Tephra and Tephrochonology
This brief summary of tephrochronology concentrates on Iceland and NW Europe and contains a number of early references from Iceland and the beginnings of crypotephrochonological studies in the British Isles. This is not a comprehensive list of all the papers from these regions/countries. For more references can be found by searching Tephrabase and consulting Lowe (2011).
Tephra is a term used to describe all of the solid material produced from a volcano during an eruption (Thorarinsson, 1944). The fine fraction of this material can travel great differences. Tephra from the 1259 AD eruption of El Chichon, Mexico, for example, has been found in both the Greenland and Antarctic ice caps (Palais et al., 1992). Small tephra shards (2-3 µm) from the eruption of Pinatubo circled the globe several times after the 1991 eruption. The interest in the study of tephra layers has proceeded on two fronts: firstly, there is interest of volcanic impact on climate and the environment and secondly, as a chronological tool. Climatological and palaeoenvironmental research has involved studies on the possible major impact of volcanic eruptions on climate, from the possible intensification of ice ages (Ramaswamy, 1992) to localised or short-term climatic change (Baillie and Munro, 1988). The large 1991 eruption of Pinatubo, for example, produced a large eruption column that had a small, but noticeable effect on the Earth's climate (Koyaguchi and Tokuno, 1993). The use of tephra layers as a chronological tool (tephrochronology) was originally developed in Iceland (Thorainsson, 1944) and has since been applied to other volcanically active areas such as Alaska, New Zealand and Mexico. This technique allows isochronous marker horizons, formed by tephra layers, to be mapped across inter-continental scale distances. These can form a dating framework against which other dating techniques can be checked and validated.
David Lowe's (2011) paper is an excellent review of the current state of tephrochronology.
Until the 1960s, tephrochronological studies in north-west Europe were restricted to Iceland, the only country with active volcanoes in the region. This work was pioneered by Sigurður Thórarinsson, who produced several seminal works (e.g. Larsen and Thórarinsson 1977; Thórarinsson 1954, 1956, 1967, 1975, 1981a, 1981b). There are many more papers and Sigurður Thórarinsson's pioneering work, aided by Icelandic colleagues, and Guðrún Larsen in particular during the 1970s, established the unparalleled historical tephrochronological record which enables accurate and precise dating of archaeological remains and environmental change over the past 1200 years. His work has now been applied across the world and we owe him a huge debt. 2012 marked the centenary of Sigurður Thórarinsson's birth and Jökull, the Icelandic Journal of Earth Sciences published a special edition to commemorate him. Larsen and Eiriksson (2008) is an informative review of Icelandic terrestrial tephrochronological.
During the 1960s, research carried out in mainland Scandinavia and the Faroe Islands produced evidence of the presence of Icelandic tephra layers thousands of kilometres from their sources (Persson, 1971). This period also saw the development of marine sediment studies, partly associated with oil exploration, and it became apparent that these contained a long record of Icelandic volcanism (Ruddiman and Glover 1972). Developments in geochemical analysis has enabled tephra layers to be identified independently of other dating methods, e.g. radiocarbon dating. Once a tephra has been geochemically identified, it can be used as a time marker horizon across continental or inter-continental distances over a wide range of depositional environments . This has led to the discovery and identification of Icelandic tephra layers across a large area, covering the period from the Late-Glacial to the present. Discoveries have been made in Norway (Mangerud et al., 1984), the Faroes (Mangerud et al., 1986; Dugmore and Newton, 1998; Wastegard et al; 2001, 2002), Scotland (Dugmore, 1989; Dugmore et al, 1992; Dugmore et al, 1995; Langdon and Barber, 2001), Northern Ireland (Hall and Pilcher et al, 2002; Pilcher and Hall 1992; Pilcher et al., 1995, 1996), Germany (Merkt et al., 1993; van den Bogaard et al, 1994, 2002; van den Bogaard and Schmincke, 2002), Sweden (Boygle, 1998; 2004; Wastegard, 1998; Bergman et al., 2004), Russia (Wastegard et al, 2000) and Greenland ice cores (e.g. Gronvold et al., 1995).
Several papers have been published which summarize and collect details on the techniques involved in tephrochronology and the information on the tephra layers. These include Haflidason et al (2000), Turney et al (2004), Dugmore et al (2004, 2012), Hunt and Hill (1993, 2001), Lowe (2011), Hayward (2012), Pearce et al (2011).
