December 12, 1997

Vents That Roar

During the past two weeks, many people have heard roaring noises from the area of Pu`u `O`o, the prominently fuming cinder-and-spatter cone 12 km south of Glenwood. Some of our best reports of this activity come from residents of both upper and lower Puna, as well as from visitors in Hawaii Volcanoes National Park.

The roars, which most listeners liken to the sound of a jet engine, issue from the throats of sporadically active vents on the southwest flank of Pu`u `O`o. These vents have produced almost no lava since summer, though occasionally they expel a brief shower of spatter or a small tongue of lava that extends no more than 30 meters.

We've been on site for a few of these brief (30-60 seconds) events, but many more go unnoticed because of the remote setting. Witnesses report dust- to fist-size rocky debris and spatter being tossed a short distance from the vents, plus Pele's hair, which can be carried some distance on the wind.

We know that these roars are caused by sudden release of gas. The magma that feeds Pu`u `O`o is gas-rich and degasses constantly as it approaches the earthís surface. If enough gas can coalesce to form a bubble, it may begin to rise through the magma. The roar results when the gas breaches the magma-air interface and escapes suddenly. If this boundary is below the ground surface in an open conduit, then the rush of escaping gas may rip rock from the conduit walls and thrust it upward. Pele's hair is spun from the magma as the gas escapes.

We can only speculate as to why these events are happening so frequently now. Part of the explanation may lie in the relative inactivity inside Pu`u `O`o crater these past two weeks. Lava is still erupting from the crater vent, but instead of spilling across the crater floor, it immediately plunges into a drainhole adjacent to the vent. Perhaps diminished degassing at this major vent shifts the task to nearby lesser vents.

Earthquake and Eruption Update, 12 December

In other news from Pu`u `O`o, another collapse pit has formed on the southwest side of the cone in the last week. From the air, we can see that the pit is funnel-shaped, roughly 50 meters in diameter at the surface and narrowing to about a quarter of that size near its bottom. A small, glowing hole was glimpsed at the floor of the pit, indicating that the pit intersects the magmatic system underlying the cone and flank vents. These pits form because this area is being undermined by the magma feeding the eruption.

The new collapse pit is reminiscent of the "Great Pit," which formed on the west slope of the cone in early 1993, and enlarged until the west wall of the crater collapsed last January. The new pit is a sign that more collapse of the cone is yet to come. The visible lava output inside Pu`u `O`o crater increased for about 24 hrs on December 8 and 9, with flows from the crater vent racing across the crater floor and filling the eastern side of the crater. This brief spate of heightened activity may have been related to the formation of the new collapse pit.

Downslope of the vents, little has changed. Lava still enters the ocean at the Waha`ula and Kamokuna sites near the eastern margin of Hawai`i Volcanoes National Park. The lava benches, where new land is building seaward, remain the most hazardous region of the volcano. Pieces of bench as large as a football field can slide into the sea without warning, so stay well inshore of the ocean entries!

There were no earthquakes reported felt during the past week.