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July 8, 1999

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Coastal steam plume likely but not constant sight

Visitors to the Big Island's southeast coast commonly see a steam plume, the telltale sign that Kilauea's eruption is sending lava into the ocean. The plume results from the boiling of seawater by the hot lava ( about 1,100 degrees Celsius, 2,000 degrees F). Although typically present, the plume is absent whenever the supply of lava to the coast is interrupted. Lengthy extinctions of the steam plume coincide with major changes at the eruption site. During these times, lava flows are forging new paths to the sea, and all lava is inland from the coastline. Lava of the current eruptive episode, episode 55, reached the ocean in July 1997. Since that time, short pauses in the eruption have occurred on 14 occasions. These pauses have ranged from 11 to 95 hours in length. In each case, supply of lava to the tube system is interrupted. Consequently, the plume vanishes. The interruption in supply of magma that leads to plume extinction is a fundamental part of the volcano's rift-system tectonics. Closing or blocking of subterranean cracks prevent magma from escaping from the Pu`u `O`o vent. At the Hawaiian Volcano Observatory we recognize immediately the beginning of some of these pauses because our summit tiltmeters show abrupt, sharp inflation of the summit magma reservoir. In other cases, however, the pause onset is subtle. In these instances, the tilt record can be interpreted only after the telltale plume has gone extinct. Thus the plume reports relayed to us are an important part of monitoring the progress of the eruption.

Plume extinction lags a few hours behind the beginning of a pause, because lava already in the tube must drain into the ocean. Observations provided to us by visitors and by the National Park staff suggest that the plume begins to diminish in as few as 3 hours and becomes fully extinct 5 or 6 hours after a pause begins. Once a pause is over and the eruption resumes, the plume may take days to reappear, depending on how quickly lava can reoccupy the tube, force any obstructions from its route, and reach the ocean. During that time, surface flows abound as lava leaks from the tube. Conceivably, lava supply to the coast could be interrupted in another way, by major obstruction within the tube system itself, downslope from the vent. This mechanism, however, has never been the cause of plume extinction. Obstructions in the tube do occur after pauses in magma supply--after the tube has been drained and lies empty. And small obstructions may occur periodically during normal flow, which may explain some breakouts of lava from the tube system. But these obstructions are too small to cause complete blockage of tube-bound lava flowing to the coast.

What is the current chance of walking to the coastal lava entry and not seeing a plume? The answer is about 4 percent of the time. Since lava renewed its ocean entry in July 1997, eruptive pauses have encompassed about 445 hours, or from 2 to 3 percent of the total eruption time. The slightly greater percentage of plume absence results from delayed clearing of the tube system, which stalls the rebirth of the telltale plume.

Eruption Update

Eruptive activity of Kilauea Volcano continued during the past week with lava erupting from Pu`u `O`o and flowing through a network of tubes from the vent to the sea near Kamokuna. A partial bench collapse occurred between 9:00 p.m. on Friday, July 2, and noon on Saturday, July 3. The public is reminded that these collapses are unpredictable and create extremely hazardous conditions with rapid subsidence of the ground accompanied by ballistic explosions of rock and surges of hot water. The steam clouds are highly acidic and laced with glass particles. For a brief period on the Fourth of July, phreatic activity at the coastal entry contended with the aerial fireworks display in Hilo Bay. Fountain heights were estimated to be as high as 150 m (500 ft).

A magnitude-3.9 earthquake was felt in Hilo and in the upper Fern Forest area at 3:42 a.m. on Friday, July 2. The earthquake was located 13 km (7.8 mi) southeast of the summit of Kilauea Volcano at a depth of 7.58 km (4.5 mi). A resident of Pahala felt an earthquake at 5:06 p.m. on Sunday, July 4. The magnitude-3.5 temblor was located 6.5 km (3.9 mi) west of Pahala at a depth of 7.63 km (4.6 mi).

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Updated: 20 July 1999