Tephra, Tephrochronology and Archaeology - a (Re-)View from Northern Europe
Riede, Felix, Thastrup, Mads Bakken, Heritage Science
Authors: Felix Riede (corresponding author) ; Mads Bakken Thastrup [1,2]
The Law of Superposition and its actualization in the form of stratigraphy constitutes the foundation of archaeological dating, albeit usually in a relative rather than an absolute manner [1, 2, 3]. Although many characteristic features within a given stratigraphy (e.g. soil horizons, loess layers) can be used to anchor such sequences in time, the process or processes that cause the formation of such similar layers are often transgressive in time and/or space thus detracting from their general utility as dating and correlation tools. In contrast, the deposition of the loose, pyroclastic volcanic material ejected during a volcanic eruption (tephra) can be considered instantaneous from a geological, evolutionary, and archaeological perspective. The term tephra comes from Greek [tau]?[phi][rho][alpha] meaning ?ashes?, although this is in many ways an unfortunate misnomer. Volcanic ash is decidedly not the product of organic combustion, but rather it is rock powder generated in an extremely high-powered environment where both native (surface) rocks as well as the magma connected to the eruption itself are torn asunder. Truly distal tephra particles are usually glassy and can retain their characteristic bubble-infused and sharp-edged form (Figure 1). Note, however, that the shape, size, degree of vesicularity, and colour of tephra particles can vary substantially.
Figure 1: Examples of tephra shards, showing some of the inherent morphological variation. All from a small kettle-hole near Lendum, northern Denmark. The source eruptions remain as yet unidentified. [see PDF for image]
Tephra is usually further divided by size classes into ash (<2 mm), lapilli (2-64 mm), and bombs or blocks (>64 mm), the latter of which can reach considerable sizes. The focus of this review is on material at the lower end of this size spectrum and its use as an analytical tool in archaeologically relevant contexts as, first and foremost, chronostratigraphic markers and, secondarily, as possible causal elements in culture-historical change. While the very finest volcanic particles ejected as part of volcanic eruptions can have considerable residence times in the atmosphere as aerosols, the larger particles such as rock fragments, pumice, crystals, and glass tend to fall out already during or shortly after the eruption. They effectively form isochrons. Especially explosive eruptions are associated with a profuse output of such material, and the frequency of such eruptions throughout the Quaternary is substantial [4, 5, 6]. Nonetheless, tephrochronological research remains a young field of specialised study. Pioneering near-field work was carried out on Iceland in the first half of the 20[sup.th] century by Thorarinsson , but it was as late as 1981 that the prospect of a superregional, far-field tephrochronology was first aired . Scientists in the UK swiftly followed Thorarinsson?s lead [9, 10], and thanks to the many Icelandic eruptions through the millennia [11, 12] and a vibrant and growing research community (see http://www.swansea.ac.uk/geography/research/environmentaldynamicsgroup/tephrainquaternaryscience/) there now exists a robust tephrochronological lattice for the Late Pleistocene and Holocene of the north-western Atlantic seaboard [e.g. [13, 14, 15, 16, 17] and references therein]. In central Europe, the botanist Firbas  suggested already in 1949 that tephra from the Late Glacial Laacher See-eruption could be used as a chronostratigraphic marker for the Allerod chronozone in Europe north of the Alps. This enthusiasm was mirrored by the archaeologist Schwabedissen?s  remark on how this tephra layer likewise could be used in archaeological investigations both in the near- as well as the far-field of the eruptive centre. Yet, it was first in the 1980s that more extensive distribution maps for the Laacher See tephra were presented , and more recent still that such data are updated and made available interactively . …