Textural evolution of pyroclasts in weakly explosive basaltic eruptions

Abstract

Fluidal eruptions are the most common style of explosive basaltic eruptions and can produce a wide range of pyroclast types and morphologies, including scoria, reticulite, and Pele’s hair. Pyroclasts may undergo textural deformation while still molten, and their final morphology is based on the complicated interactions between many different factors, including viscosity, vesicularity, surface tension, and external forces such as inertia and environmental temperature and conditions. Investigating pyroclast morphology can provide important insight into how these pyroclasts formed and evolved, and therefore, eruption dynamics and processes. In this thesis I will explore the spectrum of pyroclast textural evolution and deformation, and determine how different textures are formed based on pyroclast characteristics. First, I present results from textural analyses of highly vesicular pyroclasts from the 2021 Geldingadalir (Iceland) eruption. I found that a relatively dense rim formed around the pyroclasts through bubble loss at the edges, due to extended residence time within the fountain. Some pyroclasts also underwent substantial film failure and partial retraction in their centres, producing pyroclasts with radially variable textures. Another population of pyroclasts instead comprise a stable foam with thin films separating vesicles. These pyroclasts help us understand the competition between cooling, coalescence, and relaxation timescales. Next, I develop and characterise two non-isothermal, vesicular, basaltic magma analogues using molten sugar and glass. These analogues have easily variable vesicularities and are ideal for experiments investigating the interaction between vesicularity, deformation, and cooling. Finally, I employ these analogues in extensional deformation experiments at different vesicularities to investigate a new formation mechanism for Pele’s hair. These experiments demonstrated that Pele’s hair also form through the extension of highly vesicular (>70%) basaltic pyroclasts. Overall, I have characterised basaltic pyroclasts and analogues that cover the spectrum of basaltic pyroclast textural evolution. From this, I can conclude that highly vesicular pyroclasts resist deformation under low stresses, while substantial deformation is promoted by highly vesicular pyroclasts under high stresses. In these instances, scoriaceous pyroclasts may evolve to form different types of basaltic pyroclasts, such as Pele’s hair. This knowledge helps us understand the evolutionary history of basaltic pyroclasts at a range of vesicularities. This has implications for interpretations about pyroclast formation mechanisms, textural evolution, and overall eruptive processes.