Oblique airphoto showing Mt. St. Helens erupting at about 1130 on 18 May 1980. View is approximately to the N. Courtesy of Austin Post, USGS. Retrieved from https://volcano.si.edu/showreport.cfm?doi=10.5479/si.GVP.SEAN198005-321050.
Written by Dr. Paul F. Karrow
Scattered widely around the world there are areas of varied size that are unstable and dynamic, where the high temperatures of the Earth’s interior break through crustal vents to the Earth’s surface. These are areas where liquid rock (magma) from time to time flows out as fluid lava (eruptions) that gradually cool and harden to solid igneous (crystalline) rock. High mountains, up to thousands of metres high, may be formed, which are among the most scenic and beautiful elevated areas on Earth. Mountains formed in such situations are called volcanoes. Areas of such activity may be troubled by earthquakes as crustal fragments shift and yield. The action of earthquakes is very destructive to human activity and structures. Flowing lava causes loss of life and can wipe out buildings, roads and other human constructs.
North America has a relatively simple arrangement of volcanic activity, with a prominent north-south zone near the west (Pacific) coast, known as the Cascade Mountains. Mount St Helen’s (1980) was the most violent eruption in the Cascade Range in recent time. Mount Baker, not far south of Vancouver, B.C., showed mild signs of volcanic activity a few decades ago. More volcanic terrain extends north into Alaska and turns southwest out in the Pacific Ocean as the Aleutian Islands of Alaska. These terrains of volcanic rock and mountains form part of what has long been known as the Pacific “Ring of Fire” that encircles the Pacific tectonic plate and is one of the most prominent areas of crustal movement and instability on Earth. Unfortunately, certain mineral deposits in northern Ontario have been given the same name and this has led to geological confusion. Chile and its Andean mountain chain are recognized as the southeastern part of the better known Pacific Ring of Fire, forming much of the west coast of South America. This is known as the most tectonically active part of the Earth’s crust, with frequent earthquakes.
Volcanoes may emit their lavas quietly, but those that erupt violently and explosively attract the most attention, sending explosive gases and pulverized volcanic rock, dust, and bombs to great heights, sometimes thousands of metres in the case of volcanic ash, disrupting airline transportation and giving rise to complex classifications of types of eruptions.
Which brings us to the second part of this article: wind, and its interaction with volcanoes. Near volcanic sources around the world, wind-blown debris may be tens of metres or more in thickness, but thin to microscopic thousands of kilometres away. While in North America, including Canada, the air-born products of explosive activity are referred to as volcanic ash, e.g. Wilcox (1965), in Europe and elsewhere, the equivalent word is tephra, which originated with Icelandic volcanologists. A group of terms embrace the work, such as tephrostratigraphy, tephrochronology, and the finer grain sizes the wind distributes widely around the world such as microtephra, distal tephra, invisible tephra, and cryptotephra. This latter group has seen rapid expansion in attention this year (early 2017) with many new workers publishing papers in a variety of journals reporting the results of their laboratory analyses. Europeans have been the prime developers of this more recent work, rapidly expanding the techniques with ever increasing reach as interest grew. Some examples are summarized briefly below.
A geologist with the Swedish Geological Survey, C. Persson (1971), began work in the 1960’s, reporting the sampling of peat sequences in Norway, Sweden, and the Faroe Islands, where successions of tephra contrast vividly by colour (gray, white, or cream) with the dark enclosing organics (brown to black). Also, the tephra could be readily separated by dissolving the organics and leaving the tephra as an insoluble residue. The search for progressively finer tephra was carried forward with greater microscopic (200X to 400X) magnification following pollen analysis and radiocarbon dating with many hundreds of tephrochronology dates. Scandinavia is, fortunately, downwind (eastward) from Icelandic sources and recognition and historical records of volcanic activity were brought together early on. Eventually, tephra from the Icelandic source were recognized as far away as northwestern Russia at a distance of 2500 km. It should be noted too that Sweden was where the cyclic varves of glacial lakes were well studied from the late 1800s up to the present and provided another chronological approach which cross-linked tephra studies with neotectonics, archeology, and climate change. Concurrently, Greenland ice cores and deep-sea sediment cores provided further time records to be correlated with tephra. Thus, the March of Science could be carried forward on a broad front.
Led initially by European workers, with American and Canadian colleagues, the first publication to span North America comes from a site in Newfoundland (Pyne-O’Donnell, et al., 2012), the most easterly part of North America, with a sequence of 12 cryptotephras of ages ranging from 7500 to 600 years before present, with a further, more comprehensive, study by Pyne-O’Donnell, et al. in 2016. Other studies soon followed.
