An investigation into the explosivity of shallow subaqueous basaltic eruptions

Murtagh, Rachel M. (Rachel Maria)

Explosive basaltic eruptions present a challenge to modelling volcanic behaviour. The main focus of this thesis is the examination of pyroclastic micro-textures and morphology of the ash fraction in order to constrain the conduit conditions during ascent and at the time of fragmentation, respectively in the lead up to such explosive volcanic eruptions. Three sites, Black Point, California, Pahvant Butte, Utah and Ilchulbong, Korea, were considered. I propose that, in certain cases, conduit processes can collaborate to modify basaltic magma rheology, which can be complemented by powerful water-magma interactions to result in an intense and violent eruption.

Results of micro-textural data show: (1) vesicularity of shallow subaqueous basaltic (“Surtseyan") tephra can reach high values (up to 92% at Pahvant Butte) and vesicularity ranges are broad (e.g. from 6% to 92%, for Pahvant Butte pre-emergent mound). Quantitative values combined with textural observations (e.g. vesicle shape and pattern) indicate a strong heterogeneity of vesiculating magma. (2) vesicle number densities are high (the minimum is at Black Point, 5.4 x 102 vesicles per mm3 and the maximum is seen at both Black Point and Pahvant Butte, 4.8 x 104 vesicles per mm3). Finally, (3) extensive microlite formation is common within Surtseyan pyroclasts examined here. Results of ash morphology analyses show 75% of Black Point ash and 80% of Pahvant Butte ash fragments fall within the hydromagmatic fragmentation field. Furthermore, strong evidence for water-magma interaction (fracture-bound, stepped, cracked and etched surfaces) is seen, as is a dominance of thermohydraulic mechanism of fragmentation. Due to the thermodynamic properties of water and magma this fragmentation regime is considered the most efficient and explosive.

A model, common to the three study sites, of shallow conduit conditions and fragmentation leading to eruption is proposed here. Early nucleation at depth is followed by ascent and temporary arrest of magma resulting in a mature (relatively large equivalent diameters) vesicle population and a variable amount of microphenocrysts, both juvenile and xenocrystic. A decompression event, possibly induced by injection of a fresh batch of magma and/or, later in the eruption, flank instability and failure, induces a late stage of bubble nucleation and consequently the final ascent of the magma is rapid and thought to follow a parabolic ascent profile. Simultaneous to rapid ascent microlite nucleation occurs. This burst of nucleation is prompted by high volatile contents (Black Point 0.64-1.92 wt% and Pahvant Butte 0.80-1.29 wt% H2Ot) and decompression. Magma rheology is progressively altered, particularly the viscosity. These processes in combination are sufficient drivers of an explosive eruption. In the Surtseyan cases presented here, conduit processes act to prime the basaltic magma for explosive interaction with an external body of water, however, explosivity cannot be directly linked to the presence of external water alone.

In summary, the Surtseyan eruptions presented in this study are interpreted to have been as a result of exceptional conduit conditions which aided explosive eruptive behaviour that is uncharacteristic for basaltic magmas. It is speculated that had these eruptions occurred in a subaerial setting the outcome could have been as dramatic as well documented sub-Plinian and Plinian eruptions.

xi, 242 p. : ill. (some col.) ; 30 cm. + 1 CD-ROM (4 3/4 in.)

2011Murtagh

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