Hazards from Alaska Volcanoes

For detailed information on what it is like during ashfall, please see this page: http://volcanoes.usgs.gov/volcanic_ash/what_like_during_fall.html

Photograph of Redoubt's ash cloud, March 28, 2009, viewed while traveling between Kenai and Nikiski. Photograph courtesy of Jaden Larson.

Photograph of Redoubt's April 4, 2009 ashfall in Homer, AK. Photograph courtesy Terry Thompson.

Alaska communities downwind of an erupting volcano may encounter ash fallout. Fine ash may cause respiratory problems in some humans and animals. Heavy ash fallout may interfere with power generation and electrical equipment, damage air filters and gasoline engines, interrupt radio and cell phone transmissions, and greatly reduce visibility. The weight of very thick ashfalls may collapse roofs, especially when mixed with rain or snow. Resuspension of ash by wind could extend the unpleasant effects of ash fallout.

During the most recent eruptions in Alaska, communities have received no more than trace amounts of ashfall (1/32 in or 0.8 mm), although even small amounts of volcanic ash has resulted in transportation delays, school and business closures, and presented challenges to power and water systems. Recent eruptions of Cook Inlet volcanoes (Augustine, Spurr, Redoubt) have produced trace amounts of ash to south-central Alaska, especially the Kenai Peninsula, resulting in airport, school, and business closures, as well as ash cleanup costs and stockpiling of emergency supplies. The 2008 eruption of Kasatochi deposited a small amount of ash on Adak, and eruptions of Pavlof often result in very minor ashfall on nearby communities.

Looking a little further back, however, the largest eruption of the 20th century was the 1912 Novarupta-Katmai eruption, on the Alaska Peninsula. This eruption produced ashfall as far away as Dawson City (Yukon, Canada), Ketchikan (southeast Alaska), and the Puget Sound in Washington State. During the 3 days of the eruption darkness and suffocating conditions caused by falling ash and sulfur dioxide gas immobilized the population of Kodiak, Alaska. Sore eyes and respiratory distress were rampant, and water became undrinkable. Radio communications were totally disrupted, and with visibility near zero, ships couldn't dock. Roofs in Kodiak collapsed under the weight of more than a foot of ash, buildings were wrecked by ash avalanches that rushed down from nearby hillslopes, and other structures burned after being struck by lightning from the ash cloud. Similar conditions prevailed elsewhere in southern Alaska, and several villages were abandoned forever. Within 50 miles (80 km) of the erupting vent, animal and plant life was decimated by ash and acid rain. Bears and other large animals were blinded by ash and starved when large numbers of the plants and small animals they lived on were wiped out. Millions of dead birds that had been blinded and coated by volcanic ash littered the ground. Aquatic organisms, such as mussels, insect larvae, and kelp, as well as the fish that fed upon them, perished in ash-choked shallow water. Alaska's salmon-fishing industry was devastated, especially from 1915 to 1919, because of the starvation and failure of many adult fish to spawn in ash-choked streams ^{[4] [5] [6].}

The geological record also shows that many Alaska communities are underlain by significant amounts of volcanic ash. For example, most of south-central Alaska was covered in ash during large eruptions of Hayes volcano in the Tordrillo Mountains, 4,400–3,600 years ago ^[7]. Prehistoric eruptions of Makushin left deposits of ash that are tens of inches thick at Unalaska ^[8]. Eruptions from Mount Edgecumbe, 12,000-14,000 years ago, produced ash deposits about 3 ft (1 m) thick in Sitka ^[9]. AVO is working to understand the eruptive history of Alaska Volcanoes by studying the timing, distribution and amount of ash deposits since the last major ice sheets in Alaska retreated some 10,000-15,000 years ago.

For further information on volcanic ash fallout hazards and mitigation, visit volcanoes.usgs.gov/volcanic_ash/

Volcanic tsunami occur very rarely, but might accompany an extremely large eruption. Debris avalanches or pyroclastic flows that travel into the sea or lakes may generate unusually large waves, known as volcanic tsunami, by rapidly displacing water. Tsunamis are also occasionally produced by volcanic earthquakes, explosions, or other eruptive processes. One of our best-known volcanogenic tsunamis occurred during the 1883 eruption of Augustine volcano. During this eruption, a large debris-avalanche occurred at Burr Point on Augustine Island, displacing ocean water and creating an “earthquake wave” of 25 to 30 ft high in Port Graham on the Kenai Peninsula ^{[10] [11]}. The logbook at English Bay recorded “tidal” waves 20 ft higher than usual.

View west of the runway, helipad, and service buildings at DROT. The lahar deposit from Redoubt’s 2009 eruption is at least half a meter thick at the buildings. Photo taken March 23, 2009, by Game McGimsey, AVO/USGS.

Lahars (fast-moving slurries of water, mud, rocks, and sand) may form when hot volcanic debris melts the snow and ice that mantles most of Alaska’s volcanoes. These flows would follow streams and drainages, and may affect infrastructure or communities along these paths.

