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Kilauea Volcano's east rift zone eruption continues: most lava travels through tubes from the vent area to the coast
This update current as of December 2, 1997[Eruption updates are posted approximately every two weeks. More frequent updates will accompany drastic changes in activity or increased threat to residential areas.]
For readers familiar with events of the past few months, recent changes include these:
- The vent inside Pu`u `O`o hasn't spilled lava across the crater floor for three weeks. Instead, the top of the magma column remains 10-20 m below the bounding rampart. Magma in the vent circulates below the crusted lava surface of the crater floor--out of sight except from the air.
- Roars that emanate from vents on the southwest flank of Pu`u `O`o are heard as far as 20 km distant. These roars, often likened to the sound of a jet engine or blast furnace, leave little outward sign of their passage except for scant rocky debris and a delicate mantle of Pele's hair near the vent. Recently, however, fallout of Pele's hair has been reported from residential districts 10 km north of the vent.
- Lava from the south shield travels in tubes to the coast, an in-tube distance of about 10 km. Travel time for a particle of melt is probably about 3 hours from vent to ocean. Eruption rate is generally 500,000-600,000 cubic meters per day. Although lava will escape occasionally from the tube to form new surface flows, no such breakouts have occurred in the past four weeks.
- Ocean entries remain situated at East Kamokuna and Waha`ula, which are the distal ends of the tube system. At East Kamokuna, small collapses periodically destroy short segments of the bench. Each new embayment is slowly reoccupied by new lava as the bench attempts to maintain its seaward growth. Steam plumes rise as a nearly continuous curtain from the 500-m-long edge of the slowly prograding lava flows of the bench.
- Sulfur dioxide gas emissions from vents in the Pu`u `O`o area remain high, about 2,500 tons per day.
The 55th episode of Kilauea's almost 15-year-long east rift zone eruption continues. This episode, which began February 24, 1997, was characterized in its early months by shifting vent locations on the west and southwest flanks of Pu`u `O`o cone and by rapid enlargement of the episode 50-55 lava shield. The flow field expanded slowly until, in July, lava reached the sea. The supply of lava to the coast became restricted to tubes, and surface flow activity diminished greatly.
Oblique aerial photo looking southwest across Pu`u `O`o. Silvery flows in foreground are east-directed overflows from August and October; photo p0398 taken Nov. 14, 1997. Distance is 200 m between north summit (high point on right) and south summit (hidden in fume on left). Crater vent lies at base of dense fume cloud in Pu`u `O`o Crater. South shield is heavily fuming area south (left) of the cone.
During the last 17 weeks, eruptive activity has been concentrated at two main vents: the "crater vent" on the Pu`u `O`o crater floor and the "south shield," a lava shield about 300 m south of the Pu`u `O`o cone. The most obvious of these has been "crater vent," which originated as a spatter cone. In September, however, the spatter cone subsided into its own throat, leaving a pit. The pit is slowly enlarging and now is about 60 m in diameter. Lava froths and sloshes within this cauldron. All of these changes occurred within the already existing crater of Pu`u `O`o.
In the two weeks prior to November 3, magma issued nearly continuously from the throat of the crater vent, spilling eastward across the main crater floor. This activity has diminished greatly in the past four weeks. Indeed, during the past two weeks, the lava remains within the rampart that bounds the vent. It does circulate away from the vent, but most of the activity is hidden beneath the crust of the crater floor. The limited extent of incandescent lava has reduced the magnificent nighttime glow once so prominent from many vantage points on the east slope of Kilauea volcano.
Near-vertical aerial photo looking southwest at Pu`u `O`o; photo p0407 taken Nov. 14, 1997. Incandescent glow is top of magma column in the crater vent within Pu`u `O`o cinder-and-spatter cone. Periodic spills from the vent coat the main crater floor. East rim of main crater is near bottom of photo, and silvery flows there mark the main-crater overflow from October. Topographic relief in the crater results mainly from drainback into cracks within the crater floor but also from subsidence, which creates the low escarpments cutting the crater-mantling lava. Cross-crater width is 250 m at greatest breadth. The small vents described in the next paragraphs lie at the foot of the cone but hidden in the fume clouds.
