In the British Isles we are affected by volcanic ash, or tephra, from Iceland relatively frequently, and both the British and Irish historical and sedimentary records are rich in tephra deposits.
Tephra is a term used to describe all of the solid material produced from a volcano during an eruption (Thorarinsson, 1944), and as we have seen with the recent Eyjafjallajökull eruption, the fine ash fraction of this tephra can travel immense distances. Small tephra shards (2–3 μm) from the eruption of Mount Pinatubo, Philippines, circled the globe several times after the 1991 eruption.
Aside from Eyjafjallajökull 2010, which will certainly be a significant marker tephra in the UK for future generations, there have been at least ten Icelandic eruptions in the last 1000 years which have had an impact on the British Isles (Figure 1). For example the summer of 1783 was known as the ‘Sand Summer’, as Britain endured ash fallout for almost a year from the devastating Laki Fissure eruption, which wiped out a quarter of the population of Iceland.
Tephra gained an extremely high profile during April and May 2010, due to its reported effects on jet engines, however Earth scientists have been studying tephra for several decades, both as (1) a chronological tool in environmental dating studies, and (2) the impact of tephra from eruptions on global climate.
The use of tephra layers in both terrestrial and marine sediments as a chronological tool is called tephrochronology, and was originally developed in Iceland (Thorarinsson, 1944). 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. The technique has been applied to other volcanically active areas such as Alaska, New Zealand, western Europe and Mexico. Tephra has been found in Greenland and Antarctic ice cores, and has been used as clear marker horizons for calibrating ice core age models.
Climatic studies have proposed tephra may have intensified ice ages (Ramaswamy, 1992) and may also have caused 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).
In Iceland, close to source volcanoes, tephra may be deposited in layers many centimetres thick, and preserved in soil profiles and lake sediments. The thickness of the layers is a function of the proximity of the site to the eruption, and prevailing weather conditions during the eruption, but as a general rule, tephra deposits become more dispersed with distance from the source volcano (Figure 2).
Tephrastratigraphy has been used widely in Iceland to ‘date’ landscape events. For example, if a certain tephra layer is found on top of a landform, such as a glacier moraine, it is clear that landform is older than the tephra (Figure 3). This is a ‘relative’ dating method, and used in this way gives minimum ages, however improvements in geochemical analysis, and the subsequent development of tephrochronology has enabled events and sedimentary sequences to be directly, or ‘absolutely’dated to very precise levels.
Tephrochronology was pioneered by Sigurdur Thórarinsson in Iceland in the 1960s. In the late 60s studies carried out in mainland Scandinavia and the Faroe Islands revealed the presence of Icelandic tephra layers that had clearly travelled thousands of kilometres from their sources (Persson, 1971). With the rise in ocean floor drilling, both for oil exploration and for research, tephras were soon found preserved in the long sediment record that exists in marine basins. It has only been relatively recently that Icelandic tephras have been found in the British Isles (e.g. Dugmore et al., 1995), with key discoveries in bogs and lake basins, predominantly in Ireland (e.g. Hall and Pilcher, 2002) and Scotland (e.g. Turney et al., 1997).
Each volcano has a different geochemical signature, or ‘fingerprint’. Likewise each eruption from that volcano has different geochemical characteristics, enabling many tephra layers found in the sedimentary record to be identified using existing datasets (Figure 4). Since the settlement of Iceland in 874 AD, there has been an extremely well-documented record of eruptions, and many tephras that pre-date Icelandic settlement have been radiocarbon dated (Table 1). As a result, once a tephra has been geochemically identified, it can be used as a time marker horizon across continental or intercontinental distances over a wide range of depositional environments.
Examples of their use are:
Contact Dr Jez Everest for further information.
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