Seeking the origins of continents in the western Aleutian island arc


Elizabeth Cottrell (Smithsonian Institution), Katherine A. Kelley (University of Rhode Island), Michelle Coombs (USGS Alaska Volcano Observatory), Elizabeth Grant (University of Washington), Mattia Pistone (Smithsonian Institution), and Katherine Sheppard (UC Santa Barbara)

The origin of Earth’s continents is among the most fundamental of questions facing geoscientists today. Though andesitic in composition, continental crust shares many geochemical characteristics with basaltic lavas erupted at subduction zone arc volcanoes, suggesting that subduction zone magmatism somehow manufactures Earth’s continents. Our project goal is to examine one particular attribute shared by arc magmas and continents, unusually low iron contents (sometimes referred to as “calc-alkaline affinity”). Our work will test the roles of magmatic water content, oxygen availability, and parent magma composition on the development of low iron in arc magmas. With this goal in mind, we conducted a three-week field campaign, from September 4-23, to the far Western Aleutian islands of Buldir, Kiska, Segula, Little Sitkin, Semisopochnoi, Gareloi, Tanaga, and Kanaga, home to some of the most calcalkaline lavas on Earth. The goals of the field program were to collect samples of volcanic airfall deposits (tephra), which may preserve glass inclusions within the igneous phenocrysts that will reveal the water contents and oxygenation conditions of these end-member magmas. We conducted our field work from the home base of the R/V Maritime Maid, which anchored in four harbors among these islands, and used a Bell 407 helicopter to access field sites on these eight volcanoes. On the Maid, our GeoPRISMS team of five (Liz Cottrell, Michelle Coombs, Mattia Pistone, Elizabeth Grant, and Katherine Sheppard) was joined by a team of volcanic gas scientists supported by the Deep Carbon Observatory (Tobias Fischer and Taryn Lopez) and a team of geophysicists and field technicians from the Alaska Volcano Observatory (John Lyons, Dane Ketner, and Adrian Bender) who serviced the USGS seismic network on several of these volcanoes for the first time in more than ten years. Project PI Katie Kelley could not be in the field, but participated remotely via satellite phone and internet tools that allowed her to track the ship, helicopter, and us in nearreal time. Our voyage is a great example of how multiple teams can work together to achieve great things.


– Liz Cottrell

Preparing for Danger: Training in Anchorage

2 September 2015 | Anchorage, AK • We’re here. Anchorage, Alaska. Months of planning and training are behind us. In three days we will take a three hour jet flight to the airstrip that is both farthest East and farthest West in the United States (figure out that riddle!). From there, we will board a small boat, The R/V Maritime Maid, and steam West into the uninhabited islands of the Western Aleutians. We will only have what is in our suitcases and what we shipped months ago – and I am scared. I am scared I didn’t prepare my team. I am scared I don’t have the right equipment. I am scared I will make poor decisions. But I am most scared of this pool I’m in. This is the Learn to Return Aviation Land and Water Survival School, more commonly known as “dunker” training, with the unfortunate slogan “Be the One to Come Home!” And I’m thinking “Can’t we all come home?” In this course, we get strapped into metal seats with five-point
harnesses meant to mimic the fuselage of a plane or helicopter. We hover above the water. My instructor, Clint, barks, “Mayday Mayday! This is Echo Alpha Romeo 289 with two souls on board. We are ditching! Ditching! Ditching!” And then WHAM! The seats flip and I impact the water. I can’t see. I can’t breathe. I follow the routine. (0) Don’t panic. (1) Slide my hand. Find the door latch. (2) Open the door. (3) Anchor my hand in the door frame. (4) Slide my other hand to unfasten my belt and pull myself out. Even now, dry, thinking of Clint’s voice sends chills down my spine. One team member doesn’t pass the course. I think of my two kids and wonder if I should just get on a plane home. But somehow, tomorrow I board my flight to Adak. The R/V Maritime Maid and “2-Mike-Hotel” (the Helicopter)

3 September 2015 | Adak, AK • The heli pad on the Maid looks to be about the size of my desk at work. I wonder how it is that my first ever helicopter experience will be taking off from the back of a boat and going over the Bearing Sea to the rim of a volcano. Am I crazy? Our pilot, Dan Leary, is the most experienced pilot at Maritime Helicopters and the absolute best pilot I could ever have hoped for. I soon understand that Captain George Rains, who has sailed these waters for longer than I’ve been alive, isn’t going to take any unnecessary risks.

