During flow, lava is subjected to a combination of compressional, extensional, and shearing forces that may result in folds and fractures on the lava surface, depending on the rheologic and material properties of the lava. A series of analog experiments were performed to examine the partitioning of strain associated with shear stresses and the formation of surface fractures. Experiments simulated a range of flow conditions (increasing, decreasing, and constant effusion rates) and different surface crust rheologies (viscous, brittle, and rigid).

Strain in experimental flows displayed a combination of pure shear (α) and simple shear (γ) components, which vary in space and time. Pure shear was largely controlled by changing effusion rate. Flows with decreasing effusion rates exhibited pure shear consistent with extension (α > 1) due to progressive flow thinning. In contrast, flows with increasing effusion rates experienced pure shear associated with compression (α < 1) due to thickening and increasing flow velocity. Flows with constant effusion rates displayed simple shear (α ∼ 1) and had stable flow lengths and velocities. Simple shear in the fluid part of the flows was produced by friction along the flow base and, in the experiments with a rigid surface crust, friction below the surface crust.

In experiments with brittle crusts, fracture patterns were similar to those observed in natural lava flows (crease structures and tension gashes). As these fractures spread, they were filled by isostatic upwelling of the viscous flow interior. Flows with brittle surface crusts also produced basal breccia due to rolling of the flow front.