North America shares prevailingly westerly wind with Europe, so just as northern Europe derives its tephra from a western Icelandic source, the volcanic sources in western North America are positioned to spread their tephra eastward. Those tephra thin eastward away from the Pacific source, becoming invisible as they travel. The American tephra have been sitting quietly, waiting for attention to track the long-known visible tephra of the west into their microscopic distal equivalents to the east. In the southern hemisphere, wind patterns are reversed and tephra are spread westward from eastern sources.
Jensen et al. (2014) described tephrochronological studies that not only documented the presence of western tephra across North America, but their correlation could be traced beyond and across the North Atlantic and into northern Europe, spanning a distance of over 7000 km for a single tephra. They further concluded that more modest-size eruptions can spread tephra much further than previously thought. Patterson et al. (2017) illustrated an example of an application of tephra studies, either visible or invisible, to provide a means of detecting errors in radiocarbon dating of lake sediments, a problem that has been known to exist since the early days of radiocarbon dating, but frequently has been ignored. Thus, date lists commonly incorporate such errors and distort the history of vegetation change over time as well as historical inferences based on them. There is potentially much work needed to go back over such histories and correct them, as corrections of hundreds or even thousands of years may be involved.
The last location to be noted is in northeastern Asia, the other side of the Pacific plate and part of the Ring of Fire, where in the Kamchatsky Peninsula over 37 recently active volcanoes have spread their tephra eastward over the Bering Sea into Alaska, and become interdigitated with western North America’s tephrochronology (Ponomareva et al. 2017). This locality has rich peatland resources and a rich tephra record, supported by several photographs of the stratigraphic record which includes 41 tephra in a single section. Diverse lists record the many applications of tephra analyses in developing interdisciplinary studies with many scientific fields including archeology, geological hazards such as tsunamis, earthquakes, and landslides, and resource analysis. Tephra have long been appreciated for representing instantaneous events over large areas of the Earth and their potential to discern relationships between contemporaneous events. Many more useful studies can be expected in the future, including full global tephra being traced around the world. For skeptical readers, in 1816, Mt. Tambora in Indonesia erupted very powerfully, spreading its tephra very widely around the world, lowering global temperature by 3°C, causing unusually colourful sunsets for years and leading 1816 to be called “the year of no summer”, with snow every month in New England. Stay tuned!
References
Jenson, B.J.L., Pyre-O’Donnell, S.D.F., Plunkett, G., Froese, D.G., Hughes, P.D.M., Sigl, M., McConnell, J.R., Amesbury, M.J., Blackwell, P.G., van den Bogaard, C., Buck, C.E., Charman, D.J., Clague, J.J., Hall, V.A., Koch, J., MacKay, H., Mallon, G., McColl, L., Pilcher, J.R., 2014. Transatlantic distribution of the Alaskan White River Ash. Geology, 42, 875-878.
Patterson, R.T., Cran, C.A., Cutts, J.A., Mustaphi, C.J.C., Nasser, N.R., Macumber, A.L., Galloway, J.M., Swindles, G.T., Falk, H., 2017. New occurrences of the White River Ash (east lobe) in subarctic Canada and utility for estimating reservoir effect in lake sediment archives. Palaeogeography, Palaeoclimatology, and Palaeoecology, 477, 109.
Persson, C., 1971. Tephrochronological investigation of peat deposits in Scandinavia and on the Faroe Islands. Sverrges Geologisko Undersokning Ser CNR656 Auhandlingar Och Uppsatser Arobok 65 no. 2 Stockholm, 34p.
Ponomareva, V. Portnyagin, M., Pendea, P.F., Zelenin, E., Bourgeois, J., Pinegina, T., Kozurin, A., 2017. A full Holocene tephrochronology for the Kamchalsky Peninsula region: Application from Kamchatka to North America. Quaternary Science Reviews, 168, 101-122.
Pyne-O’Donnell, S.D.F., Hughes, P.D.M., Froese, D.G., Jensen, B.J.L., Kuehn, S.C., Mallon, G., Amesbury, M.J., Charman, D.J., Daley, T.J., Loader, N.J., Maugouy, D., Street-Perrott, F.A., Woodman-Ralph, J., 2012. High precision ultra-distal Holocene tephrochronology in North America. Quaternary Science Reviews, 52, 6-11.
Pyne-O’Donnell, S.D.F., Gwynar, L.C., Jenson, B.J.L., Vincent, J.H., Kuehn, S.C., Spear, R., Froese, D.G., 2016. West Coast volcanic ashes provide a new continental-scale Lateglacial isochron. Quaternary Science Reviews, 142, 16-25.
Wilcox, R.C., 1965. Volcanic-ash chronology. In H.E. Wright Jr. and D.G. Frey (eds.) “The Quaternary of the United States”, p. 807-816. Princeton University Press, Princeton, NJ, 912 p.