November 6, 2008

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

Halema`uma`u Crater Ringed by Remains of Old Lava Lake

Red arrows point to the "bathtub ring," or high-lava mark, from the 1967-68 eruption in Halema'uma'u Crater. The light-colored ring, composed of rocks altered by acidic gases, is obscured in places by later rockslides from the unstable crater walls.

Have you ever noticed the "bathtub ring" in Halema`uma`u? Look into the crater from Jaggar Overlook—or via HVO's Halema`uma`u Webcam—and you'll see a distinct band of light-colored rocks extending about halfway up the crater wall. Those rocks delineate the high-lava mark of a lake of molten rock erupted 41 years ago.

It all began at 2:32 a.m. on Sunday, November 5, 1967, when a line of vigorously erupting vents opened on the floor of Halema`uma`u Crater. Up to a dozen fountains spewed lava 60-75 m (200-250 feet) into the air while sulfur fumes billowed skyward in a large white cloud. Molten rock covered the entire crater floor with a lava lake, the surface of which rose as much as 4 m (12 feet) per hour. At night, incandescence reflected in the fume cloud created a glow that was visible all the way to Hilo.

When the fountains abruptly ceased 22.5 hours later, the lava lake was 31 m (103 feet) deep, but molten rock immediately began draining back into the vents. This drainback lowered the lake level by 14 m (45 feet), leaving jagged islands of crusted lava standing on the lake surface.

On November 7, a local newspaper headline declared "Eruption Officially Pau," but HVO scientists weren't convinced it was truly over. Renewed inflation at Kilauea's summit soon after the fountains died suggested otherwise. Two days later, increased harmonic tremor signaled the onset of another eruptive phase.

During the 11-day second phase, robust fountains in the center of the new lake threw lava 15-30 m (50-100 feet) into the air. This eruption differed from the previous phase in that a levee of crusted lava confined a perched pool of molten lava that grew over 30 m (100 feet) high. Lava occasionally overflowed the levee, spilling out to cover about half of the crater floor. When the eruption ended, lava again drained back into the vents, causing the lake center to drop about 6 m (20 feet).

This pattern of activity—harmonic tremor, lava extrusion and lake overflow, summit deflation, partial drainback and summit reinflation followed by a period of quiet—occurred repeatedly over the next eight months. As the lake continued to grow, the floor of the crater began to resemble late 19^th and early 20^th century paintings and photographs of Halema`uma`u. These things led HVO scientists to wonder whether Kilauea was returning to its pre-1924 activity, when Halema`uma`u maintained a nearly continuous lake of lava.

In all, 31 eruptive phases pumped 84 million cubic meters (110 million cubic yards) of lava into Halema'uma`u Crater. Phases varied in duration and intensity, ranging from a low-key phase that lasted only 8 hours, to one that continued for 68 days. By phase 16, lava fountains were characterized by massive bubbling rather than the tall bursts observed in earlier phases. Periods of quiet between eruptive phases varied from 12 hours to six days.

Not surprisingly, thousands of Hawai`i Island residents and visitors flocked to the Halema`uma`u Crater overlook to witness the eruption. During phase 1 alone, almost 22,000 people braved howling winds, chilly weather, choking sulfur fumes, and the sting of wind-blown pumice and ash for a front-row view. To manage the heavy traffic, Hawai`i Volcanoes National Park Rangers set up a one-way route around Crater Rim Drive.

The eruption boosted business on the island's hotels, tour operators, and rental car agencies. Airlines offered special excursions that circled Halema`uma`u Crater at an altitude of about 460 m (1,500 feet). Because planes were restricted to a counter-clockwise pattern, passengers switched sides after 15 minutes so they could all get a good view.

When the eruption started on November 5, 1967, the floor of Halema`uma`u Crater was 170 m (550 feet) deep. By the time the last dribble of molten rock erupted on July 13, 1968, the lava lake had filled the crater to a depth of 110-114 m (360-375 feet). The final drainback left a 15-m (50-foot) depression in the lake surface.

The aforementioned bathtub ring wasn't revealed until September 1971, when a southwest rift zone eruption drained lava from the still-molten interior of the 1967-68 lava lake. This caused the crater floor to collapse about 45 m (150 feet), exposing the high-lava mark you see today.

Activity update

Kīlauea Volcano continues to be active. A vent in Halema`uma`u Crater is erupting elevated amounts of sulfur dioxide gas and very small amounts of ash. Resulting high concentrations of sulfur dioxide in downwind air have closed the south part of Kīlauea caldera and produced occasional air-quality alerts in more distant areas, such as Pahala and communities adjacent to Hawai`i Volcanoes National Park, during kona wind periods. The have been several small ash-emission events from the vent, lasting only minutes, in the last week.

Pu`u `Ō`ō continues to produce sulfur dioxide at even higher rates than the vent in Halema`uma`u Crater. Trade winds tend to pool these emissions along the West Hawai`i coast, while Kona winds blow these emissions into communities to the north, such as Mountain View, Volcano, and Hilo.

Lava continues to erupt from the Thanksgiving Eve Breakout (TEB) vent and flows toward the ocean through a well-established lava tube. Lava breakouts in the Royal Gardens subdivision have been active throughout the past week, sending small flows several hundred yards southward onto the coastal plain. Activity at the Waikupanaha ocean entry has fluctuated over the past week. A deflation-inflation (DI) event at the summit led to a brief reduction in activity at the ocean entry on Monday, November 3. A partial collapse of the Waikupanaha lava delta occurred in the middle of last week.

Be aware that active lava deltas can collapse at any time, potentially generating large explosions. This may be especially true during times of rapidly changing lava supply conditions. Do not venture onto the lava deltas. Even the intervening beaches are susceptible to large waves generated during delta collapse; avoid these beaches. In addition, steam plumes rising from ocean entries are highly acidic and laced with glass particles. Check Civil Defense Web site (http://www.lavainfo.us) or call 961-8093 for viewing hours.

Mauna Loa is not erupting. Two earthquakes were located beneath the summit this past week. Continuing extension between locations spanning the summit indicates slow inflation of the volcano.

No earthquakes beneath Hawai`i Island were reported felt within the past week.

Visit our Web site (http://hvo.wr.usgs.gov) for daily Kīlauea eruption updates, a summary of volcanic events over the past year, and nearly real-time Hawai`i earthquake information. Kilauea daily update summaries are also available by phone at (808) 967-8862. Questions can be emailed to askHVO@usgs.gov.