached Catania.
Cultural concerns are also important in Hawai`i. The Hawaiian culture has views on volcanism that differ from those of other cultures. Many Hawaiians were disturbed by the bombing of Mauna Loa lava flows in 1935 and 1942 and by inevitable discussions on the use of bombs for any lava diversion effort; they believe the practice to be offensive to Pele, the Hawaiian volcano deity.
Any hazard mitigation effort must be looked at in terms of its total cultural, social, and economic impact. Scientists at the Hawaiian Volcano Observatory can advise on many of the technical aspects of volcanic or earthquake hazard mitigation, but the decisions on which course of action to take rest with officials of the County and State of Hawai`i.
The July 21 eruption continues to supply lava from eruptive fissure D, 2.3 km (1.4 mi) northeast of Pu`u `O`o. For the last several months this lava was directed entirely into a perched channel, consisting of separate pools often separated by bridges of cooled lava. At dawn on November 21, lava began to erupt directly from fissure D, outside of the perched channel, creating the Thanksgiving Eve breakout (TEB) flow. Lava supply to the original perched channel has been mostly redirected through this new outlet, cutting off supply to the eastward tube which had been feeding flows in the vicinity of Pu`u Kia`i through much of November.
The TEB flow over the last two weeks has reached a maximum along-flow distance of approximately 2 km (1.2 mi) from the vent. The flow extends southeast along the south flank of Kupaianaha for approximately 1.5 km (0.9 mi) and turns northeast for 0.5 km (0.3 mi) along Kupaianaha's east flank. During the most recent overflight, it was observed that the TEB flow is expanding laterally, and building vertically, with most activity limited to within 800 m (870 yards) of the breakout point. The shield at the TEB outlet has grown vertically by about 9 m (30 ft) over the last two weeks. The original perched channel has been intermittently active since the TEB flow began, with alternating overflows and lava level drops in Pond 1 indicating repeated filling and draining.
At Pu`u `O`o, no incandescence has been seen on the Webcam at night since August. The heavy fume coming from Pu`u `O`o completely obscures any view into the crater. As in years past, Pu`u `O`o likely is serving as a large chimney, beneath which lava is stored briefly and degassed substantially enroute to the erupting fissure. Sloughing of Pu`u `O`o into its own crater since late August has left numerous fresh cracks on the north rim and south flank of the cone.
Mauna Loa is not erupting. No earthquakes were located beneath the summit in the past week. Extension between locations spanning the summit, indicating inflation, continues at steady, slow rates, which have slowed further since May 2007.
One earthquake beneath Hawai`i Island was reported felt within the past week. A magnitude-3.9 earthquake occurred at 6:37 p.m., H.s.t., on Thursday, November 29, 10 km (6 miles) east of Holualoa at a depth of 29 km (18 miles).
Updated: December 10, 2007 (pnf)