Juergen Kienle

Ukinrek Maars, Alaska, II. Deposits and formation of the 1977 craters

Abstract The initial phase of the eruption forming Ukinrek Maars during March and April 1977 were explosions from the site of West Maar. These were mainly phreatomagmatic and initially transitional to strombolian. Activity at West Maar ceased after three days upon the initiation of the East Maar. The crater quickly grew by strong phreatomagmatic explosions. During the first phases of phreatomagmatic activity at East Maar, large exotic blocks derived from a subsurface till were ejected. Ballistic studies indicate muzzle velocities for these blocks of 80–90 m s −1 . Phreatomagmatic explosions ejected both juvenile and non-juvenile material which formed a low rim of ejecta ( m high ) around the crater and a localized, coarse, wellsorted ( σ φ = 1−1.5) juvenile and lithic fall deposit. Other fine ash beds, interstratified with the coarse beds, are more poorly sorted ( σ φ = 2−3) and are interpreted as fallout of wet, cohesive ash from probably milder phases of activity in the crater. Minor base surge activity damaged trees and deposited fine ash, including layers plastered on vertical surfaces. Viscous basalt lava appeared in the center of the East Maar crater almost immediately and a lava dome gradually grew in the crater despite phreatomagmatic eruptions adjacent to it. The development of these maars appears to be mainly controlled by gradual collapse of crater and conduit walls, and blasting-out of the slumped debris by phreatomagmatic explosions when rising magma contacted groundwater beneath the regional water table and a local perched aquifer. Ballistic analysis on the ejected blocks indicates a maximum muzzle velocity of 100–150 m s -1 , values similar to those obtained from other ballistic studies on maar ejecta.

Ukinrek Maars, Alaska, I. April 1977 eruption sequence, petrology and tectonic setting

Abstract 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 dominantly 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 peridotite 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 continental lithosphere.

The dynamics and thermodynamics of volcanic clouds: Theory and observations from the april 15 and april 21, 1990 eruptions of redoubt volcano, Alaska

On April 15, 1990 and April 21, 1990, two relatively small explosive eruptions occurred at Redoubt Volcano, Alaska, lasting about 4 and 8 minutes respectively. On both occasions the erupted material travelled as a pyroclastic flow down an ice canyon on the north flank of the volcano. Using slow-scan television recordings of the eruption and the seismic record in both the near- and far-field, we deduce that after a few minutes, the upper parts of these pyroclastic flows became buoyant and a large and hot ash cloud ascended off the flow. These thermals rose to a height of about 12 km, at which point they began to spread laterally, as umbrella clouds. Using a simple thermodynamic model, we estimate that the clouds had a temperature in the approximate range 600–700 K as they rose buoyantly from the flow after entraining and heating air, and melting and vaporizing ice. We also estimate that in each eruption approximately 109 kg of fine ash was injected into the atmosphere. During the April 15 eruption, a slow-scan television camera recorded the ascent of the cloud. These observations are described, analysed and compared with the predictions of a new model for the dynamics governing the ascent of such clouds. In accord with the observations, our model predicts that the cloud initially ascended rather sluggishly, since it is only just buoyant on rising from the pyroclastic flow. As it ascends, it entrains and heats up more air, and hence generates more buoyancy. Therefore, it accelerates upwards; only much higher in the cloud does the velocity decrease again, as the thermal energy of the cloud becomes exhausted. The model also predicts that the height of rise of such coignimbrite thermals is a function of the initial mass and temperature of the cloud, but is almost independent of the initial velocity. During the April 21 eruption, a sequence of photographs recorded the lateral spreading of the umbrella cloud during an interval of about 10 minutes after the eruption. These photographs are analysed and successfully compared with a simple model for the spreading of the umbrella cloud as a gravity current in a stratified environment. Using an AVHRR satellite image of the air-fall deposit from the April 15 eruption combined with a simple model, we suggest that the primary mechanism of ash dispersal was transport by the ambient wind.

Plume dynamics, thermal energy and long-distance transport of vulcanian eruption clouds from Augustine Volcano, Alaska

