Abstract
Maar-diatreme volcanoes are unique in that most “eruptive” activity takes place below the ground surface, thus forming large conduit structures filled with pyroclastic deposits that are large relative to their volcanic edifices. These small-volume volcanoes are traditionally divided into three main levels based on common divergences in geometry and internal architecture at different depths: feeder dike, diatreme structure (conduit structure) and tephra ring (surface deposits). Although maar-diatreme volcanoes worldwide show generally very similar characteristics despite many different magma compositions, there is no consensus on how these volcanoes excavate the country rock and develop during an eruption. This study aims to determine the processes and relative timing of activity taking place below the ground surface by combining detailed mapping of three exemplary exposures of diatremes at different structure levels, from dike-widening transition to well-formed diatreme, within the Hopi Buttes volcanic field. Observations from different volcanoes are readily related to one another because the field had homogeneous pre-eruption hydrology, wall-rock stratigraphy, and magma composition, with a narrow range of eruption ages.
Castle Butte Trading Post (CBTP) comprises four closely spaced narrow spatter-dikes and wider maar-diatremes ~150 m below the pre-eruptive surface. The spatter-dikes consist of bedded, variably welded deposits plus wall-rock debris in multiple NE-younging sequences demarcated by truncation surfaces. They reveal a shallow plumbing cycle of pulsating, weak, hot spatter fragmentation, concurrent wall-rock failure and periodic slips that truncated down dropped bedded deposits from repeated magma withdrawal and diversions during progressive NE fissure extension and vent stepping.
Both Standing Rocks West (SRW) and East (SRE) diatremes, exposed ~300 m below the pre-eruptive surface, are part of a single larger volcanic complex formed along a series of irregularly offset NW-SE trending dikes. SRW comprises dominantly multiple, structureless irregular columns of well-mixed, poorly sorted juvenile-rich lapilli tuff deposits that contain abundant recycled material; they truncate local marginal layered deposits and peripheral country rock breccia. SRW mostly records late-stage activity of multiple, small-volume, explosions and jets within loose pyroclastic debris, which resulted in gradual mixing, recycling and remobilization of cognate diatreme debris, incremental addition of juvenile material and a well-formed diatreme. In contrast, SRE comprises predominantly country rock lithic-rich breccia of coarse inhomogeneously mixed wall-rock blocks, cross-cut by domains of lapilli tuff deposits that are overlain by spatter deposits and cross-cut by irregularly distributed dikes. SRE shows a progressive transition from fissure to diatreme, but an overall evolution from explosive to weak eruption styles, thus reflecting an arrested diatreme.
Instead of simply representing vertical differences of a diatreme structure, CBTP, SRE and SRW reveal much volcano-to-volcano variation, and even within-eruption variation in eruption processes and intensity. Variability in eruption intensity is inferred at all scales i.e. between different en echelon dike systems, between the structures formed along segments within these systems and between the vents for individual volcanoes at the surface. These Hopi Buttes volcanoes show that the roots of weakly and strongly explosive small volcanoes are shared with changes in explosive intensity over short distances and not necessarily systematic variations in behaviour through time. The evolution of the shallow plumbing during an eruption involves local feedback effects that critically affect eruption style over short times and distances. Neither magma composition nor country-rock hydrology can be considered as the primary control on inter-eruption variation or on changes through a single eruption in the Hopi Buttes volcanic field.