# Summer Explosive Eruptions in Alaska Keep Scientists and Airlines on Edge

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Explosive eruptions on three remote Aleutian islands this summer during a three-week period generated large ash and gas-rich clouds that kept scientists at the Alaska Volcano Observatory (AVO) extremely busy assessing the activity and issuing warnings about the hazards.

Okmok eruption, July 24, 2008, taken from Ft Glenn on the east flank of Okmok Caldera. Photo by: Lonnie Kennedy.

(Public domain.)

The first eruption of the summer occurred with very little warning on July 12 at Okmok Volcano, Unimak Island, 1,383 km (859 miles) southwest of Anchorage. Okmok's most recent eruption in 1997 produced low-level ash clouds and a lava flow that traveled about 8 km (5 miles) across the volcano's broad caldera floor.

An ash and gas cloud from initial explosive activity moved southeast over a broad area of the Gulf of Alaska and caused flights to be re-routed for a few days. The explosive activity continued for several weeks, sending volcanic ash and gas to heights between 10,000 and 35,000 feet above sea level. Nine people evacuated from a cattle ranch on Unimak Island.

A volcano-monitoring network was installed on Okmok in 2002-2003 as part of the effort by AVO to monitor an increasing number of active and potentially active volcanoes in Alaska. Only local monitoring networks permit the early detection and measurement of volcanic unrest, assessment of activity during an eruption, and effective warnings of eruptions. Today, 31 volcanoes in Alaska are monitored with ground-based instruments, compared to 4 in 1989.

Next, Cleveland Volcano, 160 km (100 miles) west of Okmok, generated a short-lived explosive eruption on July 21. During the next several days, when the volcano was visible through clouds, pilot reports and satellite information indicated low-level ash emissions 10,000 to 20,000 feet above sea level. This activity is similar to brief eruptions in 2001 and 2005-2007.

The lack of a real-time seismic network at Cleveland means that scientists are unable to track local earthquake activity related to volcanic unrest. Only satellite data is used by AVO scientists to detect and track significant ash clouds (when weather allows) or identify new thermal features.

After four weeks of around-the-clock monitoring of Okmok and Cleveland volcanoes, the largest explosive eruption occurred at Kasatochi volcano on August 7, resulting in a large ash and gas cloud that spread southeastward across the Gulf of Alaska, Canada, and the northern United States. Many airline cancellations and enroute diversions were made to avoid the volcanic cloud. On August 10, dozens of flights were cancelled from Anchorage to Los Angeles, Denver, and Vancouver BC, stranding more than 6,000 passengers.

Three major eruptions occurred within the first 7 hours, and ash reached at least 45,000 feet above sea level. In addition to the ash cloud, pyroclastic flows, fast moving hot-rock avalanches, swept into the ocean on all sides of the volcano. These flows formed deposits that extended the island's shorelines and triggered a small tsunami 19 cm (7.5 inches) high, as measured at the fishing community of Adak located 80 km (50 miles) to the west.

Kasatochi Island is about 2.4 km (1.5 miles) in diameter and 314 m (1,030 feet) high. Before the eruption, the top of the mostly submerged volcano contained a lake-filled crater. Luckily, the eruption of Kasatochi was preceded by an unusually large earthquake sequence, including several magnitude-5 events, recorded by a seismic network located 40 km (25 miles) away. These earthquakes caused the emergency evacuation of two biologists working on the volcano for the Fish and Wildlife Service less than an hour before the eruption began!

The biologists had been on the island since early summer and reported feeling earthquakes in July; they did not notice any other activity in the crater lake or flanks of the volcano until August 6 and 7. Without a local network of monitoring instruments, AVO scientists could only locate very few earthquakes in a broad area near Kasatochi-an area with a long history of many tectonic earthquakes-until the larger earthquakes occurred the day before the eruption.

Clearly, had there been instruments on Kasatochi, scientists would have noticed the precursory activity in July, determined that the earthquakes were of magmatic origin beneath Kasatochi, and issued warnings several weeks before the eruption instead of 1 day, perhaps even before Okmok erupted on July 12.

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### Volcano Activity Update

Kīlauea Volcano continues to be active. A vent in Halemaumau 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 Hawaii Volcanoes National Park, during kona wind periods. There have been several small ash-emission events, lasting only minutes, in the last week. These are preceded by small seismic events, and are probably caused by tiny rock falls within the vent. In addition, an explosive eruption occurred from the vent in Halemaumau Crater at 8:13 p.m. H.s.t. on September 2, depositing ejecta onto the crater rim, Halemaumau parking lot, and Crater Rim Drive. This is the sixth explosive eruption since the vent opened in March.

Puu Ōō continues to produce sulfur dioxide at even higher rates than the vent in Halemaumau Crater. Trade winds tend to pool these emissions along the West Hawaii coast, while Kona winds blow these emissions into communities to the north, such as Mountain View, Volcano, and Hilo.

Lava continues to erupt from fissure D of the July 21, 2007, eruption and flows toward the ocean through a well-established lava tube. A summit deflation/inflation cycle starting on Monday, September 1 resulted in a weak ocean entry plume on Tuesday and Wednesday due to a short-term decrease in lava supply. Inflation, and the accompanying short-term increase in lava supply, commenced on Wednesday, September 3, producing several small breakouts on the coastal plain on the morning of Thursday, September 4. Lava continues to flow into the ocean at Waikupanaha.

Be aware that lava deltas could collapse at any time, potentially generating large explosions. This may be especially true during times of rapidly changing lava supply conditions, as have been seen lately. 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 or call 961-8093 for viewing hours.

Mauna Loa is not erupting. No earthquakes were located beneath the summit this past week. Continuing extension between locations spanning the summit indicates slow inflation of the volcano.

Two earthquakes beneath Hawaii Island was reported felt within the past week. A magnitude-2.6 earthquake occurred at 6:33 p.m., H.s.t., on Sunday, August 31, 2008, and was located 25 km (16 miles) southwest of Waikoloa Village, at a depth of 42 km (26 miles). A magnitude-3.2 earthquake occurred at 6:54 a.m. on Monday, September 1, 2008, and was located 14 km (8 miles) northwest of Kailua at a depth of 8 km (5 miles).

Visit our Web site 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