The Influence of Magma Plumbing Complexity on Low-Volume, Intraplate Volcanism

Monogenetic volcanoes in a continental field environment are typically conceived of as being constructed in single eruptive episodes, often involving low-volume single magma batches of alkali basalt. Well exposed pyroclastic successions at four eruptive centres (Udo, Suwolbong, Songaksan, Ilchulbong) in the Jeju Island Volcanic Field (Korea) allow detailed, stratigraphically controlled, geochemical investigations of juvenile ejecta. Despite the narrow ranges in overall magma variability, these sucessions reveal that multiple distinct magma batches may commonly be erupted during a single monogenetic eruption episode. Pyroclastic successions and chemical trends define two end-member behaviours. Uninterrupted depositional sequences have generally smooth chemical transitions, showing gradual changes in magmatic evolution. Discontinuous depositional sequences including truncation surfaces, slump beds, breccia horizons, asymmetric deposit geometries and deposit colour changes appear to coincide with renewals of magmatic activity by either a contrasting magma or a related magma batch showing a marked difference in evolution. Consequently, the entire chemical sequence of volcanoes showing the latter features, is oscillating and/or stepped, with chemical offests corresponding to the deposition breaks/transitions. We infer that uninterrupted eruptions result from open plumbing systems within which a single large dyke feeds the eruption of a single batch of magma that may be undergoing chemical evolution. Congested plumbing systems consist of dyke complexes that may give rise to asymmetrical eruptive centres and composite depositional sequences. If eruptions are sourced from individual dykes, stepped and oscillatory chemo-stratigraphic patterns result. The magmatic evolution, deep and shallow plumbing systems are intricately linked in the development of not only the variations in chemical composition erupted during a “monogenetic” eruption, but also the physical dynamics of the event, including whether significant intra-eruption pauses in activity may occur. Understanding the mechanisms behind such spatio-temporal eruptive behaviours during these events provides a powerful tool for understanding their hazards and potential consequences.