Characterisation of eruption and depositional processes of volcanic ash

The morphologies of pyroclasts in basaltic and silicic ash deposits and tuffs in volcanic sequences have been used by previous authors to determine eruption processes in proximal settings commonly from modern eruption deposits. However, little research has been conducted on the finest tephra fraction, ash and fine-ash, in distal settings. The study of tephra in distal localities, >100 km from source, may provide important information regarding the dispersal of ash from highly explosive large-scale eruptions, for which proximal deposits are no longer present. The grain morphologies (blocky, bubble-wall, irregular and rod-like) and vesicularity of pyroclasts can be used to determine eruption processes and distinguish between ‘dry’ magmatic and ‘wet’ hydrovolcanic eruption-styles. This research is mainly concerned with the characteristics of pyroclasts in basaltic ash/tuff likely formed by highly explosive eruptions, and whether the explosivity of these eruptions was due to magma-water interactions and hydrovolcanism. This study includes a re-examination, and detailed petrographical analysis, of tuffs in the Early Eocene Balder Formation, North Sea Basin, and Harwich Formation, SE England, where the source of tephra produced by explosive volcanism and the extent of basaltic ash dispersal have been contested. Previously, these basaltic tuffs have been attributed to large-scale hydrovolcanic eruptions that occurred during initial sea-floor spreading in the NE Atlantic during the separation of Greenland from NW Europe. These tuffs are compared and contrasted with the basaltic Saksunarvatn Ash (10.3 ka BP) of Grimsvotn Volcano, Iceland, and various non-lava lithologies of the Miocene Columbia River Flood Basalt Province, USA. The primary characteristics of pyroclasts may be modified by abrasion and alteration, particularly during additional transport and deposition. Therefore, the features created directly by the fragmentation and quenching of pyroclasts need to be distinguished from features created by the reworking of this material in the sedimentary environment. It is therefore essential to determine the characteristics of pyroclastic and volcaniclastic materials, which can be achieved by comparing data on the overall deposit characteristics, grain-types, grain-size distribution and grain-morphologies. For example, pyroclastic deposits of ash-grade are dominated by up to 100% igneous/volcanic grain-types, particularly glassy pyroclasts; in thin-section the pyroclasts have smooth-edges and may have undergone limited alteration in modern deposits. In contrast, volcaniclastic deposits may contain a wide range of clast-types, including a greater number of non-volcanic grains, and sedimentary structures such as cross-stratification; in thin-section, reworked pyroclasts of ash-grade (volcaniclastic grains) often have jagged-edges, and varying levels of alteration. This study builds upon the observations and naming schemes of previous authors and proposes a scheme applicable to distal deposits of ash and fine-ash, of both basaltic and silicic composition, including highly altered material.