Report on Veniaminof (United States) — April 1994

Bulletin of the Global Volcanism Network, vol. 19, no. 4 (April 1994)
Managing Editor: Richard Wunderman

Veniaminof (United States) Large steam plumes, lava emissions, and description of active cone and ice pit

Please cite this report as:

Global Volcanism Program, 1994. Report on Veniaminof (United States). In: Wunderman, R (ed.), Bulletin of the Global Volcanism Network, 19:4. Smithsonian Institution.

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United States

56.17°N, 159.38°W; summit elev. 2507 m

All times are local (unless otherwise noted)

The eruptive activity . . . continued through mid-May with steam plumes and lava emissions. Residents of Perryville . . . were able to see the top of Veniaminof on 21 April; a small steam plume was noted. During the week of 22-29 April, pilots and ground observers in Perryville reported occasional vigorous steam plumes emanating from near the intracaldera cinder cone. Perryville residents heard loud rumbling noises on 30 April and saw extensive glow over Veniaminof on the next evening (1 May). Large steam columns were reported by pilots and ground observers during that same period. Poor weather obscured the summit in late April and early May.

On 9 May 1994, AVO observers flew over the active cone for about 15-20 minutes around noon, and made 6-8 passes over the E flank of the active intracaldera cone. The summit of the main intracaldera cone was generally shrouded in steam and clouds (at about 2,400 m above sea level) but was obviously degassing vigorously, similar to 1983-84. Photos taken on 3 August 1993 indicated that the eruption began in the summit area.

The eruption has formed an oval-shaped NE-SW lava field about 1,000 m (N-S) x 800 m (E-W) on the SE flank of the main intracaldera cone between ~1,925 and 1,770 m elevation (preliminary measurements are based on guesses as to scale, similarity to 1983-84 features, etc.). The lava is assumed to be similar in composition to material erupted during 1983-84 and earlier eruptions, which was basaltic andesite with about 54% SiO2. Assuming an area of 0.8-1.0 km2 and an average thickness of 20 m, the volume of the lava field is 16-20 x 106 m3. The volume of magma erupted in 1983-84 was ~40 x 106 m3.

The new lava is black with some snow cover along the W margin and a few scattered snow fields low on the S and SE flanks. The lava field is composed of block lavas with well-developed levees and channels; most of the lavas appear to have moved in a general N to S direction. The N and upslope end of the lava field was apparently the source area for most, if not all, of the flows. A sharp-featured cone centered about 100 m from the edge of the ice rises 100-150 m above the surrounding ice-field and dominates this part of the field. Photographs indicate that this feature is the N half of a cinder cone; the S half apparently either collapsed or was blown away, resulting in an E-W trench and debris pile at the base of the cone. This cone, referred to hereafter as "Half Cone," was probably the locus of early activity. Except for part of its steep S slope, however, it was snow-covered, indicating a lack of recent activity. Fumes rose from the cone but no point-source fumaroles were apparent.

Three low mounds at the SW base of Half Cone were arranged in a dog-legged N-S direction with the middle mound slightly W of the others. Covered with yellow-green alteration encrustations typical of vent areas, the mounds were connected by a low fissure-like ridge covered with similar material. These mounds, and possibly the connecting ridge, have been the source of many lava flows. An incandescent bright orange-red fan-shaped lava flow was being extruded from the central mound and moving E into what looked like the E-W trench at the base of Half Cone. This W to E direction is in contrast to the general N to S movement of earlier flows. The incandescent part of the flow was an estimated 80 m long and the flow was perhaps 10 m wide at its point of issue. A line of blue gases (SO2?) extended for perhaps another 100 m to the E. Eruptive products appeared to be entirely lava; no fresh ash was seen on the surrounding ice field.

The ice pit melted by the lava field had vertical, jagged walls 30-50 m high, exposing numerous black ash layers from earlier eruptions. Fractures in the ice-field are sparsely developed parallel to the W (up-slope) wall and more extensively developed parallel to the E wall (extending perhaps 100-200 m from the wall). A series of crescent-shaped fractures in the ice field, which extended 500 m from the S end of the lava field, suggested that lava had reached the caldera floor and caused sub-glacial melting, local ice cap uplift, and subsequent collapse similar to the 1983-84 eruption. This collapse area partially overlapped the E end of the 1983-84 ice pit. No lake separated the lava flow from the ice, and there was no sign of the tunnels or ductile deformation in the ice so visible in 1983-84. Assuming a 1 km2 area and a 30-50 m thickness, 30-50 x 106 m3 of ice (<1% of the ice in the caldera) have been melted during the present eruption. An estimated 150 x 106 m3 of ice melted during the 1983-84 eruption.

Gas and steam was seen rising sparsely and discontinuously over the surface of the lava field. A vigorous fumarolic plume rose from the margin of the lava field E of the active flow. The second most active fumarole issued from the edge of the lava field N of Half Cone, and a third strong fumarole rose from the SE margin of the lava field. All three plumes were white in color and issued from the ice field at the lava-field contact. Whether each one represented an active lava flow or some combination of cooling lava, water, and ice field was unknown.

Geologic Background. Massive Veniaminof volcano, one of the highest and largest volcanoes on the Alaska Peninsula, is truncated by a steep-walled, 8 x 11 km, glacier-filled caldera that formed around 3700 years ago. The caldera rim is up to 520 m high on the north, is deeply notched on the west by Cone Glacier, and is covered by an ice sheet on the south. Post-caldera vents are located along a NW-SE zone bisecting the caldera that extends 55 km from near the Bering Sea coast, across the caldera, and down the Pacific flank. Historical eruptions probably all originated from the westernmost and most prominent of two intra-caldera cones, which reaches an elevation of 2156 m and rises about 300 m above the surrounding icefield. The other cone is larger, and has a summit crater or caldera that may reach 2.5 km in diameter, but is more subdued and barely rises above the glacier surface.

Information Contacts: AVO.