Weather Orphans the Helicopter

5 September 2015 | Constantine Harbor, Amchitka, AK • The Maritime Maid left port in Adak yesterday destined for harbor in Amchitka. At the time of our departure, the weather was beautiful with sunny, blue skies and a clear view of a steaming white fumarole at the summit of Kiska. The Maid does not sail with the helicopter parked on the deck. Instead, the helicopter normally flies when the boat is underway and they meet up again in harbor. Our plan yesterday was for the helicopter to meet us when we moored in Constantine Harbor, Amchitka but, as we sailed, fog closed in at Adak and kept the helicopter from following us. We have no scientific interest in Amchitka, so we must wait for the helicopter to join us.

8 September 2015 | Kiska Harbor, Kiska, AK • After four days of separation, we are finally reunited with the helicopter! Two days ago, we decided to lift anchor and head to Kiska harbor in the hopes of starting work, with or without the helicopter, and left a fuel cache on Amchitka so the heli could catch up with us as soon as weather permitted. We made the most of our idle time at Amchitka by taking a small skiff to shore, doing a gear shakedown, and taking some “practice” samples. Likewise at Kiska, we were able to skiff to shore yesterday and explore the area around the harbor, view and sample some distal volcanic deposits, and try not to set of any unexploded ordinances leftover from World War II. Kiska was a WWII battleground, occupied by the Japanese for a time before being re-taken by the US, and the historical remnants litter the ground and harbor.

9 September 2015 | Segula Volcano • My first day of real field work! Because of the remoteness of this region, few geological studies have been done here. Today’s target, Segula, hasn’t been visited by geologists since the 1940’s and there are only three known rock analyses from this volcano. We find a gorgeous exposed tephra section in a wide gully and greedily fill our bags with this “black gold.” By the end of the day, I realize with satisfaction that our work will return precious samples from this volcano ripe for new discoveries. – Liz Cottrell


– Elizabeth Grant

Minefield Kiska

9 September, 2015 | Kiska Island • Team tephra is in search of olivine in mafic tephra and we have split up into two sub-teams today so we can cover more ground. Mattia and I are on Kiska, the third westernmost island in the Aleutian chain. The helicopter drops us off on a relatively flat, topographic low near the northern flank of Kiska Volcano, next to a recent lava flow. As the helicopter flies off to assist the other teams, Mattia and I survey the landscape and geologic map to get our bearings. We quickly realize that what we had assumed to be a relatively easy passing is actually a literal and figurative minefield. Instead of consisting of relatively young olivine-rich basalt, the flow is actually composed of older blocks of andesite, twice as wide as we are tall and covered with plants and grasses that reach up to our waists. Not only that, but Kiska is littered with “UXO,” unidentified explosive ordinances, which could be anywhere. From our topographic low, it takes us 45 minutes to scramble the 40 meters to the top of the lava flow, and we arrive at the top sweating and out of breath. As we survey our progress, it dawns on us that we will not be able to cross the rest of this flow; it’s too large and too dangerous. From the comfort of the ship’s galley, we had routed our path across the map’s page, talking about sampling along the way. In reality, the unexpected size of the lava flow provided us with some much-needed perspective about the unforgiving scale of nature and the long-reaching consequences of human activity.


– Katie Kelley

The Virtual Aleutians

10 September 2015 | Narragansett, RI • I wish I were there. And I don’t. Staring at the computer screen, I wonder for the umpteenth time if I made the right decision to stay home. My baby daughter, Miranda, is only four months old, so I couldn’t have gone. Still, I can’t help but have this internal debate daily. It is mid-afternoon here and they will be starting their day in the field soon. I login to a website to check the location of the Maid and the helicopter, both of which pop up on an animated map of our field area. When the helicopter is in flight, it lights up pink and its little propeller turns as it moves across the screen. Watching this is the most exciting part of my shore-based experience.

My phone rings and the caller ID shows “Liz Sat Phone.” Liz says it is raining and I can hear the raindrops over the phone. It seems always to be raining there; she says the volcanoes make their own weather. We quickly debrief on yesterday’s work and go over a plan for the coming day’s activities: our party will deploy one team to Kiska, and the other to Buldir. Buldir is the riskiest flight of the trip, partly because the flight itself is extremely dangerous and partly because they might not find anything useful when they get there, so they are risking life and limb possibly for naught. I am incredibly nervous for them. After we hang up, I login to another website to track Liz’s InReach device, which sends her location every ten minutes. I leave the office just as the helicopter leaves the Maid for Buldir.

