Geology topics
Janet Schaefer
Publications and source records attributed to Janet Schaefer.
Geologic map of Okmok Volcano
The geologic map and description of map units presented here cover approximately 880 km2 of northeastern Umnak Island, Aleutian Islands, Alaska. This report focuses on Okmok Volcano and its eruptive products and updates the mid-20th-century geologic map of Byers (1959). Mapped deposits reflect the state of the volcano just prior to the 2008 eruption. Published information about other portions of Umnak Island geology, including Mount Recheshnoi and Mount Vsevidof, can be found in Byers (1959). The 2008 eruption and its deposits are described in Larsen and others (2009, 2013, 2015). Okmok Volcano is one of 54 historically active volcanoes in the Alaska–Aleutian volcanic arc that stretches across southern mainland Alaska and the Aleutian Islands (fig. 1; Wood and Kienle, 1990; Miller and others, 1998; Cameron and others, 2020). The highest point of the modern Okmok Caldera is along the caldera’s northern rim, 967 m in elevation, and formally named “Mount Okmok” (U.S. Board on Geographic Names, www.usgs.gov/core-science-systems/ngp/boardon-geographic-names/domestic-names). Okmok Volcano dominates the northeastern portion of Umnak Island, which is 100 km southwest of Unalaska/Dutch Harbor and 1,400 km southwest of Anchorage (figs. 1, 2). The Port of Dutch Harbor on Unalaska Island produces the highest volume of seafood for any port in the United States (see fisheries.noaa.gov/resource/document/fisheries-united-states-2018-report). Unalaska city and the Port of Dutch Harbor have been impacted by ash fall and drifting ash clouds from Okmok Volcano’s explosive eruptions as recently as 2008. Holocene and late Pleistocene volcanic rocks and deposits of Okmok Volcano rest upon glaciated Tertiary volcanic and sedimentary rocks (Byers, 1959). The first geologic mapping expedition to Okmok Volcano was by the U.S. Geological Survey (USGS) after the 1945 eruption, largely in response to concerns about volcanic hazards to U.S. military activities in the Aleutians Islands (Byers and others, 1947, 1959; Byers and Brannock, 1949; Byers, 1955, 1959, 1961). The State of Alaska conducted further mapping and geochemical studies as part of its geothermal exploration program in the 1980s (Nye, 1983; Nye and Reid, 1986; Motyka and others, 1993). Additional modern geological work focused on Okmok Volcano and the rest of Umnak Island to address the geochemistry and origin of primary Aleutian arc magmas and subduction zone mass recycling (Marsh, 1982; Brophy and Marsh, 1986; Nye and Reid, 1986; Myers and Marsh, 1987; Miller and others, 1992; Fournelle and others, 1994; Kay and Kay, 1994). In 1998, the Alaska Volcano Observatory (AVO) began a multi-year effort to expand geophysical monitoring in the central Aleutians Islands, including at Okmok Volcano. As part of this effort, AVO geologists from the University of Alaska Fairbanks Geophysical Institute (UAF/GI), the Alaska Division of Geological & Geophysical Surveys (DGGS), and USGS also began a renewed effort to document Okmok Volcano’s recent eruption products. The project started with reconnaissance fieldwork to document and sample products from the 1997 eruption within Okmok Caldera. This evolved into an effort to produce an updated geologic map of Okmok Volcano and gather more information about its eruptive history and hazards. Three significant eruptions occurred at Okmok Volcano in 1958, 1997, and 2008—after fieldwork had been conducted for the original 1:63,360-scale geologic map produced by Byers (1959)—resulting in new volcanic deposits not previously described. Okmok Volcano is one of the most frequently active volcanoes in the Aleutian volcanic arc. Seismic and geodetic monitoring indicate ongoing unrest at Okmok Volcano since at least 1997. Geodetic observations of inflation before and after the 1997 and 2008 eruptions indicate a nearly continuous input of new magma from a depth consistent with frequent eruptions of basalt and basaltic andesite magmas over the past 200 years (Larsen and others, 2013; Lu and others, 2000, 2003, 2005; Mann, 2002; Mann and others, 2002). To better understand the likelihood and character of future eruptions from Okmok Volcano, it is necessary to understand its past behavior, including eruptions since the first geologic map was published by Byers (1959).