Abstract Augustine, an island volcano in Lower Cook Inlet, southern Alaska, erupted in January, 1976, after 12 years of dormancy. By April, when the eruptions ended, a new lava dome had been extruded into the summit crater and about 0.1 km3 of pyroclastics had been deposited on the island, mainly as pyroclastic debris avalanches and pumice flows. The ventclearing phase in January was highly explosive and we have been able to document 13 major vulcanian eruptions. The timing, thermal energy, mass loading of fine particles and the horizontal dispersion of these eruption clouds were determined from radar measurements of cloud height, reports of pilots flying in plumes, satellite photography, seismic records and infrasonic detection of air waves. A lower estimate of the mass of fine (r The vulcanian eruption clouds stayed intact for at least 700 km downwind. Satellite images in both visible and infrared wavebands, showing the Gulf of Alaska just after sunrise on January 23, reveal a series of puffs strung out downwind from the volcano, 20–30 km in diameter and with their tops at altitudes of about 8 km, overlying a continuous plume at altitude 4 km. Each puff corresponded to a seismically and infrasonically timed eruption. A substantial portion of the material injected into the atmosphere between January 22 and 25 was rapidly transported by the subpolar jet stream through southwestern Canada and the western United States, then northeast across the States into the Atlantic. The clouds were observed passing over Tucson, Arizona, on January 25 at an elevation of 7 km. Several of the eruptions penetrated into the stratosphere. Sun photometer measurements, taken at Mauna Loa, Hawaii, six weeks after the eruption, showed an increased stratospheric optical thickness of 0.01 (wavelength 0.5 μm), which decayed in about 5 months. The maximum column mass loading of the veil was 4–10 × 10−7 g cm−2. The mass of the veil, spread-ever a fourth of the earths surface, is 10 to 100 times larger than can be accounted for by assuming that injected ash and converted sulfate particles from the 13 main Augustine eruptions are the only components contributing to the stratospheric turbidity observed at Mauna Loa.

Geophysical studies of Erebus volcano, Antarctica, from 1974 December to 1982 January

Abstract Seismic, infrasonic, and magnetic induction recordings and eruption observations during expeditions to Mount Erebus in December-—January of 1974–75, 1975–76, 1978–79,1978–81 and December 1981 are reported. Erebus IS umque in its high-latitude location within a tectonic plate and its persistent lava lake of phonolitic composition. In an aseismic region, more than 100 volcanic earthquakes per day occur in the energy range of 0.2-200000 J (ML–2 to 1). Foci Iie mostly within the edifice of the vo1cano with a concentration of B-types near the crater. Preliminary Pvelocity determinations are 1.6 km/s in the summit cone (from crater explosions),4.5 km/s in the main edifice (from distant earthquakes), 6.5 km/s in the underlying basement, and 7.5 km/s at about 7 km below sea level (from refraction surveys). The b-value is 1.5 for earthquakes below 100 J and 0.6 for the larger earthquakes which accompany the average 3.6 ± 2.7 strombolian eruptions per day. Large earthquakes (and eruptions) are about 10 tim...

Volcanism in the eastern Aleutian arc: Late quaternary and holocene centers, tectonic setting and petrology

Abstract Cale-alkaline volcanism and oceanic plate subduction are intimately linked in the eastern Aleutian arc. The volcanic arc is segmented: larger caldera-forming volcanic centers tend to be located near segment boundaries. Intrasegment volcanoes form smaller stratocones. Ten of the 22 volcanoes that make up the 540 km long volcanic front in the eastern Aleutian are have erupted in recorded history and another six show hydothermal activity. The geometry of the Benioff zone in the eastern Aleutian arc has been defined by earthquake data from a local, high-gain short-period seismograph network. The Benioff zone dips at an angle of about 45° beneath the volcanic arc and reaches a maximum depth of 200 km. Based on the alignment of volcanoes, the eastern Aleutain arc can be subdivided into two main segments, the Cook and Katmai segments. A misorientation of 35° of the two segments reflects a change in strike of the underlying Benioff zone and implies a lateral warping of the subducting plate. The Cook segment volcanoes line up closely on the 100 km isobath of the Benioff zone. The Katmai segment volcanoes lie on a cross-cutting trend with respect to the strike of the underlying Benioff zone. Depths to the dipping seismic zone beneath volcanoes of the Katmai segment vary by 25% from 100 to 75 km. In the Katmai segment there is also good geophysical evidence that crustal tectonics plays an important role in localizing volcanism. Narrowly spaced linear groups of volcanoes appear to be positioned over a deep crustal fault that underlies the volcanic front. Transverse arc elements divide the arc into subsegments and localize larger magma reservoirs at shallow levels in the crust. Intrasegment volcanoes in both the Cook and Katmai segments erupt andesite and minor dacite of remarkably uniform composition despite differences in depths to the Benioff zone. Segment boundary volcanoes erupt lavas with a wider range of compositions (basalt to rhyolite) but are still calc-alkaline, in contrast to volcanoes in similar tectonic settings near segment boundaries in the central Aleutains. Greater crustal thickness in the eastern Aleutian arc, coupled with structural traps in the crust, allow magma ponding at shallow crustal levels. Differentiation at shallow depths yields dacite and even rhyolite.