Of course, Liz tried to contact me from Buldir while I picked up my children from day care, during the only ten-minute window of my day when I had to pocket my phone. They made it safely there, which is a relief, and I briefly text with Liz’s husband, who is also closely tracking her steps, about how wild it is to watch her walking around. When I get home, we setup my laptop at the dinner table (the only time a computer has ever been allowed at the table, mind you) so my whole family can watch the “action” as it happens. My three year-old daughter has learned the names of all of the volcanoes on the itinerary and asks where Liz and the helicopter are today.


– Mattia Pistone

Kiska Volcano: Ascensus ad coelum et descensus ad Inferos

10 September 2015 • Thirty minutes prior to sunrise. From the vessel bow, while sipping hot tea, I observe that low-elevation clouds still seal the sky. These are not ideal conditions for helicopter flight but this is our last day on Kiska Island and, despite numerous attempts on the flanks, we have yet to find any of the rocks (mafic tephra) that we are looking for. We can only hope to find a fissure in the cloud barrier and find a way to the volcano summit. The wind is with us, however, and it is rapidly clearing up the sky from the dusty clouds. Today, the first team to be deployed by helicopter is the gas team (Tobias Fischer, Taryn Lopez) supported by Adrian Bender, the sedimentologist of “tsunamites” and “stormites,” who is going to be the “radio antenna man” in contact with the Maid while the group collects dangerous volcanic gases on the southwest flank of the volcano summit. The helicopter is fully packed with people, backpacks, and field instruments. There is only one free seat for one person with a backpack. After briefing with the other members of team tephra, it is unanimously decided that I will join the gas team today. I will be tasked with finding and returning rocks from the volcano summit back to the research vessel. I am thrilled! After taking off, our helicopter pilot Dan Leary is like a hawk looking for prey; he finds a spot with broken clouds and steers into it. Thanks to the ascending winds increasing while approaching the hidden east flank of the volcano, we are promptly above the clouds – the sky can also be blue here in the Aleutians. While ascending, the northwest wind is too strong to make any attempt for landing at the volcano summit. Therefore, we are all deployed at about a thousand feet below the volcano summit. It looks like we will have to reach the summit only with some effort and sweat. After a short briefing about work tasks and timetable, and radio communications, the gas team and myself initiate our hike up. Lava flows, loose volcanic bombs and blocks dominate the landscape. After several days at low elevation, I can enjoy this hike without fear of encountering UXO. I am at the top of the crater rim and the landscape in front of me is gorgeous! The volcano crater is in front of me and I feel so small and insignificant. Clusters of loose rocks cover the internal flanks of the volcano. The crater floor is filled with fine volcanic material; it looks like mud. The western side of the volcano has no rim and from there, low-elevation clouds ascend and enter this amphitheater while the northwestern winds blows. I am the lonely spectator of this volcanic show and feel like an explorer – I am the first one to set foot in this volcanic crater… well, after the geologist Robert Coats, who most probably came here during his mapping work in the early 50’s… but, for sure, I am the first Italian here! That’s exciting! Now, back to work. I report the GPS coordinates and field
observations in my book and start to hammer the samples I need. Any sample I take looks beautiful and full of precious information. I wish to take any and all specimens with me since this is the first and last chance we have to collect rocks in the crater of Kiska Volcano during this field mission. But I have to face the reality: I am by myself and cannot carry too many rocks. How time flies! I am quick to collect samples, observations, and data because I have to go back soon. We have now 70 kg of rocks to hike to our helicopter rendez-vous location and I anticipate a very negative reaction from the gas team, who have worked hard for many hours. Instead, I receive generous support, which is typical of an enthusiastic team of people. This is the best reward after a long day of work between volcanic rocks and wind gusts. Together, we march back to the pick-up point. I think this is the greatest day of the field mission here in the Aleutians.


