Late Cenozoic Volcanism in the Aleutian Arc: Examining the pre-Holocene record on Unalaska Island
What: Volcanoes of the Aleutian arc have erupted intermittently during the last three million years, and biostratigraphy indicates that the marine sediments have accumulated in a subduction zone setting since the Miocene, however, previous studies of Aleutian volcanoes have focused primarily on the main stratocones and satellite cinder cones that have erupted in the Holocene. On Unalaska Island, Makushin Volcano and its satellite vents have erupted on older lavas and pyroclastic deposits, yet these Pleistocene lavas, which cover approximately 600 km2, are virtually unstudied. We propose to map and sample the Pleistocene pre-Makushin lavas, which will facilitate subsequent lab-based investigations into the timing and development of the pre-Makushin magmatic system.
When: July 8-August 4
Where: Near Makushin Volcano, Unalaska Island, in the Aleutian island chain.
Who: Six students and two project leaders: Kirsten Nicolaysen (Whitman College) and Rick Hazlett (Pomona College)
Project Description and Goals
Our primary goal is to provide significant field and petrologic research experiences for the participating students including developing field research skills, formulating hypotheses and designing sampling campaigns. Students will engage in post-field laboratory investigation and presents the analysis their results in comparison to published work. The trip will also provide opportunity to meet staff at the Alaska Volcano Observatory and participate in the unique culture of Aleutians that combines influences from native Unangan peoples, Russian Orthodox traditions, and American enterprise.
Previous investigations of the Aleutian arc indicate a spectrum of magma formation conditions from direct slab melting in the western portion of the arc to a system dominated by sediment and fluid input in the east. Compositionally the Aleutian lavas range from magnesian andesites similar to early continental crust to more typical calc-alkaline lavas. Although regional compositional variation has been investigated by examining Holocene lavas and pyroclastic deposits (e.g., Kelemen et al., 2003; Singer et al., 2007), few studies extend this to the earlier Pliocene and Pleistocene volcanic deposits.
Located in the eastern portion of the arc, Makushin is the glaciated Holocene volcano (2,036 m high) northern peninsula of the island of Unalaska. Because Makushin is approximately 28 km from the town of Unalaska and the major fishing port of Dutch Harbor, the Holocene volcanic histories of Makushin and its young satellite volcanoes have been vigorously investigated (e.g., Nye et al., 1986; McConnell et al., 1997, Bean, 1999; Begét et al., 2000; T. Plank, in preparation).
Makushin and its satellite vents are built upon bedrock designated the Tertiary Unalaska Formation (Tu, See Figure), which consists of undifferentiated lavas, volcaniclastic deposits and volcanogenic sandstones (Drewes et al., 1961; Nye et al., 1984). Additionally the Unalaska Formation is cross-cut by numerous dikes and sills. Overlying this stratigraphically, Nye et al. (1986) and McConnell et al. (1997) distinguish the older Pleistocene lavas and pyroclastic deposits (Qom, See Figure) as being erupted from earlier phases of Makushin or from vents now covered by younger lavas. This formation encircles Makushin, though it is primarily exposed to the northeast, north, and west of the main stratovolcano. Five preliminary dates of this unit provide a wide range of ages from as old as 2.49±0.48 Ma to as young as 45±11 ka (Nye et al., 1986; McConnell et al., 1997). Other than these preliminary dates, these units are virtually completely unstudied though their combined area covers approximately 600 km2 on the northern peninsula of Unalaska. We propose to map and sample a portion of this area by establishing a base camp at Wide Bay. Although the primary target is investigating the pre-Holocene lavas, dikes and sills, the nearby Holocene vents of Wide Bay Cone and Table Top Mountain may also inspire some student projects.
There is flexibility to identify and develop project proposals during the fieldwork. However, because the weather may curtail the number or length of field days, we have carefully considered these potential projects, several of which have publication potential. In addition to detailed mapping of the pre-Makushin lavas (map unit Qom), this proposed expedition offers the following projects:
- Geochronology: The timing of the emplacement of both the Pre-Makushin volcanic deposits and the mapped dikes and sills are constrained only by field relationships. Acquiring 40Ar/39Ar dates of the rocks of the study area is a crucial aspect of the project.
- Geochemistry and petrogenesis of Pre-Makushin lavas will provide one or two projects. In particular students would analyze the geochemical differences between the pre-Makushin lavas with those from the Holocene vents.
- Pyroxene thermobarometry (e.g., Putirka et al., 2003) reveals the crystallization depths and potentially constrains the stall points in the path of magmas en route to the surface.
- Petrogenetic investigation of unstudied sills and dikes and their relationship to Holocene and Plio-Pleistocene lavas will contribute to understanding the shallow magma supply system beneath Makushin volcano.
- Comparison of stable isotope signatures of Holocene lavas from Wide Bay Cone and Table Top Mountain relative to pre-Makushin lavas may reveal changing proportions of sediment and water in the mantle melt genesis region.
- Paleomagnetism: In addition to measuring relative polarity in the field to help constrain relative chronology of lavas, a paleomagnetic study may investigate flow directions of Plio-Pleistocene lavas to help constrain vent locations and/or and rotation of the arc crust due to the obliquity of subduction.
- Detailed stratigraphy of Holocene tephra deposits in the study area would elucidate the interplay between Makushin, Wide Bay Cone and Table Top Mountain and would build on current understanding of pyroclastic hazards already described elsewhere on the island by Beget et al., 2000.
- Little recent work on the glacial history or the interplay between tectonic uplift and sea level change has been published (e.g., Black, 1986). Although this is not the expertise area of the co-PI’s, a motivated student with appropriate guidance at their home institution could choose a project in this area.
Weather in the Aleutians can be quite bad to say the least. Participants should expect to camp and hike in wet conditions for approximately three weeks. We will mitigate working in cold, wet conditions by taking two, fifteen-foot long Weatherport tents for cooking and office work. These tents have a steel frame covered by rubberized canvas and can be anchored using ratcheting straps. Additionally, we provide each pair of students with a four-season Northface VE-25 tent that can hold 5 people.
In addition to the typically rainy and windy weather, we will be hiking long distances and climbing up to 2,000 feet in elevation each day. It will be helpful if you participate in a fitness program prior to departure.
Both project leaders have extensive experience in mapping and sampling in the challenging weather conditions of the Aleutian Islands. Richard Hazlett has participated in field investigations of Okmok Volcano with Dr. Jessica Larsen of the University of Alaska, Fairbanks and AVO. Hazlett contributes particular expertise in field mapping and physical volcanology. Kirsten Nicolaysen has spent five field seasons in remote islands of the central and eastern Aleutian arc (Yunaska: 1992, 1993; Chuginadak/ Cleveland: 2002, 2004; Adak: 2006). Three of these field seasons were 2-5 weeks long and involved setting up Weatherport base-camps. Nicolaysen’s post-dissertation research program has focused primarily on the dating and petrogenesis of individual volcanic centers (Coats Caldera on Yunaska Island and Cleveland Volcano on Chuginadak) as well as comparison of the Hf isotopic compositions of these volcanoes to the regional setting (e.g., Myers et al., 1994; Nicolaysen, 1994; Nicolaysen et al., 2003; 2005; Singer et al., 2007). Nicolaysen contributes expertise in geochemical analysis including major, trace and Sr, Nd, Pb and Hf radiogenic isotopic compositions as well as Ar geochronology and clinopyroxene thermobarometry.
Ideally, students should have taken at least three of the following courses: mineralogy, petrology, geomorphology, sedimentology and stratigraphy, volcanology, geophysics, or geochemistry. Prior participation in field camp is recommended.