New Publications from the Tree Ring Lab

Four new studies from the Wooster Tree Ring Lab have recently appeared in Ecology, Journal of Geophysical Research – Biosciences, The Holocene and Chemosphere.

Brian Buma lead the study published in Ecology that described the results of revisiting a classic ecological succession site in Glacier Bay National Park and Preserve. The article is titled100 years of primary succession highlights stochasticity and competition driving community establishment and stabilityWe blogged about some of the fieldwork for this study a few years ago here.

Abstract: The study of community succession is one of the oldest pursuits in ecology. Challenges remain in terms of evaluating the predictability of succession and the reliability of the chronosequence methods typically used to study community development. The research of William S. Cooper in Glacier Bay National Park is an early and well‐known example of successional ecology that provides a long‐term observational dataset to test hypotheses derived from space‐for‐time substitutions. It also provides a unique opportunity to explore the importance of historical contingencies and as an example of a revitalized historical study system. We test the textbook successional trajectory in Glacier Bay and evaluate long‐term plant community development via primary succession through extensive fieldwork, remote sensing, dendrochronological methods, and newly discovered data that fills in data gaps (1940’s to late 1980’s) in continuous measurement over 100+ years. To date, Cooper’s quadrats do not support the classic facilitation model of succession in which a sequence of species interacts to form predictable successional trajectories. Rather, stochastic early community assembly and subsequent inhibition have dominated; most species arrived shortly after deglaciation and have remained stable for 50+ years. Chronosequence studies assuming prior composition are thus questionable, as no predictable species sequence or timeline was observed. This underscores the significance of assumptions about early conditions in chronosequences and the need to defend such assumptions. Furthermore, this work brings a classic study system in ecology up to date via a plot size expansion, new baseline biogeochemical data, and spatial mapping for future researchers for its second century of observation.

Photo taken in the West Arm of Glacier Bay close to where the Cooper plots were “rediscovered”.


Dr. Ben Gaglioti (University of Alaska – Fairbanks)  just lead another innovative study. This time Ben has assembled a time series of traumatic resin ducts (TRDs) in mountain hemlock that  is a record of past winter conditions and the strength of the Aleutian Low. The article appeared in the Journal of Geophysical Research: Biogeosciences.

The study included tree-ring records from four wild outer coast sites along the the Gulf of Alaska. This is the first work to use these TRD features in tree-rings as a proxy for winter storminess.

a – The clearly-stressed trees used in this study. b – Careful observations and measurements from increment cores were taken to work out the timing of maximum wind stress and storminess. c, d –  Examples of Traumatic Resin Ducts in the tree rings.

Once assembled, the decadal variability of the winter time record was clearly related to the Pacific Decadal Oscillation (see below). This new record is the first of its kind and gives us a new record of wintertime variability from the North Pacific.

Figure above shows a frequency diagram TRDs compared with indices of winter Pacific decadal variability. The records compare favorably giving us confidence in this new proxy technique and Ben’s interpretations.

Ben used some new sites and some from  the archives of the Wooster Tree Ring Lab as part of this study and his new technique is one that the Wooster lab can adopt and learn as we continue to analyze new collections and re-analyze our past collections.


Rob Wilson (University of St. Andrews) lead a study from the Yukon. He used blue intensity tree-ring records from white spruce to improve dendroclimatic temperature reconstructions from the southern Yukon.

The study is titled: Improved dendroclimatic calibration using blue intensity in the southern Yukon. and the abstract reads like this: In north-western North America, the so-called divergence problem (DP) is expressed in tree ring width (RW) as an unstable temperature signal in recent decades. Maximum latewood density (MXD), from the same region, shows minimal evidence of DP. While MXD is a superior proxy for summer temperatures, there are very few long MXD records from North America. Latewood blue intensity (LWB) measures similar wood properties as MXD, expresses a similar climate response, is much cheaper to generate and thereby could provide the means to profoundly expand the extant network of temperature sensitive tree-ring (TR) chronologies in North America. In this study, LWB is measured from 17 white spruce sites (Picea glauca) in south-western Yukon to test whether LWB is immune to the temporal calibration instabilities observed in RW. A number of detrending methodologies are examined. The strongest calibration results for both RW and LWB are consistently returned using age-dependent spline (ADS) detrending within the signal-free (SF) framework. RW data calibrate best with June–July maximum temperatures (Tmax), explaining up to 28% variance, but all models fail validation and residual analysis. In comparison, LWB calibrates strongly (explaining 43–51% of May–August Tmax) and validates well. The reconstruction extends to 1337 CE, but uncertainties increase substantially before the early 17th century because of low replication. RW-, MXD- and LWB-based summer temperature reconstructions from the Gulf of Alaska, the Wrangell Mountains and Northern Alaska display good agreement at multi-decadal and higher frequencies, but the Yukon LWB reconstruction appears potentially limited in its expression of centennial-scale variation. While LWB improves dendroclimatic calibration, future work must focus on suitably preserved sub-fossil material to increase replication prior to 1650 CE.

