Documenting the Geology of Volcanoes and Volcanic Terrains

This chapter outlines the major principles and challenges in facies analysis, field survey mapping, developing stratigraphic architecture, and documentation of the geology of volcanoes and volcanic terrains, both modern and ancient. The stratigraphic record of almost all volcanoes can be very incomplete, depending on the preservation potential of each eruptive unit. This is because of the prolonged inter-eruptive periods compared with the short duration of eruption events, and due to the erupted nature and volume of the deposits (e.g. large versus small volume, coherent versus clastic units). Although much of sedimentary stratigraphy is based on correlation of laterally continuous “layer-cake” type units, in volcanic terrains the stratigraphic architecture is much more complex, particularly in proximal, high relief areas on large volcanic edifices, where highly discordant contacts are common. We assess the major factors that influence the preserved geometry of stratigraphic units, including the effects of eruption geometry, effects of high rates of erosion, volcano-tectonic (as opposed to regional tectonic) deformation, and provide a current review of the different approaches (e.g. lithostratigraphic, event-sequence, and seismic sequence or architectural elements) to representing volcanic stratigraphy. Identifying event sequences, or “genetic packages”, bound at their tops and bottom by unconformities is essential to understanding the history, evolution, and chrono-stratigraphy of volcanic terrains. However, unconformities are common in volcanic terrains, so it is equally significant to establish their rank or hierarchy and temporal or chronological significance. Routine methods used for the correlation of volcanic successions are briefly summarised, as well as additional tools now being employed to map modern volcanoes and volcanic terrains such as satellite and geophysical imagery. In ancient volcanic terrains facies models have been and remain the chief method to represent volcanic facies architecture and lithostratigraphy.