– Katherine Sheppard

On the Edge

10 September 2015 | Kiska Harbor, Kiska, AK • Buldir is a tiny speck of an island about halfway between the much larger masses of Kiska and Attu, all the way out in the far Western reaches of the Aleutian chain. How far out? Let’s just say we didn’t have our passports with us, so we couldn’t except to legally get much further west. There are 45 miles of open water to the east and west of Buldir, which was about a 45-minute flight for our Bell 407 helicopter. This is a perfect amount of time to reminisce about three things: we would be among only a few geologists to visit the volcano in decades, we only had one day to do as much work there as we possibly could, and if anything went wrong while on the island we were all royally screwed. Our day on Buldir had the potential to be the most dangerous day of the trip, mostly due to the long over-water helicopter flight and the remote location. If the helicopter landed but couldn’t take off due to bad weather,
we would be stuck out there until the weather cleared or the boat came to get us. As anyone familiar with the weather in the Aleutians knows, this could take a very, very long time.

As it turned out, we landed, did our work and I soaked in the glory of feeling like a real life explorer. The fog stayed at a respectful distance, the wind stayed manageable, and we were able to take off at the end of the day with minimal excitement. When we landed on the boat 45 minutes later, however, we were ecstatic. We had found amazing tephra samples that suggested an explosive, volatile-rich history for Buldir. Not at all as we were led to expect before the trip, so a resounding success! Only when we had unpacked all the rocks and rid ourselves of our protective gear did our helicopter pilot turn to Liz, let out a breath he seemed to have been holding all day, and say “I am never, ever, ever doing that again.”


– Liz Cottrell

12 September 2015 | Semisopochnoi • With the stressful overwater flight to Buldir behind me, I find myself relaxed and enjoying the flight to Semisopochnoi Volcano. A survey from the air reveals beautiful sections of tephra cut from the vegetated landscape. Michelle and I are able to sample meticulously here all day.

17 September 2015 | Gareloi • I am standing waist-deep in olivine scoria and loving it. Katherine and I fill our sampling bags to bursting and I know we have gotten what we came for. I then hike up to the crater rim – just to take a peek. I am stunned at what I see… a crater lake and active fumaroles! This appears to be a new development since the last time geologists visited this place in 2005 and I am reminded that these are indeed very active volcanoes.


– Michelle Coomb

20 September 2015 | Kanaga • Gas team and two tephra teams got set on Kanaga in the morning, and then boat transited from Hot Springs Bay on Tanaga to the Bay of Islands on the west side of Adak. This was the most spectacular day of the trip so far. Kanaga was completely out and cloud free and I took many beautiful photos. Kanaga is a great island and volcano – deep blue lake in the caldera, spectacular lava flows, deep green grass everywhere. Kanaton Ridge is just screaming out for more and better work, as is the entire island. Visited a few tephra sections as guided by CW’s paper and found some big lapilli pumice falls. No mafic scoria to speak of, unlike Tanaga, and much to our team’s disappointment. I think I found two mafic ashes that may be from Tanaga, which will be interesting to see. We ended the day around 5 pm at the hot springs.■

 

“Report from the Field” was designed to inform the community of real-time, exciting GeoPRISMS -related research. Through this report, the authors expose the excitement, trials, and opportunities to conduct fieldwork, as well as the challenges they may have experienced by deploying research activities in unique geological settings. If you would like to contribute to this series and share your experience on the field, please contact the GeoPRISMS Office at info@geoprisms.org. This opportunity is open to anyone engaged in GeoPRISMS research, from senior researchers to undergraduate students.
We hope to hear from you!

Reference information
Seeking the origins of continents in the western Aleutian island arc
GeoPRISMS Newsletter, Issue No. 36, Spring 2016. Retrieved from http://geoprisms.org

Investigating older rocks in the oceanic Aleutian volcanic arc east of Adak


Peter Kelemen (Columbia University, LDEO) on behalf of Merry Yue Cai (Columbia University, LDEO), Emily H.G. Cooperdock (UT Austin), Steve Goldstein (Columbia University, LDEO), Matt Rioux (UC Santa Barbara), and Gene Yogodzinski (University of South Carolina)

Benefiting from the NSF GeoPRISMS community platform in the Aleutian volcanic arc in the summer of 2015, our group from the University of South Carolina and Columbia University had a
matchless opportunity to study and sample outcrops of pre-Holocene volcanic and plutonic rocks on Unalaska, Umnak, and Atka Islands. Speaking for myself, at the age of 59 and having worked in the field in a lot of spectacular places – every year for forty years – this was one of the most memorable and rewarding field seasons of my life.