Perhaps the most remarkable activity during the past two weeks has been the numerous roars reported by residents and visitors throughout the area. These roars, which may be compared to the sound of a jet engine, issue from the throats of sporadically active vents on the southwest flank of Pu`u `O`o. The few close-hand observations describe dust- to fist-size rocky debris being tossed a short distance during such an event. Lately the area near these small vents has received much Pele's hair, filamentous volcanic glass as long as several centimeters. Unusually widespread fallout occurred on the weekend of November 28-30, when Pele's hair was reported at Napau Crater campground, 5 km west-southwest of the vents, and in the Glenwood district, 10 km to the north.
Our preliminary assessment explains these events as a consequence of sudden gas escape. The magma is gas rich and degasses constantly in the near-surface environment. 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.
Seismogram from the Steamcrack seismic station, located roughly 3 km from the vent. Time proceeds from left to right and from top to bottom. The small rectilinear bumps denote one-minute intervals; interval between lines is 15 minutes. Cigar-shaped seismic signals may correspond to gas-escape events, on basis of evidence from episodes in the early 1980s. The few roaring events we've witnessed recently, however, were too small to record a seismic trace, and timing for larger events is too poorly known to allow correlation between seismic signal and gas escape.
The other main vent, the south shield, is the source of the flows entering the ocean at the Waha`ula and East Kamokuna sites near the east boundary of Hawai`i Volcanoes National Park. The flows are encased within lava tubes for most of their length and are visible only through skylights in the roof of the tube.
The tubes discharge their lava at the shoreline. The hot lava, about 1,150 degrees Celsius when it reaches the ocean, generates dense plumes of steam upon contact with seawater. The new lava builds benches beyond the low seacliffs that bound the south coast of the Big Island.
View southwest across coastal plain, showing lava benches at Waha`ula (landward of steam plume at left) and East Kamokuna (at right). Distance between plumes is about 970 m. These benches are new land built since July 1997. Photo p0372, Nov. 14, 1997.
Small explosions periodically disrupt the rapidly chilling lava and throw it onto the bench, constructing low nearshore (littoral) cones. These small explosions pose a minor threat for visitors. A far greater threat exists, however; these benches may collapse into the sea without warning, triggering large steam explosions that hurl dense rock and molten spatter tens of meters inland.
Such a collapse occurred in early November (four weeks ago), lopping 4.75 acres (1.92 hectares) of existing episode-55 bench into the ocean at East Kamokuna. (No one was on the bench, so only land, not life, was lost.) Since then, a new lava flow from the beheaded tube is building a shelf at the foot of the new cliffs created by the collapse. These features may be discernible in the previous photo. The righthand steam plume is at East Kamokuna. The new lava flow, 40-50 m wide, is adjacent on the right side of that steam plume. The word "cliffline" and a white leader point to the cliff that formed as a result of the collapse. Thus, the new lava flow is filling an embayment created during the abrupt destruction of unstable land. Steam plumes rise as a nearly continuous curtain from the 500-m-long edge of the slowly prograding lava flow.
More recently, smaller collapses have pared slices of the newly emplaced lava flows. The largest occurred on Monday, Nov. 24, when 0.65 acres (0.26 ha) subsided into the ocean. A tour guide on site at the time shared with us his realization that these collapse events happen quickly--too quickly for visitors to escape if they're within the area of collapse.
The situations just described should serve as ample warning: No one should venture onto the benches, no matter how stable the new land may appear.
Eruption-viewing opportunities change constantly, so those readers planning a visit to the volcano should contact Hawai`i Volcanoes National Park for the most current eruption information (808-985-6000). Additional photographs and descriptions of east rift eruptive activity may be found on the University of Hawai`i's web site.
On Sunday, November 16, east Hawai`i and especially Hawai`i Volcanoes National Park were engulfed in one of the worst episodes of vog this year. Gentle southeasterly winds blew much of the sulfur dioxide (SO2) emissions from Kilauea's east rift zone--about 2800 tons per day (t/d)--directly across the heavily visited Kilauea caldera area of the National Park. The current SO2 emission rate of 2500 t/d is about 40 percent higher than the average for the last several years during continuous eruption. Summit SO2 emissions from Kilauea remain in the range of 50-100 t/d.
This map current as of December 2, 1997
Eruption-viewing opportunities change constantly, refer to the HVO home page for current information. Those readers planning a visit to Kilauea or Mauna Loa volcanoes can get much useful information from Hawai`i Volcanoes National Park.
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