Abstract
The Crater Peak flank vent of Mount Spurr volcano erupted June 27, August 18, and September 16-
17, 1992. The three eruptions were similar in intensity (vulcanian to subplinian eruption columns reaching
up to 14 km Above Sea Level) and duration (3.5 to 4.0 hours) and produced tephra-fall deposits (12,
14, 15 x 10 6 m3 Dense Rock Equivalent [DRE]) discernible up to 1,000 km downwind. The June 27 ash
cloud traveled north over the rugged, ice- and snow-covered Alaska Range. The August 18 ash cloud was
carried southeastward over Anchorage, across Prince William Sound, and down the southeastern shoreline
of the Gulf of Alaska. The September 16-17 ash plume was directed eastward over the Talkeetna and
Wrangell mountains and into the Yukon Territory of Canada. Over 50 mass-per-unit-area (MPUA) samples
were collected for each of the latter two fall deposits at distances ranging from about 2 km to 370 km
downwind from the volcano. Only 10 (mostly proximal) samples were collected for the June fall deposit
due to inaccessible terrain and funding constraints. MPUA data were plotted and contoured (isomass
lines) to graphically display the distribution of each fall deposit. For the August and September eruptions,
fallout was concentrated along a narrow (30 to 50 km wide) belt. The fallout was most concentrated
(100,000 to greater than 250,000 g/m2) within about 80 km of the volcano. Secondary maxima occur at
200 km (2,620 g/m2) and 300 km (4,659 g/m2), respectively, down axis for the August and September
deposits. The maxima contain bimodal grain size distributions (with peaks at 88.4 and 22.1 microns) indicating
aggregation within the ash cloud. Combined tephra-volume for the 1992 Mount Spurr eruptions
(41 x 10 6 m3 DRE) is comparable to that (tephra-fall only) of the 1989-90 eruptions of nearby Redoubt
volcano (31-49 x 106 m3 DRE).
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First posted July 15, 2002
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