The older rocks in the Aleutian volcanic arc are notable because they include the most extensive outcrops of plutonic rocks in any oceanic arc, worldwide. Aside from the visionary work of Sue and Bob Kay and their colleagues, these plutonic rocks have not received much attention since pioneering USGS studies were completed in the 1950’s (Umnak), 1960’s (Unalaska), and 1970’s (Atka). This prior work demonstrated that the Eocene to Miocene plutonic rocks east of Adak Island were more strongly “calc-alkaline”, with higher SiO2 at a given Mg/Fe ratio, compared to the “tholeiitic”, Holocene volcanic rocks on the same islands.

In a recently published pilot study using USGS samples (Cai et al., Earth Planet. Sci. Lett. 2015), we found that these plutonic rocks are also isotopically distinct from the lavas on the same islands, demonstrating that the two suites were generated by melting of two distinct sources.

Our field season in the summer of 2015 was designed to investigate whether these differences in source composition were the result of :

  1. temporal evolution of the arc, in which case Miocene to Eocene lavas should have isotope ratios similar to those of calc-alkaline plutons, and perhaps will also mirror the calc-alkaline compositions of these plutons, or
  2. distinct processes, in which viscous, SiO2– and H2O-rich, calc-alkaline, andesitic magmas tended to stall and form mid-crustal plutons, while relatively low viscosity, SiO2– and H2O-poor, tholeiitic, basaltic magmas tended to erupt on the surface, in which case Miocene to Eocene lavas may be isotopically (and compositionally?) distinct from coeval plutons.

To this end, we hoped to sample coeval plutonic and volcanic arcs on several Aleutian islands where the plutons are well-exposed.

Our starting plan was to set up fly camps in the alpine terrain on the islands, which is underlain by extensive outcrops of granodiorite and diorite plutons. We assumed that we would have difficulty obtaining ages on highly altered volcanic rocks, whereas it would be relatively easy to date zircons from the large plutons. Thus, we expected to sample volcanic rocks where they are intruded by plutons of known age. Frankly, my expectations about the field work were not high. I imagined we would be semi-lost in perennial fog, while disconsolately scraping moss, lichen, and tundra grasses off texture-less, fine-grained, grey-green outcrops, and spending a lot of time arguing about whether a specific sample was volcanic, plutonic, or even sedimentary!

Merry Cai, Steve Goldstein, Gene Yogodzinski, and I flew to the commercial airport in Dutch Harbor on Unalaska Island on August 5, where we were joined by pilot Sean Charlton in Pollux Aviation’s R44 helicopter. Sean had flown out from the mainland, with floats fully inflated. I had never used such a small, gasoline-powered helicopter, with an engine not much larger than a lawnmower, so I was a bit skeptical at first. We initially focused on the Shaler pluton on Unalaska, which is the largest in the Aleutians, and hence in any oceanic volcanic arc, worldwide. The weather was quite good when we were there, which allowed us to fly every day. Everyone says the Aleutian weather is bad and unpredictable, and of course, it is, but not always. Working there can often be quite nice. We set up a couple of fly camps, and ranged through the beautiful alpine terrain, examining complex border facies of granodiorite, diorite and volcanic hornfels. We also took advantage of the helicopter on re-supply days to make ground stops along the coast. There, we found exceptional outcrops, including surprisingly fresh volcanic rocks with chilled margins, suitable for 40Ar/39Ar geochronology.

After a while, the exceptional coastal exposures, coupled with the convenience of the helicopter, induced a change in our plans. We moved into the hotel in Dutch Harbor, and flew every day. It
turned out that world-class sea cliff outcrops, coupled with wave cut terraces that offered ideal helicopter landing sites at all but the highest tides, provided a spectacular opportunity for us to conduct comprehensive sampling.

As the photos accompanying this article show, the Aleutian sea cliffs revealed spectacular sequences of pillow lavas, pyroclastic deposits, and columnar-jointed sills. Indeed, photos in the USGS Bulletins showed some of these exceptionally well-exposed features, but in earlier years, without a helicopter, these outcrops were very difficult to access from small boats. In addition, there were few opportunities to obtain reliable ages on the lavas. With the helicopter, and some confidence about 40Ar/39Ar dating of fine-grained volcanic rocks, we were in heaven. Further, as it turns out, our samples from the many sills intruding the volcanics will provide plenty of opportunities to check the Argon ages using U/Pb in zircon.

