Rheology of Basaltic Lava Tubes: Disequilibrium two-step cooling deformation experiments

Lava cooling rate mainly determines how far lava can travel from its source. Lava in insulated tubes cool slowly and flows farther, whereas lava in open channels cools quickly and solidifies sooner. This study explores how the rheology of a lava changes as it transitions from being contained in a tube to surface breakout flow. Using two-step cooling deformation experiments (CDEs), we simulated a shift from a slow cooling rate (0.5 °C/min; tube-like) to faster rates (1, 2, 3, 4, and 5 °C/min; breakout-like) with a constant shear rate of 1 s −1 . A second set of CDEs was performed at cooling rates from 0.5 to 5 °C/min and variable transition temperatures (1185, 1175, 1165, 1155, and 1145 °C). We found that lava subjected to slow cooling achieves significantly lower viscosities and can flow to lower temperatures before solidifying. Additionally, for lava at a comparison temperature of 1132 °C, a higher breakout transition temperature (e.g., 1185 °C) resulted in lower viscosity than a lower transition temperature (e.g., 1145 °C). We conclude that a lava's thermal history is a key factor influencing its viscosity, which explains the enhanced mobility of tube-fed lava even after exposure. • Two-step cooling rate deformation experiments can mimic natural lava cooling histories; this work replicates a lava tube breakout scenario with the transition from a slow conductive cooling-dominate to a fast radiative cooling-dominated regime. • Lavas undergoing a tube-like (slow to fast) cooling can sustain a lower viscosity than a channel-like (fast to slow) cooling, resulting in lavas from a lava tube breakout flowing more easily. • The thermal history of lava tube is a key factor in the viscous evolution which explains the enhanced mobility of lava tube breakouts when compared to channelized lava flows.