Internal strain within lava flows from analogue modelling

Internal strain within lava flows can be measured in the field from deformed bubbles or vesicle foliations, preferred crystal orientation or AMS studies. These measurements have important implications for the determination of vertical velocity profiles and flow directions. Recent field studies show very contrasting strain patterns especially in vertical profiles. An experimental study was conducted to relate variations in internal strain to different boundary conditions during flow. Results show that viscous flow in a straight channel along a basal slope produces three successive stretch trajectories in vertical flow planes. These trajectories are related to the distribution with time of a simple-shear component and a pure-shear component. Viscous flow in a straight channel along a horizontal base reveals convex stretch trajectories in the vertical flow plane. Radial spreading at the mouth of a straight channel reveals concentric stretching perpendicular to the radial direction of displacement in the upper part of the flow. Strain components deduced from experiments may also explain fold formation during motion. These experimental results provide a few end-member strain patterns which may facilitate the interpretation of strain measurements in natural lava flows in terms of emplacement mechanisms.