As it turned out, the R44 helicopter was perfect for us, fitting easily into small landing spots, often within ten meters of the Pacific surf. Unfortunately, Steve Goldstein came down with shingles and had to convalesce in Dutch Harbor, sampling volcanic rocks from the extensive road network when he could. However – sorry Steve! – this did reduce our helicopter-supported group to three, who just fit into the three passenger seats in the R44, enabling ultra-efficient field work. We would leapfrog along the coast, setting out one or two people at each landing spot, and scheduling pickups a few hundred meters further along the coast.

In the middle of August, we moved from Unalaska to Umnak Island, where we were fortunate to stay in a bunkhouse at Bering Pacific Ranches, Ltd., near the abandoned WWII airfield at Fort Glenn. This is a fascinating operation; while we were there, Ranch owner Pat Harvie and his crew were preparing to round up thousands of “organic, free-range” cattle from across the island, using a fleet of R22 helicopters, plus a lot of bailing wire and duct tape. These animals were destined for shipment to Canadian markets in the late summer and early fall. We all hope this visionary operation ended in great success!

From this spectacular basecamp, we spent several productive days sampling along the north and southeast coasts of the island, with a side-trip to the rim of the giant Okmok caldera during a clear spell. We also used the opportunity to access the westernmost peninsula of Unalaska Island, completing our extensive sampling there. All too soon, it was time to leave the Ranch. We returned to Dutch Harbor, where we met Captain George Rains, the crew of the R/V Maritime Maid, and pilot Dan Leary with Maritime Helicopters’ Bell 206 Long Ranger. We also rejoined a rejuvenated Steve Goldstein, together with his daughter, Emily Cooperdock, who had flown up to join us. This increase in our group size corresponded with the change from the four-seat R44 to the six-seat 206, and as a result we remained a highly efficient, helicopter-supported team!

We moved into comfortable quarters onboard the Maid and, delayed by weather, spent a few more days living on the ship in Dutch Harbor, continuing to sample on Unalaska Island. Until this point, we had not lost a single day to weather, though we had gotten wet on a couple of days.

However, our transit to Atka Island, and our work there, were substantially delayed by wind, then fog. A side benefit was a spectacular morning at anchor among the Islands of Four Mountains,
where we photographed the perfect strato-volcanoes there while we waited for the helicopter to catch up with the ship. Finally, the weather cleared and we spent a highly productive day and a half
racing along the western peninsula of Atka Island, acquiring a fantastic set of samples, including previously dated intrusions that span the range from the youngest (9 Ma) to oldest (39 Ma) plutons known in the arc east of Adak.

We then set out for Adak Island. In the airport there, we greeted the next group who would use the GeoPRISMS community platform onboard the Maid, led by Liz Cottrell. We wished them all the best and, sadly, began the long trip home.

In addition to the pilots, and the crew of the Maritime Maid, we would like to express deep gratitude to Program Manager Jenn Wade at NSF, who worked tirelessly to make the community platform concept come alive, and to Christie Haupert, Alaska Science Project Manager for Polar Field Services, Inc., who provided flawless logistical support.

Preliminary data on a few 2015 samples, Unalaska Island.

PS: Since that time, we’ve been working hard processing our samples and obtaining initial data. On the left is a plot of some early, major element analyses of our samples from Unalaska Island. Note that, as for the USGS samples we analyzed for our pilot study (Cai et al., Earth Planet. Sci. Lett., 2015), most of our 2015 plutonic samples are calc-alkaline and most of our 2015 mafic lava samples are tholeiitic, despite the fact that the 2015 lavas and plutons are approximately coeval. This suggests that the chemical differences documented by Cai et al. (2015) are present among coeval igneous rock suites in the Aleutians, and did not arise as a result of temporal evolution of both volcanic and plutonic magmatism.

