Event Name : Ukinrek Maars 1977/3
|Start:||March 30, 1977 ||Observed|
|Stop:||April 9, 1977 ± 4 Hours||Observed|
|Lava flow: ||
|Pyroclastic flow, surge, or nuee ardente: ||
|Lava dome: ||
|Maar, tuff cone, tuff ring: ||
|Tephra plume: ||
|Eruption Product: || basalt ||
|ColHeight: ||6500 m ||
|MaxVEI: ||3 ||
|Duration: ||11 days ||
From Kienle and others (1980): "The first explosions were observed on March 30, 1977, about 4:00 a.m. Alaska Standard Time (AST), from Egegik, 65 km northwest of the eruption site. A fine ash fall occurred that day at Larsen Bay, Kodiak Island, 160 km east of the maars, and a sulfurous haze layer lay over the city of Kodiak, 250 km to the east. Fine ash fell over some 20,000 - 25,000 square km, but significant ash accumulation was restricted to a radius of about 3 km from the maars. During the day, the Federal Aviation Administration (FAA) in King Salmon received reports from pilots of steam and ash columns reaching heights of 300-600 m. Two King Salmon pilots independently overflew the eruption site in the late afternoon (17:00 AST) and witnessed a steam and ash plume rising 1500 m before billowing out and eventually reaching about 6500 m, the maximum height reported during the eruptions. The source of the explosions was a single vent, 30-35 m in diameter, located at an elevation of approximately 70 m above sea level on a short ridge about 3 km southwest of Gas Rocks at the southern shore of Becharof Lake and called West Maar in this paper.
"Poor weather conditions prevented observations on March 31 and April 1, but at 11:00 AST, April 2, a commercial airline pilot observed a 3000-m-high ash plume in the vicinity of Gas Rocks. A new vent, about 60 m in diameter and located 600 m east of the original vent at about the same elevation, was discovered by an FAA pilot at 17:30 AST; the original vent had partially filled with water and was quiescent. The new vent was ringed by about seven tephra layers which were thickest on the NNE side of the crater rim.
"Reports indicate that the second crater, here called East Maar, had increased in diameter to about 100 m by 10:00 AST, April 3. At 14:30 AST Alaska Department of Fish and Game (ADFG) personnel photographed a yellowish-orange pool of lava at the bottom of the second crater. Another bush pilot observed that incandescent material was thrown up to 30 m above the lava pool and that large boulders, up to 1 m and more in diameter, were ejected to a height of 300 m.
"On April 4 the weather was poor and it snowed. In the late morning hours of April 5 an ash cloud up to 4000 m high reached King Salmon, 95 km to the north, where trace amounts of ash accumulated through mid-afternoon. At 16:30 AST, a spectacular eruption from East Maar was witnessed by ADFG personnel during a helicopter flight near Gas Rocks. The eruption cloud was initially of a white color, probably rich in steam and poor in solids, a phase which was followed by vigorous ejection of ash and rock debris, culminating in a mushroom-shaped cloud. The column was approximately 500 m in diameter near its base and reached a height of 3000 m before billowing out. This eruption was followed by a series of smaller phreatomagmatic explosions. Many of them had a companion ground wave (base surge?) of gas and tephra which appeared to be directed laterally down a trough leading to the WNW away from the crater. During the day wind-blown air fall ejecta was dispersed principally to the northwest and north.
"By mid-afternoon on April 6 fresh magma had apparently drained from the East Maar and the activity had decreased to steaming, with vapor plumes reaching heights of 1000 m. This milder activity allowed clearer examination of the East Maar at this stage of its development. Well-bedded tephra layers, amounting to a total thickness of several meters, surrounded the crater which was conically shaped but skewed, having a nearly vertical south-southeastern wall. The deepest part of the crater, and also the source of the initial eruptive activity, was at the base of this wall. Activity increased significantly by early evening when heavy steam and ash laden phreatomagmatic clouds were periodically ejected from East Maar.
"Magma had returned to the crater by 09:30 AST, April 7. During a University of Alaska (UA) overflight, an orange-red lava pool, about 20-30 m in diameter, was observed at the bottom of East Maar. Incandescent material was again thrown up to a height of about 30 m above the magma pool. Turbulent dark grey ash laden clouds were periodically (every 5 minutes) ejected from a second vent located northwest of the first vent but still within East Maar, with plumes rising to 1500 m. The quiet intervals were characterized by intense steaming. The source of the steam appeared to have been subsurface groundwater coming into contact with magma. * * * The height of the magma level within the crater was variable: during one 15-minute period, the magma completely drained from the crater, an ash explosion occurred, then the magma returned to its previous level, about 50-60 m below the southern rim of the crater. * * * No further eruptions were reported until the early morning hours of April 9, when a fisherman saw violent explosions from a distance of 30 km. These explosions marked the end of the eruption and subsequent activity was confined to steaming and degassing."
Kienle and others (1980) also estimate the total volume of erupted material: "A total bulk volume of 26x10^6 cubic m is probably an underestimate for the deposits within a 5-km radius of the maars. The volume of non-juvenile ejecta should be approximately that of the craters. East Maar is estimated to have a crater volume of 4x10^6 cubic m and that of West Maar is 0.25x10^6 cubic m. The dome in East Maar above the crater floor has a volume of 0.9x10^6 cubic m. The total volume of the ejecta represents perhaps 10x10^6 cubic m of dense rock which is substantially greater than the combined crater volume, 4.3x10^6 cubic m. The excess volume, 5.7x10^6 cubic m of dense rock, is mostly accounted for by that of juvenile airfall material."
Self and others (1980) provide more detailed estimates of the volumes of the erupted juvenile material: "The volume of the juvenile 'strombolian' deposit plus a rough estimate of the juvenile component of the other deposits gives a dense rock estimate of the order of 0.5x10^6 cubic m. The basalt lava flow extruded onto the floor of East Maar contains about 1x10^6 cubic m of dense rock. Hence, about 1.5 x10^6 cubic m were ejected in 11 days activity. For the lava flow alone a very slow extrusion rate of 1.8 cubic m s^-1 is indicated using 47 hours as the duration of the 'strombolian' phase.