The Figure above shows the location of the Yukon study site and includes various other sites the Wooster lab has worked on in Alaska. Rob has been a great help in the efforts at the Wooster Tree Ring Lab facilitating our lab’s ability to perform these analyses.


Mary Garvin (Biology, Oberlin College) lead the study using tree-rings and chemical analyses entitled: A survey of trace metal burdens in increment cores from eastern cottonwood (Populus deltoides) across a childhood cancer cluster, Sandusky County, OH, USA.  

Abstract: A dendrochemical study of cottonwood trees (Populus deltoides) was conducted across a childhood cancer cluster in eastern Sandusky County (Ohio, USA). The justification for this study was that no satisfactory explanation has yet been put forward, despite extensive local surveys of aerosols, groundwater, and soil. Concentrations of eight trace metals were measured by ICP-MS in microwave-digested 5-year sections of increment cores, collected during 2012 and 2013. To determine whether the onset of the first cancer cases could be connected to an emergence of any of these contaminants, cores spanning the period 1970–2009 were taken from 51 trees of similar age, inside the cluster and in a control area to the west. The abundance of metals in cottonwood tree annual rings served as a proxy for their long-term, low-level accumulation from the same sources whereby exposure of the children may have occurred. A spatial analysis of cumulative metal burdens (lifetime accumulation in the tree) was performed to search for significant ‘hotspots’, employing a scan statistic with a mask of variable radius and center. For Cd, Cr, and Ni, circular hotspots were found that nearly coincide with the cancer cluster and are similar in size. No hotspots were found for Co, Cu, and Pb, while As and V were largely below method detection limits. Whereas our results do not implicate exposure to metals as a causative factor, we conclude that, after 1970, cottonwood trees have accumulated more Cd, Cr, and Ni, inside the childhood cancer cluster than elsewhere in Sandusky County.

Figure – shows the extent of the cancer cluster that coincides with more accumulated Cd, Cr and Ni in the tree-rings.

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Drought in Zimbabwe and Other Climate Woes

One of my colleagues shared this article from Truthout with me because the title was about how Alaska has no sea ice within 150 miles of its coastline for the first time in recorded history of Alaskan sea ice. That’s another checkpoint in the decline of summer sea ice, for sure, but the article included many signs of global warming changing our climate and impacting animals and people. One of the litany of climate woes they cite was a drought in Zimbabwe.  In July this year, the Zimbabwean government declared an emergency because taps were running dry in the capital of Harare and water levels were so low that hydroelectric power ceased, causing widespread blackouts.

The city of Harare reported producing 450 million liters of water per day for 4.5 million people — so 100 liters per day per person on average. (The above article, though, cites cynical estimates that the real number is 1/4 of that.) To put this in perspective, Americans average over 300 liters of water per day.  In 2018, Cape Town, South Africa barely avoided “Day Zero” during their own drought. “Day Zero” was the day that municipal taps would be turned off and residents would receive a ration of 25 liters or water per day.  They prevented that level by millions reducing use to about 50 liters per day for a prolonged period. Cape Town sounds worse, then, even without reaching Day Zero.

Figure 1: Screenshot of Google Maps showing Zimbabwe in Southern Africa.

