Field Observations of Tephra Fallout Deposits

Many styles of volcanic activity result in the generation of volcanic ash, which is then available to be lofted into the atmosphere and transported to depositional settings away from the vent (Fig. 1). Ash from eruptions of all scales poses hazards to the natural environment, and to humans and their support systems, both when airborne and on the Earth’s surface (e.g. Rose and Durant, 2009; Wilson et al., 2011; 2014; Browning et al., 2015). Ash from large explosive volcanic eruptions may also form chemically-distinct tracers, that can be traceable over a wide area (>104 - 106 km2) to form time-horizons that can be dated, and correlated across marine and terrestrial settings (e.g. Pyle, 1999; Fontijn et al., 2014). As a consequence, there has been considerable interest in the search for and recognition of volcanic ash (or tephra, more generally) in stratigraphic contexts for a host of applications - from the reconstruction of past eruptions and their impacts, to the development of precise chronologies for use in palaeoclimate reconstruction, paleoseismology and archaeology (e.g. Lowe, 2011; Matthews et al., 2012; van Daele et al., 2015; Davies, 2015). In this chapter we focus on the use of volcanic ash to reconstruct volcanic eruptions of the past - giving a brief introduction to the conventional modes of field analysis of these deposits; how they may be modelled and interpreted. We also point out some areas where current knowledge is still lacking and more work remains to be done