Petrology and geochemistry of three early Holocene eruptions from Makushin volcano, Alaska
Makushin stratovolcano, Alaska, produced three, highly explosive, andesitic eruptions between ~ 9292 and 6215 yBP. Those eruptions are informally named the CFE (“crater-forming eruption”), Nateekin, and Driftwood Pumice, and they deposited significant tephra fallout in the present-day port of Dutch Harbor and City of Unalaska area. The focus of this study is to examine the geochemistry and petrology of those eruptions to better understand Makushin volcano hazards, andesite petrogenesis and eruption triggering by mafic recharge processes. The CFE, Nateekin, and Driftwood Pumice samples range from basaltic andesite to dacite but are predominantly andesitic (SiO 2 = 55.6 to 63.5 wt%). The CFE deposits are slightly compositionally stratified, with the top CFE samples slightly more mafic (55 to 60 wt% SiO 2 ) than the basal deposits (58 to 60 wt% SiO 2 ). Disequilibrium mineral compositions and textures in the CFE, Nateekin, and Driftwood Pumice samples, combined with two pyroxene thermometry and An-rich plagioclase microlites (An 80 ) found only in the top of the CFE deposits, provide evidence for repetitive mafic recharge triggering those eruptions, consistent with prior studies. We compare the Makushin geochemical data with data from select satellite vents and cones in the Makushin Volcanic Field (MVF) from prior studies, to examine possible genetic relationships. The geochemical data and Rhyolite-MELTS models run at crustal storage conditions (2 kbar, fO 2 = Ni-NiO, and 1.5 and 3.5 wt% H 2 O) indicate that no single parental magma supplies the MVF satellite cones and Makushin volcano. Instead, two component mixing models better fit the MVF geochemical array. Our Makushin results compare well with models of predominantly andesitic volcanoes that require mafic recharge to mobilize the andesites and trigger eruptions.
A post-eruption study of gases and thermal waters at Okmok Volcano, Alaska
We report here on the first focused study of gas discharges and thermal spring waters at Okmok Volcano since the 2008 phreatomagmatic eruptions. Results include the first compositional gas data from Okmok with minimal air contamination and the first data on magmatic carbon in Okmok spring waters. Chemical and isotopic analyses of the waters and gases are used to assess the character of Okmok fluids eight years after the eruptions ceased. Gases from vents on intracaldera Cone C have high concentrations of H2 and contain H2S rather than SO2, demonstrating the influence of a hydrothermal system, while isotope values of carbon ( 10.2 to 8.9‰) and helium (~8 RA) confirm the presence of magma-derived volatiles. Estimates of equilibrium temperatures for the Cone C gas are ~230 ± 30 ºC. A much cooler reservoir with a maximum temperature of ~55 ºC feeds the intracaldera warm springs. Based on discharge measurements of creeks draining the caldera, the total heat output of the warm springs is estimated to be about 32 MW. Gas data from a single location of steaming ground at the Geyser Bight geothermal area southwest of the Okmok Caldera are given. The gas is typical of geothermal gases with high concentrations of H2S and an air-corrected helium isotope ratio of 7.15 RA.
Short-term forecasting and detection of explosions during the 2016–2017 eruption of Bogoslof volcano, Alaska
We describe a multidisciplinary approach to forecast, rapidly detect, and characterize explosive events during the 2016–2017 eruption of Bogoslof volcano, a back-arc shallow submarine volcano in Alaska’s Aleutian arc. The eruptive sequence began in December 2016 and included about 70 discrete explosive events. Because the volcano has no local monitoring stations, we used distant stations on the nearest volcanoes, Okmok (54 km) and Makushin (72 km), combined with regional infrasound sensors and lightning detection from the Worldwide Lightning Location Network (WWLLN). Pre-eruptive seismicity was detected for 12 events during the first half of the eruption; for all other events co-eruptive signals allowed for detection only. Monitoring of activity used a combination of scheduled checks combined with automated alarms. Alarms triggered on real-time data included real-time seismic amplitude measurement (RSAM); infrasound from several arrays, the closest being on Okmok; and lightning strokes detected from WWLLN within a 20-km radius of the volcano. During periods of unrest, a multidisciplinary response team of four people fulfilled specific roles to evaluate geophysical and remote-sensing data, run event-specific ash-cloud dispersion models, ensure interagency coordination, and develop and distribute of formalized warning products. Using this approach, for events that produced ash clouds ≥7.5 km above sea level, Alaska Volcano Observatory (AVO) called emergency response partners 15 min, and issued written notices 30 min, after event onset (mean times). Factors that affect timeliness of written warnings include event size and number of data streams available; bigger events and more data both decrease uncertainty and allow for faster warnings. In remote areas where airborne ash is the primary hazard, the approach used at Bogoslof is an effective strategy for hazard mitigation.