Two interesting ripples in this story, though:

First, the socioeconomic and political situation in Zimbabwe is less stable than in South Africa. Long-time despotic leader Robert Mugabe died on September 6, 2019. News outlets in the USA as diverse as Fox News and Vox have maligned him as a brutal dictator who oppressed his country and brought economic ruin. Before his death, reports of protests in the streets by those opposed to the regime (an especially the economic turmoil) trickled through Western media outlets (examples at the Guardian and NPR). This turmoil clearly goes beyond one drought, and the acuteness with which the drought is impacting Zimbabweans may have more to do with social vulnerability than physical hazard.

Second, is this really about climate change? Every news outlet I’ve read seems to have no problem attributing the drought to climate change despite providing no evidence that this drought is unprecedented or part of a larger pattern. There is evidence that droughts have become more common in parts of the world, such as the Mediterranean (see this NY Times article), but not everywhere. The answer for Southern Africa is: maybe. According to a study in the journal Natural Hazards, droughts in Southern Africa became more severe over the the course of the 20th century, in part because of long term global warming. However, using a combination of evidence from observational studies, the latest report from the Intergovernmental Panel on Climate Change (IPCC) did not find significant increases in the annual maximum in the number of consecutive dry days in Southern Africa.

    Figure 2: Trends in the maximum number of consecutive dry days each year (CDD) 1951 -2010. The plus signs indicate significant trends.


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Thirty-third annual report of the Wooster Earth Sciences Department now available

Thank you to Patrice Reeder for her epic work on this report, from its design and art to content. Click away and enjoy!

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Geomorphology – A Walk in the Park

The class stands in front of one of the unconformities in Wooster Memorial Park (aka Spangler). There was some discussion if this is a disconformity (yes) or a nonconformity (maybe yes). The lodgment till at the base is overlain by a fluvial gravel. The high relief of the contact is due to high mineralization – iron, manganese and other oxides; interestingly, many of the clasts were coated with over a centimeter-thick rind.

Two class members (in the distance) work on the fluvial sediments sitting on top of the Mississippian bedrock. This is the Great Unconformity in the Spangler Gorge. The Fluvial sediments are capped with mill pond (legacy) sediments. Others drill in stainless steel erosion pins into the bedrock channel. Morgan in the foreground points one of the pins out.

Evan examines a debris flow that moved into the fluvial point bars sediments. He found organics in the flow and we have sampled and sent wood samples out for radiocarbon analysis. Results are pending, but they will give us information on the geologic evolution of the gorge.

The Schmidt Hammer is being used to measure  rock strength. Several measurements of the bedrock will be taken to “map out” the relative rock strength of the gorge floor (bedrock stream).

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Two Records in Arctic Melt This Summer

There is perhaps a bit too much media hype about July 2019 being the warmest month on record. If you go to the source — the European Copernicus Climate Service article,  the official statement is that “July 2019 was on a par with, and possibly marginally higher than, that of July 2016”.  This is important to note because NASA and NOAA have yet to provide their own summaries for July, and because of uncertainties in the surface temperature record and varying techniques for measuring global temperature among groups, it’s possible July 2016 will remain the record based on other scientific agencies. However, it’s still been a hot summer overall, with June 2019 very clearly the hottest June ever.

This warmth has been felt in the Arctic, as well. Surface melt on the Greenland Ice Sheet has been particularly high this summer, and it recently experienced its highest daily melt area ever recorded, based on data from the National Snow and Ice Data Center. Over 60% of the ice sheet was melting at the surface. In the world of sea ice, it is unlikely that we break the all-time record from 2012 for the annual daily minimum sea ice extent, as that coincided with the strongest storm ever recorded in the Arctic (which helped break up and melt the ice pack).  However, there has never been less sea ice in the Arctic Ocean in July than in 2019.


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Last fieldwork of the summer: A local section with a mystery limestone

It was a delightful August day in northeastern Ohio with pleasant temperatures and thunderstorms that obligingly went around us. Nick Wiesenberg, our geology technician, and I were invited by Dr. Nigel Brush (Ashland University) and friends to examine an outcrop of the Cuyahoga Formation (Lower Carboniferous) near Jeromesville, about a 25 minute drive west of Wooster. Nigel has been investigating a strange limestone in a thick shale sequence that doesn’t seem to fit into our known stratigraphy for the region. I’m always up for a new limestone to look at! As a bonus, the shales here have excellent fossils, especially in concretionary beds. The outcrop, with Nick for scale, is about ten meters high. It is on land owned by the Bricker family, who are geology enthusiasts with beautiful rock and fossil collections.

