Journal of Volcanology and Geothermal Research, 7 (1980) 11-37 11 © Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands UKINREK MAARS, ALASKA, I. APRIL 1977 ERUPTION SEQUENCE, PETROLOGY AND TECTONIC SETTING JUERGEN KIENLE', PHILIP R. KYLE 2 , STEPHEN SELF', ROMAN J. MOTYKA' and VOLKER LORENZ4 ' Geophysical Institute, University of Alaska, Fairbanks, AK 99701 (U.S.A.) 'Institute of Polar Studies, Ohio State University, Columbus, OH 43210 (U.S.A.) Department of Earth Sciences, Dartmouth College, Hanover, NH 03755 (U.S.A.) 4 Institut fur Geowissenschaften, Johannes Gutenberg-Universitilt, 6500 Mainz (Federal Republic of Germany) (Revised version accepted September 3, 1979) ABSTRACT Kienle, J., Kyle, P.R., Self, S., Motyka, R.J. and Lorenz, V., 1980. Ukinrek Maars, Alaska, I. April 1977 eruption sequence, petrology and tectonic setting. J. Volcanol. Geo- therm. Res., 7: 11-37. During ten days of phreatomagmatic activity in early April 1977, two maars formed 13 km behind the Aleutian arc near Peulik volcano on the Alaska Peninsula. They have been named "Ukinrek Maars", meaning "two holes in the ground" in Yupik Eskimo. The western maar formed at the northwestern end of a low ridge within the first three days and is up to 170 m in diameter and 35 m in depth. The eastern maar formed during the next seven days 600 m east of West Maar at a lower elevation in a shallow saddle on the same ridge and is more circular, up to 300 m in diameter and 70 m in depth. The maars formed in terrain that was heavily glaciated in Pleistocene times. The groundwater contained in the underlying till and silicic volcanics from nearby Peulik volcano controlled the domi- nantly phreatomagmatic course of the eruption. During the eruptions, steam and ash clouds reached maximum heights of about 6 km and a thin blanket of fine ash was deposited north and east of the vents up to a distance of at least 160 km. Magma started to pool on the floor of East Maar after four days of intense phreatomagmatic activity. The new melt is a weakly undersaturated alkali olivine basalt (Ne = 1.2%) showing some transitional character toward high-alumina basalts. The chemistry, an anomaly in the tholeitic basalt-andesite-dominated Aleutian arc, suggests that the new melt is primitive, generated at a depth of 80 km or greater by a low degree of partial melting of garnet peri- dotite mantle with little subsequent fractionization during transport. The Pacific plate subduction zone lies at a depth of 150 km beneath the maars. Their position appears to be tectonically controlled by a major regional fault, the Bruin Bay fault, and its intersection with cross-arc structural features. We favor a model for the emplacement of the Ukinrek Maars that does not link the Ukinrek conduit to the plumbing system of nearby Peulik volcano. The Ukinrek eruptions probably represent a genetically distinct magma pulse originating at asthenospheric depths beneath the conti- nental lithosphere. Onlvenity enntri .nutinns Series rairbantz.3, idc:ka 9970 13 Jima Island in 1957 (Corwin and Foster, 1959) and in the Nilahue River Val- ley, Chile in 1955 (Muller and Veyl, 1957; Zuniga, 1956; flies, 1959). Pos- sibly a maar was also produced by a very large eruption of Ksudach volcano in Kamchatka in 1907 (Vlodavetz and Piip, 1959), by the interaction of magma and water from a pre-existing caldera lake. Maars form when the crust is perforated by phreatic or phreatomagmatic explosions that result when rising magma contacts groundwater at shallow depth. Phreatic is used here to denote explosions involving steam alone or steam and country rock; phreatomagmatic denotes explosions involving steam and magma (with or without country rock). By far the most common occur- rence of maars is in association with basic volcanism where groundwater inter- cepts magma that would, without the intervention of water, egress to the sur- face to produce strombolian explosions, scoria cones and lava flows. However, maars can be formed by other magma types, including carbonatitic, interme- diate or even acidic melts (Lorenz, 1973). Possibly most maars are the surface expression of diatremes (Lorenz, 1975; Lorenz et al., 1970; McCallum et al., 1976). Between March 30 and April 9, 1977, two maars formed near the crest of a short east-west-trending ridge in generally low-lying glacial terrain, 2.0 km south of the shore of Lake Becharof on the Alaska Peninsula (Kienle et al., 1978). The rim of the maars are located about 70 m above the lake level. The eruption site lies behind the Aleutian volcanic arc, 13 km northwest of Mt. Peulik, a 1525-m-high stratovolcano which erupted last in 1852. Fig. 1 shows the location of the maars and their geologic setting. Quaternary Volcanics Volcanoes FTI Mesozoic Shallow Marine ' Sandstone a Shale 1..1 Jurassic Batholith —Fault UKINREK MAARS Pilot4 Point Regional Ash Fallout (7• 77 n , r _ _, 0 Fig. 1. Map of the upper Alaska Peninsula, showing the location of the Ukinrek Maars (star), Quaternary and Recent volcanoes and simplified geology after Beikman (1978). The minimum area of fine ash fall is also indicated. 