The volcanically active areas of Iceland can be divided into 4 volcanic zones. These are the Snæfellsness Zone, the Reykjanes-Langjokull Zone, the Northern Zone and the Eastern Zone. Öræfajökull, south-east Iceland, however does not belong to any of these zones. Jakobsson (1979) defines a volcanic system as a "spatial grouping of eruption sites in a certain period of time, with particular characteristics of tectonics, petrography and geochemistry". Volcanic systems generally start off as fissure swarms (e.g. Veidivötn), producing mainly basaltic rocks. In time more evolved rocks are produced and activity often becomes concentrated in one area. A caldera, central volcano and high temperature thermal field eventually follows (e.g. Katla Volcanic System). Systems generally have a life of between 300,000 and 500,000 years, but central volcanoes may reach an age of over 2 million years. As each volcanic system is partly defined on the geochemical property of the products it has produced, this helps in identifying the source volcanoes of tephra layers found thousands of kilometres from Iceland. So far all of this work has involved major element analysis of glass shards and this is the data stored on this database. More details on volcanic system found in Iceland are available. Many volcanic systems in Iceland are covered by thick ice-caps, which have a major impact on eruptions.
The eruptions of Eyjafjallajökull (2010) and Grímsvötn (2011) reminded the world that even relatively modest eruptions can have a major impact on modern societies at some distance from the volcano. These eruptions have led to a much better understanding of the mechanisms by which tephra is transported through the atmosphere and what controls its deposition in distal locals. The work of Davies et al (2010), Stevenson et al. (2012, 2013), Swindles (2010) and Tesche et al. (2012) are well worth reading.
As well as providing important information about volcanic histories, tephrochronology can also provide invaluable data on environmental change. This can range from establishing rates of Icelandic soil erosion (e.g. Dugmore et al., 2009), the impact of the arrival of plague on farming and society in Iceland (Streeter et al., 2012) and using incomplete or non-perfect tephra sequences to inform us geomorphological processes (Dugmore and Newton, 2012)
Tephrochronology in NW Europe is not restricted to Icelandic tephras. Tephra deposits from the Laacher See eruption (c. 13,000 years BP) have been used with increasing frequency as a tephrochronological marker in the context of both environmental as well as archaeological research (e.g., Blockley et al., 2007; Blockley et al., 2008; Blockley et al., 2008; Riede, 2007; Riede, 2008; Riede and Bazely, 2009).
Central Mexico
Intense volcanic activity in the Transmexican Volcanic Belt (TMVB), which stretches across central México, has occurred since the Oligocene. Volcanic activity continues through to the present day (e.g. Popocatépetl since 1994) representing a major potential hazard. Tephra falls from eruptions over the historical period (since 1521 AD) are reported to have effected large areas, some several hundred kilometres from the source volcano, e.g. Volcán de Colima (Martin del Pozzo et al., 1995). Volcanic activity in central Mexico is associated with the subduction of the Cocos and Rivera Plates beneath the North American plate. Although the volcanic activity in central Mexico forms an arcuate distribution, it is not parallel to the subduction, especially the part of the TMVB which lies about 19° north. The reasons for this are not entirely clear.
Present day volcanic activity can be roughly devided between large stratovolcanoes and smaller scale volcanoes such as cinder cones, maars, sheild volcanoes and lava flows. From a tephrochronological point of view tephra produced from stratovolcanoes and cinder cones are the most important features. The major stratovolcanoes in central Mexico include Nevado de Colima (4,240 metres), Volcán de Colima (3,820 metres), Nevado de Toluca (4,575 metres), Iztaccihuatl (5230 metres), Popocatépetl (5,465 metres), La Malinche and Pico de Orizabo (5,610 metres). For example, Volcán de Colimat is not only the most active volcano in Mexico, but also in North America (Bretón González et al., 2002; Luhr et al., 2010). The Michoacán-Guanajuato Volcanic Field (MGVF) is dominated by extremely dense volcanic activity in the form of cinder cones and shield volcanoes, with over 900 cinder cones found in an area of 40,000 km2 (Hasenaka and Carmichael, 1985). The two youngest cinder cones are Jorullo (AD 1759-1774) and Parícutin (AD 1943-1952). The Chichinautzin monogenetic field is another area of dense monogenetic activity with about 50 cinder cones produced in the last 50,000 years.
Whilst many studies of volcanic deposits in Mexico have occurred close to the source volcano (e.g. Luhr et al., 2010), more distal lacustrine-based tephrochronological records have been established across central Mexico. These range from Basin of Mexico (Ortega and Newton, 1998), Upper Toluca Basin (Newton and Metcalfe, 1999) and several lake basins in Michoacán (Davies et al., 2004; Telford et al., 2004; Newton et al., 2005).
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