The shale cliff is steep and has been cut rapidly by the stream (Quaker Springs Run). assisted by occasional dredging of the channel for gravel. The limestone in question is about a meter over our heads here. That’s me, Nigel Brush, and Jeff Dilyard. Photo by Nick.

The shale beds contain siderite concretionary beds loaded with fossils. The most common are spiriferid and spiriferinid brachiopods preserved as internal and external molds and occasionally their original calcite. The taxonomy of these brachiopods is well known, but their paleoecology is still to be determined. (Future Independent Study projects!)

Here is a piece of that limestone eroded from the cliff into the stream. You can immediately see how different it is from the surrounding muddy shales. It is cross-bedded, coarse-grained, and loaded with shale intraclasts (the gray chips), which are mud flakes that were incorporated into the rock while it was being deposited. It looks very much like a storm deposit, possibly associated with a sudden drop in sea level (and rise again).

We took a piece of the above limestone and cut it with a rock saw to begin assessing its composition and fabric. The round grains you see are almost entirely crinoid fragments, mostly stem and arm fragments. The gray chips are the shale intraclasts. Clearly this rock has a story to tell! We will now make acetate peels of this limestone for microscopic study.

One primary question we must address is just where this unit fits in the local stratigraphy. We know it is the Cuyahoga Formation, but which member? It could be the Wooster Shale, Armstrong, or Meadville Shale. One clue would be the conodonts present in the rock. Conodonts are phosphatic dental elements of long-extinct swimming animals. They have considerable biostratigraphic value and so can be used to place units in a stratigraphic column. Since they are phosphatic, they can be collected from limestones by dissolving the carbonate matrix with formic acid and sifting through the remaining insoluble residue. I did this for my dissertation many years ago, and most recently Dean Thomas (’17) dissolved Ordovician limestones for these little critters. I started the process this evening in the paleo lab. Above you see two basins which contain limestone fragments in a 10% solution of formic acid. They are bubbling furiously as the carbonate dissolves. In a few days I’ll sieve the residues and see if we have any conodonts. You’ll be the first to know.

Thank you, Nigel Brush, for the invitation and all your expert fieldwork. It was a fun day as the summer wanes and our first classes of the fall rush towards us.

[Update: A quick snapshot of the conodont elements recovered (December 14, 2019)]


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Shamrock Glacier, Neacola Mountains, Alaska

While in the Neacola Mountains of Alaska last month, we flew over Shamrock Glacier. This first image is from the head of the glacier, where crevasses have been filled in with snow during the accumulation season.

Farther down the north-flowing glacier, we see the merging of the east and west branches and a fine example of a medial moraine.  However, also note on the far lower-right the recently exposed rock. This part of the glacier is shrinking in size, becoming a narrower ice stream.

The toe of Shamrock Glacier is just plain beautiful, ending at a small lake that is dammed from two larger lakes by a ring of moraines. Estimates in a blog post by Mauri Pelto are that the glacier extended all the way out to that moraine as recently as 1950.

In fact, back in 2015, Mauri showed a Landsat satellite image showing the retreat of Shamrock Glacier away from its moraine from 1987 to 2014.  Updating this with the most recent July 2019 imagery, you can see the continued retreat of Shamrock Glacier just in the past four years.  A big section on the left has calved off, and the glacier has slipped off a rise on the right.

Finally, taking a view from the ground, we can better see how the glacier has not only retreated back, but also thinned and narrowed over the past few years. (The 2015 line is based on a photo from Jerry Pillarelli It’s still a pretty glacier, and the sound of calving icebergs while eating lunch  is always welcome.  However, it won’t be long before it retreats upslope sufficiently to no longer calve off into the lake.