15 C Fig. 2. (a) Easterly view across the two Ukinrek Maars on April 3, 1977, 10 AST, West Maar in foreground with hot black ejecta blanket and steam rising from the lake, East Maar in phreatomagmatic eruption in the background (photograph by Larry Conyers, CITGO). (b) "Mushroom" cloud produced by phreatic/phreatomagmatic explosion on April 5, 1977, 15:30-16:00 AST. Note the "cap" of white steam followed by a column of dark ash and steam (photograph looking south by Jim Faro, ADFG). (c) Vigorous phreatomagmatic activity at the East Maar on the evening of April 6, 1977, 17:00 AST (photograph looking south by R. Russel, ADFG). 17 66 •8 71.4 B D 61 9 NI 67.8 BM2 70.2 73 0 67.2 BM1 71626112 • 72• 78.177A3 ,75 7 •69.2 • 71.8 0 79.16 IF 79 1 • 75.0 BM1A 78.3 72.2 75 96" Agglomerate 0 50 100 meters WEST MAAR EAST MAAR Fig. 4. Map of the Ukinrek Maars (survey by R. Motyka), also showing the thickness of the rim deposit. Accuracy of absolute elevations, given in meters above sea level, is unknown; elevation differences between surveyed points are 0.5 m or better. Date of survey: August 30, 1977. 21 the Ukinrek Maars are small (see also Table 1) and once again the lack of ero- sional modification is evident. We were surprised to find very high temperatures within the ejecta of two distinct scoriaceous* fall lobes on the high southeast and southwest rims of West Maar (Fig. 6a, b). The highest temperature measured was 805°C near its rim at 1.1 m depth, temperatures on a profile between the two maars decreas- ed from 210°C at 15 cm depth at the rim of the West Maar to 20°C at the same depth at the rim of the East Maar (D.J. Lalla, personal communication). Similar high temperatures at a distance of 250 m west of the West Maar are in- dicated by the fact that willow trunks were completely charcoaled at a depth greater than 20 cm below the tephra surface. The high temperatures on the rim probably indicate very rapid accumulation of spatter and scoriaceous lapilli and bombs during the latter stages of West Maar activity, suggesting that magma also reached the surface at this maar. We have no visual observations to confirm magma in the crater of West Maar when it was active during the first three days of the eruption. However, irregular dike-like intrusions were observed in the till exposed in the southern crater wall during a 1979 visit by one of the authors (Lorenz). In April 1977 West Maar contained a lukewarm shallow (4.6 m deep) lake of slightly acidic (pH — 6) water (Fig. 6a). Subsequently, the lake drained (Fig. 6c) sometime between our second visit, May 20-29, and our third visit in late August. The water of the remaining hot spring (next to the person in Fig. 6c) was sampled by G. McCoy and the U.S. Geological Survey on August 24: It had a pH of 6.3 and a temperature of 81°C. The gas in the thermal wa- ters contained 98% CO 2 with a 13C composition of —6.36%o, a range that is common for CO 2 from the mantle (Barnes et al., 1978). By July 1979 a shal- low lake had again formed within the northern half and at the location of the former spring two lines of gas bubbles were observed with headings of N47°E and N151°E. Ejecta and maar volumes The two maars are surrounded by deposits of fine, grey-tan-colored ash fall, grey medium-coarse vesicle-poor lapilli and lithic fall, black juvenile scoriace- ous fall including many cauliflower bombs (Lorenz, 1974a), spherical bombs and ribbon bombs, and lithic (country rock) ejecta blankets (Figs. 2a, 6a, f). No accretionary lapilli have been observed. However, vesiculated tuffs (Lorenz, 1974b) were deposited by base surge against obstacles. Several fall units compose any one section and reflect the changing style of activity (Fig. 6d). The crater walls of West Maar (Fig. 6b, c) expose till; the walls of East Maar (Fig. 6d—f) are not readily accessible but appear to consist of till sheets and interbedded silicic pyroclastic (pumice?) Peulik volcanics, as viewed from the crater rim. *Within the scoriaceous deposits of both West and East Maar a great number of cauliflower bombs indicate a phreatomagmatic component.