Injection, transport, and deposition of tephra during event 5 at Redoubt Volcano, 23 March, 2009
Among the events of the 2009 eruption at Redoubt Volcano, Alaska, event 5 was the best documented by radar, satellite imagery, and deposit mapping. We use the new Eulerian tephra transport model Ash3d to simulate transport and deposition of event 5 tephra at distances up to 350 km. The eruption, which started at about 1230 UTC on 23 March, 2009, sent a plume from the vent elevation (estimated at 2.3 ± 0.1 km above sea level or a.s.l.) to about 16 ± 2 km above sea level in 5 min. The plume was a few kilometers higher than would be expected for the estimated average mass eruption rate and atmospheric conditions, possibly due to release of most of the eruptive mass in the first half of the 20-minute event. The eruption injected tephra into a wind field of high shear, with weak easterly winds below ~ 3 km elevation, strong southerly winds at 6–10 km and weak westerlies above ~ 16 km. Model simulations in this wind field predicted development of a northward-migrating inverted “v”-shaped cloud with a southwest-trending arm at a few kilometers elevation, which was not visible in IR satellite images due to cloud cover, and a southeast-trending arm at > 10 km elevation that was clearly visible. Simulations also predicted a deposit distribution that strongly depended on plume height: a plume height below 15 km predicted ash deposits that were located west of those mapped, whereas good agreement was reached with a modeled plume height of 15–18 km. Field sampling of the deposit found it to contain abundant tephra aggregates, which accelerated the removal of tephra from the atmosphere. We were able to reasonably approximate the effect of aggregation on the deposit mass distribution by two methods: (1) adjusting the grain-size distribution, taking the erupted mass < = 0.063 mm in diameter and distributing it evenly into bins of coarser size; and (2) moving 80–90% of the mass < = 0.063 mm into a single particle bin ranging in size from 0.25 to 1 mm. These methods produced an area inside the 100 g m − 2 isomass lines that was within a few tens of percent of mapped area; however they under-predicted deposit mass at very proximal (< 50 km) and very distal (> 250 km) locations. Modeled grain-size distributions at sample locations are also generally coarser than observed. The mismatch may result from a combination of limitations in field sampling, approximations inherent in the model, errors in the numerical wind field, and aggregation of particles larger than 0.063 mm.
Eruption of Alaska volcano breaks historic pattern
In the late morning of 12 July 2008, the Alaska Volcano Observatory (AVO) received an unexpected call from the U.S. Coast Guard, reporting an explosive volcanic eruption in the central Aleutians in the vicinity of Okmok volcano, a relatively young (~2000-year-old) caldera. The Coast Guard had received an emergency call requesting assistance from a family living at a cattle ranch on the flanks of the volcano, who reported loud "thunder," lightning, and noontime darkness due to ashfall. AVO staff immediately confirmed the report by observing a strong eruption signal recorded on the Okmok seismic network and the presence of a large dark ash cloud above Okmok in satellite imagery. Within 5 minutes of the call, AVO declared the volcano at aviation code red, signifying that a highly explosive, ash-rich eruption was under way.
Late pleistocene and holocene caldera-forming eruptions of Okmok Caldera, Aleutian Islands, Alaska
This chapter contains sections titled: Introduction Geologic setting Field and Analytical Methods Results Discussion Conclusions