Description: For 220 pages of information on the three 1992 eruptions of Crater Peak, please see the USGS Bulletin 2139, available online at: https://www.avo.alaska.edu/pdfs/B2139.pdf ; this file is 9 MB.
From Eichelberger and others (1995): "Following the June [June 27, 1992] eruption, seismicity remained low through the first half of August. Only one shallow and two deep earthquakes were recorded between August 12 and 17. Because the closest operational seismic station at that time was 5 km from the vent, several attempts were made to reestablish a seismic station on the crater rim. These were unsuccessful because of poor weather conditions. At 3:37 p.m. ADT on August 18, a 16-minute episode of weak tremor and several LP [long-period] events began, but these rather obscure events were not identified until post-eruption analysis of the data. At 3:48 p.m. ADT, a pilot reported an ash-rich plume. With confirmation of this plume at 4:25 p.m., AVO began a calldown announcing level of concern color code Yellow. The main eruption began at 4:42 p.m. ADT, when strong tremor was recorded by all Mount Spurr seismic stations. AVO began a calldown announcing color code Orange at 4:47 p.m. ADT, but repeated the calldown process 11 minutes later to raise the color code to Red. By 4:58 p.m. ADT, a subplinian column had risen through low clouds to a height of 11,000 m, and it ultimately reached nearly 14,000 m. From an aircraft only 2.5 km away, AVO staff observed and videotaped a dark roiling cloud that was periodically surrounded by lenticular shock waves. Large bombs were thrown 800 m above the vent. Small-volume pyroclastic flows of breadcrusted blocks descended the east and southeast flanks of Crater Peak; these flows formed coarse, clast-supported lobate deposits with steep-fronted margins. Other flows mixed with snow and ice high on the cone and became lahars. A late fusillade of mostly lithic ballistic projectiles, some as large as 1 m, were hurled as far as 10 km southeastward (Waitt and others, this volume [USGS Bulletin 2139]). More than 170 lightning strikes were detected by the AVO lightning detection system (LDS) during the second half of the eruption (Paskievitch and others, this volume [USGS Bulletin 2139]). The eruption ended at 8:10 p.m. ADT.
"Upper level winds moved the eruption plume east-southeast directly over Anchorage, where it deposited as much as 3 mm of sand-sized ash (Neal and others, this volume [USGS Bulletin 2139]). A satellite image 44 minutes after the onset of the eruption shows the plume extending 80 km east from the volcano over an area of 20,000 square km. Three hours after onset of eruption, the leading edge of the plume was 300 km southeast of Mount Spurr, and its area had grown to 21,000 square km.
"Ashfall forced the closing of Anchorage International Airport for 20 hours (N.W. Gibson, Anchorage International Airport, written commun., 1993). Air quality alerts were issued in Anchorage during the fallout period and also on the following day, as vehicle traffic stirred up ash again (R.B. Stewart, Office of Emergency Management, Municipality of Anchorage, written commun., 1993). Reworked windblown ash continued to reduce air quality until the first snow of autumn, and then it reappeared during the summer of 1993."
Eichelberger and others (1995) state that this eruption produced 52 million cubic m of tephra (14 million cubic m Dense Rock Equivalent).