David T. Lescinsky

The rheological parameters of a lava flow can be inferred by studying structural features on the solidified lava surface. We have developed a technique using the S‐transform (a localized Fourier transform) to analyze and interpret digital elevation data. We examined structural features of a silicic lava flow, the Medicine Lake dacite flow in California. The S‐transform accurately identified and located both repeating and single‐instance structures. Several classes of structures were identified in the data, including the following: localized short wavelength features (1.3–3.6 m in wavelength) identified as large blocks and repeating medium wavelength features (5–8, 10–16, 18–26, 30–36, and 56–67 m in wavelength) identified as multiple generations of crustal folding. Fold wavelengths determined by the S‐transform and a model of compressional folding associated with near surface viscosity gradients were used to evaluate the rheology of the Medicine Lake dacite flow. The similarity of the amount of strain during folding events and the wavelength ratio of successive generations of folding suggest that the flow rheology remained relatively constant during folding of the flow surface. Repeating long wavelength features (67–70‐ and 105–140‐m wavelengths) were identified as the spacing of crease structures. Crease structures were recognized by the absence of short wavelength features at the location of their smooth flanks and by a central spike corresponding to the central trough. The limitations and applicability of this approach lie in the accuracy and resolution of the digital elevation data used.

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.

Structures resulting from lava and ice interaction are common at glaciated stratovolcanoes. During summit eruptions at stratovolcanoes, meltwater is produced and travels freely down steep slopes and thin permeable valley glaciers, eroding the ice and enlarging preexisting glacial drainages. As a result, eruptions in this environment have produced few catastrophic floods. Lava flowing into the open channels and voids in the glaciers becomes confined and grows thicker, filling the available space and producing steep‐sided bodies with smooth, bulbous contact surfaces. Quenching of lava against ice or by water forms small‐scale features such as tensional fractures and glass. As the amount of meltwater in contact with the lava increases, the type and abundance of smaller‐scale features become similar to those produced during subglacial eruptions into meltwater lakes. Identification of large‐ and small‐scale lava‐ice contact features in the field can be used to reconstruct paleoglacial extent and, combined with geochronology of lavas, to determine past paleoclimate. An understanding of lava‐ice interaction allows us to better assess the hazards posed by future eruptions at glaciated volcanoes.