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Wooster undergraduate researchers will share findings at international conference

Wooster, OH – Team Geochemistry finished a highly productive summer research season with two conference abstracts that we submitted to the 2019 Fall Meeting of the American Geophysical Union. AGU will be an excellent opportunity for the team to hone their presentation skills, network with potential employers and graduate school advisors, and explore the wide range of disciplines in the geosciences. Read about our work in the abstracts:

Ok and Bræðravirki offer us opportunities to explore past climates and interactions between volcanoes and glaciers, helping us predict what might be in store for Iceland and other ice-covered volcanoes across the world. (Photo Credit: Hannah Grachen)

A Geochemical Study of Bræðravirki Ridge (Western Volcanic Zone, Iceland) Reveals Regional Glaciovolcanic Variations and Complex Tindar Construction 

By Hannah Grachen, Simon Crawford-Muscat, Billy Irving, Meagen Pollock, Benjamin R. Edwards, Shelley A. Judge, Layali Banna, Kendra Devereux, Marisa Schaefer

Summary: As global ice recedes in response to climate change, volcanoes that erupted under the ice are becoming more exposed. We studied one such subglacial volcano in Iceland called Bræðravirki Ridge, and found that Bræðravirki is chemically different from other nearby subglacial volcanoes. We also found that there were several packages of volcanic ash and lava that were erupted at different times. These findings show that subglacial eruptions are complicated, and we might be able to use these types of observations to understand more about the history of volcanoes and glacial ice in an area.

Although tuyas in the Western Volcanic Zone (WVZ) of Iceland have been studied in detail, little work has been done on the numerous smaller tindars there. This study compares Bræðravirki Ridge, a 3-km long tindar on the southeastern flank of Ok shield volcano in the WVZ, to regional tuyas and creates a model for ridge formation based on combined mapping and geochemical analyses. Bræðravirki is dominated by palagonitized lapilli tuff with scattered intrusions, rare exposures of intact pillow lavas, and multiple tuff/lapilli tuff units. Whole rock samples were measured for major and trace elements by XRF and ICP-MS. Mineral compositions were measured by SEM-EDS. Major elements show that Bræðravirki is relatively enriched in SiO2 and depleted in CaO, FeO*, and MgO compared to regional data. Within the ridge, vitric tuff/lapilli tuff units are more evolved (MgO 6.29-6.66 wt.%) than other units (MgO 6.94-7.72 wt.%). Preliminary Rhyolite-MELTS (v. 1.0.1) models are consistent with the two compositional groups being genetically related by 3 kb fractional crystallization of a common parent magma. The MELTS models generate plagioclase (An61 to An70) and two pyroxene (low- and high-Ca) compositions that are observed in the SEM-EDS mineral data. Incompatible trace elements show limited variation between the two compositional groups (Nb/Y 0.37-0.41), suggesting stable melting conditions. We propose a two-stage eruptive model that begins with an explosive phase, followed by a second explosive-effusive phase that forms intrusions and pillows. The second phase is hypothesized to be initiated by a recharge event, emplacing the higher-MgO units. This study demonstrates that small tindars can be constructed through multiple eruptive events that shift in eruptive style and that glaciovolcanic edifices in the same region can have significant compositional differences, possibly providing insights into the understanding of their timing, magmatic history, and paleo-ice conditions.

Microscope photo of one of the granitic rock samples that was classified using the new and old naming systems.

New Igneous Classification System Produces Consistent Rock Names and Illuminates Modal Data

By Hannah Grachen, Anna Cooke, Charley Hankla, Ethan Killian, Cody Park, Layali Banna, Kendra Devereux, Meagen Pollock

Summary: Rock names are useful for understanding what a rock is made of, how it formed, and how it might be useful. For rocks that form from molten magma, the current naming system doesn’t communicate all of the relevant information. Earlier this year, scientists proposed a controversial new naming system to make rock names more meaningful. We tested the new naming system and found that it worked well at the microscopic scale but was more confusing when using the naked eye. We found that the new rock names contained more useful information than the old rock names, and we think that the new system could become the future of rock classification.

