Volcanic ash resuspension from the Katmai region: a hazard long after 1912

Volcanic ash is a hazard even when nothing is erupting. In strong winds, loose ash deposited earlier can be picked up and reworked into dust clouds. Kristi L. Wallace and Hans F. Schwaiger of the Alaska Volcano Observatory (AVO), U.S. Geological Survey, describe how this happens in Katmai National Park and Preserve, Alaska, where it has been observed and documented many times over the past several decades and has likely gone on since the 1912 Novarupta-Katmai eruption.

Figure 1. True color composite satellite image of the Valley of Ten Thousand Smokes (VTTS) and surrounding volcanoes. Image produced by Steve J. Smith; modified from AVO image ID 2152.

The 1912 source

The 1912 eruption was the largest in the world in the 20th century. It produced approximately 4 cubic miles (17 cubic km) of ash deposits and 2.6 cubic miles (11 cubic km) of pyroclastic material that filled nearby valleys. On June 6, 1912, after at least a week of unusually frequent and strengthening earthquakes that led families at Katmai Village to evacuate for good, the skies darkened around 1 PM, and for the next 60 hours the sun disappeared. Novarupta threw out 30 times more ash and magma than the 1980 eruption of Mount St. Helens. Kodiak Island, 100 miles (160 km) downwind, fell into darkness so complete that a lantern at arm's length was barely visible, for three days. Townspeople, some temporarily blinded by sulfurous gas, crowded onto the U.S. Revenue Cutter Manning, while one foot of ash (30 cm) fell in three closely spaced periods. Roofs collapsed, ash avalanches wrecked buildings, lightning from the ash cloud set some on fire and water became undrinkable. The summit of Mount Katmai, some 6 miles (10 km) away, collapsed into a caldera 1.9 miles (3 km) wide and 2,000 feet (600 m) deep, and expeditions funded by the National Geographic Society led to Katmai National Monument in 1918.

When and how ash is resuspended

Volcanic ash is volcanic material fragments under 0.08 inches (2 mm) across. In the VTTS, ash can be reworked into dust clouds in spring and fall, or whenever strong northwesterly winds cross the snow-free landscape, and it is especially susceptible when the ground is very dry. People downwind have seen the clouds, and web cameras and satellites have recorded them.

Figure 2. A web camera image of resuspended ash in the VTTS on September 29, 2014. AVO image ID 80721.

Figure 3. MODIS Aqua 1-km-resolution true color image, October 5, 2014, courtesy of NASA: a cloud from high winds scouring dry, unvegetated deposits stretches across Shelikof Strait to western Kodiak Island. AVO image ID 133541.

Figure 4. NOAA satellite image from the same event, October 5, 2014, showing brightness temperature differences between two infrared channels that reveal airborne ash (blue area, upper right). The plume reached an estimated 4,000–6,000 ft above sea level, drifting southeast toward Kodiak Island. AVO image ID 80451.

Since AVO began recording them in 2003, resuspension events have been observed about 30 times.

Figure 5. Katmai-area resuspension events known to AVO, 2003-2018.

Events often trigger ash alerts for AVO and National Weather Service (NWS) staff.

Hazards

Figure 6. Backscatter electron image of resuspended 1912 ash picked up by high winds on November 1, 2015 and carried to Larsen Bay on Kodiak Island. AVO image ID 94841.

Resuspended ash is mainly volcanic glass shards and is physically identical to eruption ash, so its plume looks like an explosive eruption's. Even dilute clouds can threaten human health and aircraft: ash can abrade cockpit and cabin windows, damage electronics and erode and stick to engine parts. Such plumes have not been well studied, and the effects of their fallout are not well known. It is not known whether trace fallout is an air quality or public health concern. Near the VTTS, though, dust storms are likely hazardous to visitors and staff. Avoiding exposure is best, especially for children, the elderly and people with respiratory or cardiovascular disease. In a VTTS dust storm, an industry-approved disposable N95 respirator is advised until it ends, and goggles and long-sleeved clothing limit eye and skin irritation.

Monitoring and forecasting

AVO and the NWS watch for ash clouds in near-real-time satellite data. AVO reviews data daily and has automated alarms; the NWS monitors continuously. Both pay special attention when high winds are forecast for Katmai.

AVO has modified its ash dispersion and deposition model, Ash3d, to include resuspension. Ash begins to remobilize once surface wind stress passes a threshold. If the lower atmosphere is stable the ash stays near the ground, but vertical mixing can loft it several thousand feet and carry it southeast. Using past events' wind, stability, precipitation and snow cover, Ash3d with weather forecasts can anticipate events, and for events with good satellite coverage it has been possible to estimate the amount of ash and the cloud height, both crucial for aviation hazard. Ash3d runs automatically at AVO and sends text and email alerts.

Figure 7. (a) Satellite-derived cloud load for a resuspension event on October 5, 2015; (b) the Ash3d simulated cloud load for the same event.

AVO and the Alaska Department of Environmental Conservation Air Quality Division placed PM-10 particulate monitors in Larsen Bay and the city of Kodiak during the fall and spring of 2015-2016. Only moderate events occurred, and a few degraded air quality, with PM-10 values up to 60 µg/m3, below the EPA 24-hour PM-10 standard of 150 µg/m3. Samples show the ash is mostly glass shards that still look pristine after more than 100 years, much like eruption deposits.

How the public is told

The NWS issues forecasts and statements: ash hazards to aircraft, volcanic ash advisories and ashfall forecasts. The state air quality division issues air quality guidance. AVO issues Information Statements during Katmai resuspension events, which state clearly that no eruption is under way, so it issues no formal Volcanic Activity Notices.

Sources

• Kristi L. Wallace and Hans F. Schwaiger, U.S. Geological Survey, Alaska Volcano Observatory, Volcanic Ash Resuspension from the Katmai Region, Alaska Park Science 18(1), National Park Service; rewritten in hubnx's own words. A photograph showing a local official is left out.