In addition to our main line of inquiry, outlined above, we are evaluating the potential for study of detrital zircons in volcanoclastic sediments, while Emily Cooperdock is preparing a proposal to study the uplift and denudation history of the Aleutians via U-Th-He thermochronology as well as fission track and 40Ar/39Ar analyses. ■

A) Schematic map of the Aleutian island arc, sampled areas are highlighted in black. B) Wt% SiO2 versus Fe/Mg ratio of studied Aleutian igneous rocks. By convention, the Fe/Mg ratio is calculated using wt% MgO and FeO, with all Fe as FeO. C) Present-day Nd and Pb isotope ratios of Aleutian igneous rocks vs. longitude and vs. age. Circles are central and eastern Aleutian volcanics: Yellow = Rat and Delarof Islands, Green = Adak and Kanaga, Blue = Atka, Purple = Umnak, White = Unalaska. Error bars are smaller than the symbols. In 3) and 4), the Holocene volcanics are separated by location only without age differences. Figures from Cai et al., Earth Planet Sci. Lett. 2015.

“Report from the Field” was designed to inform the community of real-time, exciting GeoPRISMS -related research. Through this report, the authors expose the excitement, trials, and opportunities to conduct fieldwork, as well as the challenges they may have experienced by deploying research activities in unique geological settings. If you would like to contribute to this series and share your experience on the field, please contact the GeoPRISMS Office at info@geoprisms.org. This opportunity is open to anyone engaged in GeoPRISMS research, from senior researchers to undergraduate students.
We hope to hear from you!

Reference information
Investigating older rocks in the oceanic Aleutian volcanic arc east of Adak
GeoPRISMS Newsletter, Issue No. 36, Spring 2016. Retrieved from http://geoprisms.org

Islands of Four Mountains to Unimak: From the slab to the surface


Report retrieved from the website of the Department of Terrestrial Magnetism Carnegie Institution of Washington and the Facebook page of the field mission (IFM-Unimak 2015: From the Slab to the Surface)

Scientists have a relatively good understanding of the processes occurring in the upper portions of the Earth’s crust that lead to volcanic activity. However, much remains unknown about
how these shallow processes are controlled by the large-scale tectonics and deep mantle processes that are ultimately responsible for volcanism.

A NSF-funded group led by DTM seismologist Diana Roman headed to Alaska for three weeks,two of which were spent on the research vessel Maritime Maid, to collect seismic data in the Islands of the Four Mountains and tephra samples throughout the eastern Aleutians. The group included Roman and DTM postdoc Amanda Lough, as well as Dan Rasmussen, Alex Lloyd, and Terry Plank from Columbia University’s Lamont-Doherty Earth Observatory, Pete Stelling from Western Washington University, and John Power, John Lyons, Christoph Kern, and Cindy Werner from the U.S. Geological Survey.

The goal of their work is to determine how the amount of water dissolved in magma affects where, and for how long, magma is stored in Earth’s crust. This information is critical for accurately
forecasting volcanic eruptions and understanding the large-scale processes that lead to volcanism in Earth’s subduction zones. The volcanoes targeted in this study have a wide range of magma water contents, magma storage depths, and depths of seismic activity, making them ideal candidates for this research.

Roman led another trip in the summer of 2016 to retrieve seismic equipment from the Islands of the Four Mountains. ■

“Report from the Field” was designed to inform the community of real-time, exciting GeoPRISMS -related research. Through this report, the authors expose the excitement, trials, and opportunities to conduct fieldwork, as well as the challenges they may have experienced by deploying research activities in unique geological settings. If you would like to contribute to this series and share your experience on the field, please contact the GeoPRISMS Office at info@geoprisms.org. This opportunity is open to anyone engaged in GeoPRISMS research, from senior researchers to undergraduate students.
We hope to hear from you!

Reference information
Islands of Four Mountains to Unimak: From the slab to the surface
GeoPRISMS Newsletter, Issue No. 36, Spring 2016. Retrieved from http://geoprisms.org

Magnetotelluric & seismic investigation of arc melt generation, delivery, and storage beneath Okmok volcano


Ninfa Bennington (U. Wisconsin-Madison), Kerry Key (Scripps Institution of Oceanography), and USGS Investigators Matthew Haney and Paul Bedrosian

Okmok Volcano - Umnak Island in the eastern Aleutian islands of Alaska
Fore more information about the project, videos, photos, updates, visit the blog at http://okmok.ucsd.edu/

Okmok is one of the most active volcanoes in the Aleutian arc and hosts a 10 km diameter caldera. The subdued topography of Okmok, relative to other Aleutian volcanoes, improves access and permits dense sampling of the volcanic edifice. We have selected Okmok as the site of study for this project due to frequent volcanic activity and the presence of a crustal magma reservoir as inferred from previous seismic studies. At least two caldera forming eruptions are recognized and Okmok is believed to be representative of volcanoes both within the Aleutian arc and worldwide, where long periods of effusive eruptions are punctuated by much larger explosive caldera forming eruptions.

