Porosity and permeability variations in volcanic conduits

Abstract

Porosity and permeability variations in high viscosity lava domes are the result of collapse and shear processes during eruption. In depth field mapping of Ngongotaha Dome (Rotorua Caldera) and Ruawahia Dome (Tarawera Volcanic Centre) has produced eruption models that can help explain these variations from styles of eruption of the domes. In addition, high temperature, low stress experiments have been used to quantify the effect of compression on both crystalline and non-crystalline rocks. These experiments have shown that while porosity collapses in a predictable fashion (based upon initial vesicularity and crystal content), permeability will be highly variable during compression as bubbles close (acting to reduce permeability) and cracks open (acting to increase permeability).

Both Ngongotaha and Ruawahia Domes have erupted in a fashion that previous modelling cannot adequately explain. Flow band variations suggest that instead of a simple flow-like eruption, both domes erupted from a series of lobes and sheets that erupted from a fissure-controlled conduit. In particular, Ngongotaha exhibits obsidian sheets bounded by meter scale breccia zones indicating that the sheets moved laterally relative to each other, producing thick breccia zones between them. These breccia zones provided pathways for gas escape, leading to the formation of minerals that have annealed the breccia. The orientation of flow bands in each dome can be linked back to the shape and orientation of the conduit, which in turn is controlled by the regional structure of the host caldera and the overall structure of the rifting of the Taupo Volcanic Zone.

The models produced give new insights into the eruption of lava domes, and the evolution of the porosity and permeability within. Understanding the processes of dome growth, bubble collapse and permeability evolution is key when predicting the behaviour of lava domes, which are one of the common volcanic landforms in the Taupo Volcanic Zone.