Lahars from Redoubt’s 1989-90 and 2009 eruptions damaged oil production storage facilities within the Drift River valley on the west side of Cook Inlet, resulting in decreased oil production during and after the eruptions.

Kiska's gas plume, sourced from multiple, individual fumaroles. Note deposition of sulfur surrounding the gas vent. Photo taken September 10, 2015, by Taryn Lopez, AVO/UAFGI.

Cindy Werner and Christoph Kern sample gases and waters from the Geyser Bight geothermal area, Umnak Island, on August 16, 2015. Photo by John Lyons, AVO/USGS.

Summit fumaroles at Makushin, August 3, 2012. Photo by Janet Schaefer, AVO/ADGGS.

Volcanic vents often emit steam and gases, including hydrogen sulfide, carbon dioxide and sulfur dioxide, in concentrations that are potentially harmful to humans. Reports of sulfur dioxide and hydrogen sulfide smell (rotten eggs) are common from mariners, pilots and people on the ground from Alaska volcanoes but these concentrations do not pose a significant health hazard. Lava flows can also emit large amounts of steam and gas. Dangerous invisible, odorless volcanic gases can collect in low-lying areas near fumaroles, making it inadvisable to descend into craters, caves, or fissures occupied by fumaroles.

A pyroclastic flow sweeping down the north flank of 4,206 ft (1,282 m) high Augustine Volcano. The eruption cloud is carried to the east by prevailing winds. Photograph by M.E. Yount, U.S. Geological Survey, March 30, 1986.

Pyroclastic flows (incandescent flows of ash, gas, and volcanic rock) and surges (hurricane-force blasts of turbulent hot gas and ash clouds) can race down slopes at speeds as great as 330 ft per second (100 m per second), travel 6 to 18 miles (10 to 30 km) from the vent, and overtop hills and other topographic obstacles. Large pyroclastic flows that enter water may cause a tsunami.

Breadcrust bomb (ballistic) from a Holocene eruption of Augustine. Photographs from geology field work at Augustine, August 2015. Photo by Michelle Coombs, AVO/USGS.

Explosive eruptions can blast pebble- to boulder-sized or larger fragments of rock, ice, or pumice into the air, where they travel on arcuate, ballistic trajectories away from the vent. These projectiles, called ballistics, fall at high speed, and can injure or kill people, and crush or damage equipment and buildings. Most ballistics will fall within a few miles of the vent.

Kanaga Volcano, seen from the east, with blocky 1906 lava flow. Photo taken September 20, 2015, by Michelle Coombs, AVO/USGS.

View of lava dome within Cleveland's summit crater, August 4, 2015, showing concentric rings and radial fractures in dome surface, surrounding elevated, hot dome core. Photo by John Lyons, AVO/USGS.

Flows of molten rock (lava) may erupt from Alaska volcanoes. These flows will probably move slowly and pose little hazard to humans. Lava flows may develop steep, blocky fronts that collapse on occasion. Avalanching of blocks from a flow front could be hazardous to someone nearby. If lava reaches the ocean, explosions and unstable ground are possible.

Before and after images of a small rockfall/landslide on Iliamna, captured on station IVE's webcam. Left image: view from IVE on October 2, pre-rockfall. Right image: view from IVE on October 3, after the rockfall. The debris ran about 2 km down the upper Red Glacier. The slide happened early UTC on Oct 3 2014. Iliamna’s typically-active fumaroles are also visible in both images. Photo credit: AVO’s IVE webcam, and Max Kaufman, AVO/UAFGI.

Alaska volcanoes often have steep cliffs that are prone to rockfalls and small landslides. Hydrothermal alteration may create additional rock instabilities, especially at volcanoes like Iliamna. The frequency and size of these events is likely to increase during times of eruption or earthquakes.

Debris avalanche deposits from the 1883 eruption of Augustine Volcano form small hills and islets at Burr Point on the north shore of Augustine Island. Photograph by S. McNutt, Geophysical Institute, University of Alaska, July 31, 1992.

A debris avalanche is a rapidly moving mass of rock debris produced by a large-scale landslide from the summit area of a volcano. The steep slopes of stratovolcanoes, with unconsolidated mixed material, are especially prone to debris avalanches. Debris avalanches may travel several kilometers before coming to rest, or they may transform into more water-rich lahars, and travel tens of kilometers downstream. These events are often, but not always, accompanied by volcanic eruptions.

A directed blast is a laterally directed explosion of the volcano caused by rapid release of internal pressure. Most directed blasts are caused by a slope failure of newly erupted lava domes or sector collapse of the summit edifice into a debris avalanche, resulting in rapid depressurization of a shallow magma body. Directed blasts can destroy structures throughout a large radial zone extending 6 to 15 mi (10-25 km). In the United States, our most well-known example of a directed blast occurred during the May 18th, 1980 explosion of Mount St. Helens, in Washington. More information on that event is available here: http://pubs.usgs.gov/gip/msh/lateral.html