We are applying geophysical techniques to characterize the magmatic system beneath Okmok. During the summer of 2015, we collected onshore and offshore magnetotelluric (MT) data and installed a temporary year long seismic deployment. The seismic instruments will be retrieved in summer 2016. These new geophysical data will be used to test hypotheses regarding the role of slab fluids in arc melt generation, melt migration within the crust, and the crustal magmatic plumbing and storage system beneath Okmok Caldera.

Offshore MT Field Deployment

After numerous delays due to thick fog typical of the Aleutians, the entirety of the offshore MT crew arrived to Dutch Harbor, AK and was assembled on the R/V Thompson. A video of the research vessel captured by a morning drone flight can be viewed here. On June 18, 2015, the team departed Dutch Harbor for their first offshore MT site. The offshore crew spent the day preparing receivers so that there were only a few remaining steps to complete before deploying them over the side of the ship. By June 20, all 54 MT receivers had been deployed well ahead of schedule.

With the MT deployment complete, the team collected multi-beam bathymetry data on the upper forearc slope south of Umnak Island using the ship’s EM302 multi-beam echo sounder. On top of mapping the bathymetry of the ocean floor, the intensity of these recordings can be used to help determine the nature of the seabed (e.g. sediments versus hard rock). On June 21, the offshore team transited back to Dutch Harbor via Umnak pass. This return route included spectacular views of Mount Makushin Volcano on Unalaska Island.

Onshore MT and Seismic Deployments

On June 23, having returning to Dutch Harbor from offshore MT work, co-PI Key and Scripps graduate student Georgiana Zelenak joined up with the rest of the onshore team (PI Bennington, UW-Madison post-doc Summer Ohlendorf, and USGS collaborators Matthew Haney and Paul Bedrosian) and departed for Umnak Island. The onshore work was based out of Bering Pacific Ranch at Fort Glenn, an abandoned WWII military base, with a helicopter transporting the seismic and MT teams and equipment during the 19 days of field operations.

After the team arrived at Fort Glenn, the camp house was set up and seismic and MT equipment were prepared for the start of field operations the following day. Seismic and MT field operations commenced on June 23 and extended until July 11. The seismic team installed thirteen temporary broadband seismometers both in and around the Okmok Caldera. In tandem with the Alaska Volcano Observatory’s twelve permanent seismic stations, there are now twelve seismic instruments within or at the rim of the caldera and 14 seismic instruments outside the caldera. The temporary array will record seismic data until its retrieval in summer 2016. Onshore magnetotelluric data were collected in a 3D array using a combination of long-period and wide-band MT systems, with 19 stations within the caldera and ten stations outside. Following the completion of onshore fieldwork, part of the onshore team (PI Bennington, Summer Ohlendorf, and USGS collaborators Matthew Haney and Paul Bedrosian) caught a charter flight back to Dutch Harbor. Co-PI Kerry Key and graduate student Georgie Zelenak hitched a more unconventional ride when the rescue boat from the RV Sikuliaq picked them up from Umnak Island.

Offshore MT Instrument Recovery

Following completion of onshore MT work, the offshore MT team made a six day cruise on the new R/V Sikuliaq to recover offshore MT instruments. Of the 54 offshore deployments, 53 instruments were successfully recovered while one instrument was lost in Umnak pass due to strong tidal currents in the shallow water. ■

“Report from the Field” was designed to inform the community of real-time, exciting GeoPRISMS -related research. Through this report, the authors expose the excitement, trials, and opportunities to conduct fieldwork, as well as the challenges they may have experienced by deploying research activities in unique geological settings. If you would like to contribute to this series and share your experience on the field, please contact the GeoPRISMS Office at info@geoprisms.org. This opportunity is open to anyone engaged in GeoPRISMS research, from senior researchers to undergraduate students.
We hope to hear from you!

Reference information
Magnetotelluric & seismic investigation of arc melt generation, delivery, and storage beneath Okmok volcano
GeoPRISMS Newsletter, Issue No. 36, Spring 2016. Retrieved from http://geoprisms.org