Volcanic diatreme deposits commonly feature a significant percentage of country rock lithic fragments that result from explosive processes. Where extracted from country rock with known stratigraphy, they provide “tracer” particles that potentially provide information about how much excavation took place, at what depths below the surface, during a diatreme-forming eruption. Current understanding of subsurface processes in diatreme volcanoes is poor, and understanding how large, deep-sourced lithics are erupted to the surface is a current challenge. The transport of country rock lithic particles in subsurface diatreme systems has not been investigated experimentally in detail, and the aim of this thesis is to conduct experimental investigations into subsurface diatreme-forming processes, and also to find out what information can be extracted from lithic populations as a whole. In the current diatreme literature, more focus has been awarded to lithics situated within tephra rings than those in diatreme structures due to the better availability of tephra ring deposits. The proportional lithic populations in their tephra rings do not agree with the inferred depths of individual diatremes, however, and deep-sourced lithics are underrepresented in tephra ring lithic layers.

Published research results on maar-diatreme-forming processes agree that there is a significant proportion of lithics that do not become ejected onto the surface; thus, a lot can be learned by focusing on the lithic fragments contained within the subsurface diatreme structure. Searching for patterns among the sources, sizes, shapes, and final distribution of lithics reported in the field, and conducting analogue lab-scale experiments observing the movement and final resting place of lithics within a representative diatreme structure aids in illuminating the physical phenomena of maar-diatreme-forming processes. 

This work contains a meta-analysis of lithics found worldwide at maar-diatreme and kimberlite sites; it also reviews the current understanding of the mechanisms transporting lithics within diatreme structures, and the observable signatures of these within surficial tephra deposits and diatreme infill. My experimental work primarily addresses the transport of larger fragments (lithic-block analogues) in multiple separate explosions, explosion series, and briefly sustained jets. This experimental work differs from earlier studies using compressed air or water alone to create a steady state of fluidisation as it involved injecting compressed gas and “juvenile”-representative beads into host glass beads to simulate discrete blasts of entrained juvenile-product-representative particles into a host medium, and seeded larger-sized glass beads within the host layers to represent lithic fragments in order to investigate the behaviour and transport of lithic particles within a mixed diatreme system.

The meta-analysis of the compiled data reveals no consistent trends among any of the characteristics of lithics or their positions within the deposits analysed. This conclusion is tentative, due to the lack of data available—more comprehensive studies on lithic fragments at diatreme and ejecta ring sites are needed for a more robust conclusion. The experimental results show that sustained jets are not very effective in mixing blocks from different levels compared with a series of explosions, which also induce mixing over a much larger footprint through time. While sustained jets are efficient in transporting particles from the gas source to eruption at the surface, discrete explosions both drive particles upward and trigger inward collapse of a surrounding zone of material through which the blasts vent to the surface. For a given amount of gas release, far more subsurface particle motion is induced with repeated blasts than with a sustained jet. 

This thesis provides a two-pronged data-based and experiment-based investigative approach that sheds further light on these debated processes and offers a good theoretical and experimental stepping stone for further research.