Ash plume at Shishaldin Volcano on April 23, 1999, following second major eruptive episode

An eruption column from Shishaldin Volcano, located on Unimak Island (see map) 1,100 km southwest of Anchorage, rose to over 45,000 feet above sea level on April 19, 1999. Using data from newly installed seismometers and from civilian meteorological satellites, scientists of the Alaska Volcano Observatory tracked an increasing unrest at the volcano for 2 1/2 months and issued several eruption warnings. A second smaller eruption episode on April 22-23 sent an ash plume at least 15,000 ft above sea level. The eruption warnings were issued in the form of information releases using a "color code" that has proven effective in the past 10 years for alerting the aviation community to potential volcano ash hazards from restless volcanoes in the North Pacific region.

Eruptive activity begins
Lava fountaining activity was first observed by in the summit crater by scientists on April 17, and two days later an explosive eruption sent a column of volcanic ash to at least 45,000 ft. above sea level (spectrogram of eruption onset). The eruption lasted about 7 hours and an eruption cloud moved to the north at higher elevations and to the south at lower elevations. The volcano was placed at color code RED for a 21-hour period.

Following this explosive event, lava fountaining continued at the summit crater. A smaller and shorter-lived explosive event started at 9 p.m. on April 22; the color code was changed again to RED for the next 10 hours. As determined by satellite data, a narrow eruption cloud from this event moved south from the volcano. Seismic activity at the volcano then declined so that on April 28, scientists changed the color code back down to YELLOW.

Between early April and mid May, scientists at the Alaska Volcano Observatory worked 24 hours a day to monitor the volcano's seismic activity and examine satellite data for evidence of thermal features and eruption clouds. Many ash bursts have occurred with little or no warning since late April, but the resulting ash plumes rarely reached 15,000 ft. above sea level. Thus, scientists kept the color code at YELLOW.

See description of how seismologists analyze seismicity and tremor using spectrograms, which show the intensity of ground shaking in specific frequency bands.

New networks make warnings possible
Such warnings were not possible 4 years ago. Seismometers were first installed on Unimak Island in 1996 and significant improvements have been made since then in using civilian meteorological satellite data for near-real time analysis of thermal features on volcanoes and tracking of eruption clouds (see summary of satellite monitoring). This effort to expand the volcano monitoring capability of the Alaska Volcano Observatory is driven by the serious threat that volcanic ash poses to the expanding number of aircraft, passengers, and cargo traversing the North Pacific Ocean.

Sequence of 3 satellite images of the eruption cloud that formed during the eruption on April 19, 1999, and animation of satellite images provided by the Alaska Volcano Observatory.

Additional information

Shishaldin Volcano. General description of the volcano, and images and maps provided by the Alaska Volcano Observatory.

• Volcanic Ash -- Danger to Aircraft in the North Pacific  PDF  HTML
• Can Another Great Eruption Happen in Alaska?  PDF  HTML

Alaska Volcano Observatory