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Volcanowatch

January 8, 2009

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


Where did the rest of the rocks go? More went down than up

 This plume, erupting from Halema`uma`u in 1934, is remarkably similar in appearance to the plume erupting from the crater today. USGS photograph.

Now that the 2008 Halema`uma`u eruption has ended (or is in repose), the total volume of solid material (tephra) thrown out from the new vent can be estimated. This volume, though tiny, is interesting, for it bears on the origin of the vent itself.

Calculation of the volume is complicated, however. We did it in two parts, one for the 8 explosive eruptions (March 19, April 9 and 16, August 1 and 27, September 2, and October 12 and 14) and the other for the nearly continuous ejection of tephra between those events. Each approach first calculates the mass (weight) of the ejecta, from which the volume can be determined.

We used a mathematical model, developed 20 years ago for eruptions hundreds of times larger than ours, to estimate the weight of tephra ejected during the 8 explosive eruptions. For each one, we weighed all the tephra that landed in 10 or more small areas, each generally 1 square meter (10.8 square feet) or less. Weights from these areas were then fed into the model to calculate the total weight of all the tephra erupted. The volume can be calculated by knowing the density of the tephra. A weight of 1,000 kg (2,200 pounds) is approximately equivalent to a volume of 1 cubic meter (1.3 cubic yards).

The largest explosive eruption, on September 2, produced 800 cubic meters (1,050 cubic yards), about the volume of the display section of the Jaggar Museum. The smallest, on October 14, ejected about 7 cubic meters (9 cubic yards), about half of a modest bathroom with shower. The total volume ejected by all 8 explosive eruptions is about 1,640 cubic meters (2,150 cubic yards).

These were tiny events! For comparison, the volume of ejecta in the largest lava fountain of the 1959 Kīlauea Iki eruption was about 400 times the volume of the September 2 eruption. The two largest known explosive eruptions from Kīlauea, about 1200 and 400 years ago, each had a volume more than 25,000 times that of the September 2 event.

The volume of the nearly continuous tephra fall between the 8 main events was calculated from the weights that fell into 10 plastic buckets on the ground between the vent and the parking lot. Each morning, the contents of the buckets were weighed. For several days, we calculated an average ratio between the weight in the buckets and the total weight of the deposit by using the tedious mathematical model. Using this ratio, we could calculate the deposit weight from the daily collections. A volume was estimated from that weight. The volume ejected between the explosive eruptions is only about 300 cubic meters (390 cubic yards), 18 percent of the volume erupted by those much larger events.

The total volume of ejecta from the 2008 eruption is about 1,940 cubic meters (2,540 cubic yards), about three times the volume of the Jaggar Museum display area.

The tephra consists of two different materials. One, derived from the lava itself, is called juvenile; Pele's tears and spatter are examples. The other, broken from older rocks exposed in the walls of the vent, is called lithic. Ongoing studies show about equal amounts of juvenile and lithic tephra in the deposits, about 970 cubic meters (1,270 cubic yards) each.

Now the fun begins. The volume of the crater created at the vent is 200,000-300,000 cubic meters (260,000-390,000 cubic yards), but the volume of the lithic ejecta is only 970 cubic meters (1,270 cubic yards), a discrepancy of 200-300 times! Clearly, most of the rocks removed to form the vent crater must have collapsed into the underground rather than being tossed onto the ground surface.

What did the rocks collapse into? Was it a cavity, did they fall onto a "conveyor belt" that carried them away, or what?

Underground voids are anathema to many geologists. But, limestone caves exist to depths of 2.2 km (7,200 ft), and deep mines go down to nearly 3.5 km (11,500 ft). Why couldn't temporary cavities a few hundred meters deep exist in Kīlauea? The deep mines are engineered, but Nature is a better engineer than Homo sapiens.

Pit craters, as along the Chain of Craters, are thought to form by collapse into a void. Perhaps we have witnessed the formation of a small pit crater, twice the diameter of Devil's Throat, without realizing it.

Activity update

Kīlauea Volcano continues to be active. A vent in Halema`uma`u Crater is emitting elevated amounts of sulfur dioxide gas but producing no 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.

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 erupting from the Thanksgiving Eve Breakout (TEB) vent at the eastern base of Pu'u 'O'o continues to flow to the ocean at Waikupanaha through a well-established lava tube. Beakouts from the lava tube were active in the Royal Gardens subdivision and on the coastal plain in the past week. Ocean entry activity has continued throughout the past week, with a minor short-term reduction in activity following a deflation-inflation cycle on December 31-January 1.

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. The Waikupanaha delta has collapsed many times over the last several months, with three of the collapses resulting in rock blasts that tossed television-sized rocks up onto the sea-cliff and threw fist-sized rocks more than 200 yards inland.

Do not approach the ocean entry or 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. Call Hawai`i County Civil Defense at 961-8093 for viewing hours.

Mauna Loa is not erupting. Six earthquakes were located beneath the summit this past week. Continuing extension between locations spanning the summit indicates slow inflation of the volcano, combined with slow eastward slippage of its east flank.

Two earthquakes beneath Hawai`i Island were reported felt within the past week. A magnitude-2.8 earthquake occurred at 5:53 a.m., H.s.t., on Monday, January 5, 2009, and was located 5 km (3 miles) northeast of Na`alehu at a depth of 12 km (8 miles). A magnitude-2.1 earthquake occurred at 1:17 p.m. on Wednesday, January 7, and was located 8 km (5 miles) northeast of Waiki`i at a depth of 14 km (9 miles).

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. Kīlauea daily update summaries are also available by phone at (808) 967-8862. Questions can be emailed to askHVO@usgs.gov. skip past bottom navigational bar


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Updated: January 11, 2009 (pnf)