In the February 2019 issue of GSA Today, Glazner et al. proposed a new classification system for igneous rocks, citing shortcomings within the standard International Union of Geological Sciences (IUGS) classification system, in use since 1974; chiefly its inability to convey modal data and its arbitrary division of interrelated rocks into disparate types. Their proposed system recognizes rock classifications on ternary diagrams to be “fuzzy” rather than distinct, therefore better representing the continuous nature of rock sequences and lending their system the term “fuzzy classification scheme”. They suggest a system of limited root names combined with modal percentage numbers and significant accessory minerals to provide more informative and straightforward rock names. To test this claim, we have named a suite of six thin sections and hand samples of granitic rocks from Songo Pluton, North Jay Quarry, Mount Waldo, and Deer Isle, using both the IUGS and “fuzzy” classification schemes. We also obtained modes on thin sections through quantitative image analysis using Adobe Illustrator and ImageJ, this being an important step in our process due to the difficulty of differentiating alkali feldspar and plagioclase in our samples. We determined that while the comments on Glazner et al.’s paper are critical, in the tests we performed, some of those criticisms, such as the non-replicability of the proposed system due to interpretive bias, are insubstantial. In thin section, the fuzzy system naming led to more consistent naming results than in hand sample observations, while the opposite was true in hand sample. One of the things we would suggest after conducting our tests is to include boundaries to distinguish the names in a better way. Overall, the proposed system’s ease of data communication is an improvement on the IUGS system that warrants more attention and modification.

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Team Geochemistry’s Grand Finale

Wooster, OH – Team Geochemistry wrapped up their grand adventure last week. The summer research experience left us with fond memories of trips to Dickinson College and Iceland, knowledge of lots of new analytical techniques, and many new friendships, not to mention the important findings from a critical field site that we’ll be presenting at the December meeting of AGU. Here are some parting thoughts on our bittersweet ending from guest blogger Kendra Devereux (’21):

After returning from Iceland, team geochemistry has been hard at work prepping their samples. In just two short weeks, all 23 whole rock samples from the field have been cleaned, powdered, sieved and turned into fused glass beads and pressed pellets.

Hannah, Layali, and Kendra spent the first week back from Iceland sieving their powdered samples.

Layali happily sieving!

Hannah happily sieving!

Hannah finished her time for the summer last week and is busy moving to Florida with her family, leaving Layali and Kendra on their own for their last week of work. Layali and Kendra spent this past week making 23 fused glass beads and 23 pressed pellets

Pressed pellets and glass beads are all ready to be analyzed with the XRF next week!

As a celebration of all their hard work, Dr. Pollock, Layali, and Kendra enjoyed “Taste of Wooster” downtown on Thursday night.

A delicious chocolate marshmallow whoopie pie from The Blue Rooster.

After 8 weeks together, team geochemistry have said goodbye until the 2019-2020 school year begins!

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Wooster geologists working at site of first glacier memorial in Iceland

Iceland – Big news this week as researchers dedicate the first-ever monument to Okjökull glacier, memorializing the first Icelandic glacier to lose its glacier status to climate change. Ice loss in Iceland is dramatic; researchers expect all of Iceland’s glaciers to go extinct in the next ~200 years, with drastic changes to Icelandic culture and economy. As the ice retreats and reveals the underlying land, scientists can investigate ancient volcanoes that erupted thousands of years ago under the glacier. Wooster geologists were with a team from Dickinson College on the scene of Ok volcano this summer, exploring a glaciovolcanic ridge that is now exposed on the southeast flank of Ok volcano.

View of Ok shield volcano from the south. Bræðravirki is the glaciovolcanic ridge on the southeast flank of Ok volcano. (Photo Credit: Hannah Grachen)

Another view of Bræðravirki ridge from the east. The summit of Ok volcano is off of the photo to the right. (Photo Credit: Hannah Grachen)

Most of Bræðravirki ridge is made of yellow ash that was erupted explosively and has been consolidated into a rock called tuff. The patterns in the rock tell us about how it formed. (Photo Credit: Hannah Grachen)

The tuff has a distinct yellow color that forms when the volcanic ash reacts with hot water. We also see glassy black rocks in the ash that we think are volcanic bombs, which form when lava is ejected violently during an explosive eruption. (Photo Credit: Hannah Grachen)

The ridge also has irregularly shaped bodies of massive rock. These are light gray and extremely hard, with a unique pattern of cracks that forms as the molten rock cools and solidifies. We think these are fingers of magma that get injected into the growing ash pile. (Photo Credit: Hannah Grachen)

Ok volcano and Bræðravirki ridge give us the opportunity to explore past climates and interactions between volcanoes and glaciers, helping us predict what might be in store for Iceland and other ice-covered volcanoes across the world. (Photo Credit: Hannah Grachen)

Look for us at the December 2019 meeting of the American Geophysical Union in San Francisco, CA (USA), where we will be presenting our findings from Bræðravirki ridge.

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