The influence of moisture on ash strength: implications for understanding volcanic stratigraphy

Ash layers within extensive pyroclastic density current (PDC) deposits can be important in understanding the evolution of explosive eruptions. If interpreted as ashfall deposits, they may be used to identify hiatus episodes and determine how many pyroclastic density currents were generated during an eruption. However, such layers, which often contain ash aggregates indicating the presence of moisture, may be variably deposited, preserved and eroded, and there has been little study of their characteristics. This paper investigates the geomechanical properties of ash layers through a comparison of field observations and experimental analysis. We present new field evidence of intriguing relationships between ash aggregate layers and overlying ignimbrite facies within the 273 ka Poris ignimbrite of Tenerife. We identify three types of interactions, each displaying distinct erosional and remobilisation behaviours associated with varying moisture conditions. To complement these field observations, we performed direct shear box, ring shear and drop tests under both low (1 wt.%) and high (> 15 wt.%) moisture conditions. We find that fine pyroclastic material increases yield strength with moisture compared to dry conditions. Furthermore, we see shear thickening behaviours in fine ash at high moisture content. These behaviours show how ash layers formed under low and high moisture conditions are more likely to be preserved than dry layers. This may lead to misunderstanding in flow unit interpretation and, therefore, assessments of eruption frequency. Furthermore, dry, loose ash material may promote flow bulking, increasing mobility, runout and flow velocities in PDCs.