Crease structures: Indicators of emplacement rates and surface stress regimes of lava flows
Crease structures are features commonly found on lava flow surfaces and consist of a fracture with curved walls that extend outward from a linear valley. These crease structures are found on flows of nearly all compositions and crystallinities. We have mapped the distributions of crease structures on many flows in the western United States and found that (1) axial length is not dependent upon composition and crystallinity; (2) adjacent crease structures are generally aligned in an en echelon pattern; (3) crease structures located adjacent to flow margins are generally perpendicular to these margins; and (4) at Mount St. Helens, Washington, large lobe-bisecting crease structures are found on lobes situated on slopes of less than 20 degrees. A primary surface feature found on many crease structures is striations. Striations are sets of long stripes on the walls of the crease structure that extend approximately parallel to the axis of the central valley, and they appear to be analogous to those found on the faces of cyclically fractured basalt columns. Observations of developing crease structures on 6 of the nearly 20 Mount St. Helens dome lobes show that they form throughout the extrusion of flows situated on slopes of less than 20 degrees, but only at the very beginning and/or end of extrusion of flows on steeper slopes. These observations imply that crease structures form when the lava flow is forced to spread laterally, either as the flow advances over a flat area, or as the down-slope movement stagnates near the end of extrusion (causing the rate of spreading to exceed the downslope rate of flow). This lateral spreading of lava results in the concentration of tensile stress along a line oriented perpendicular to the direction of spreading. The cooled crust of the extrusion is therefore torn apart about this line of tensile stress concentration, forming a central valley that exposes hot, ductile material from the flow interior to the atmosphere. The presence of striations on many crease-structure walls implies that the emplacement mechanism is similar to that suggested for columnar basalts, where each striation is produced by incremental fracturing. The stress responsible for crease structure formation is due to both cooling and lateral spreading of lava. Calculations show that stress due to spreading is three orders of magnitude higher than that due to cooling. Fracture advance times were calculated using a simple one-dimensional conductive cooling model adjusted for the special geometry of the crease structure. The model closely approximates measured crease-structure emplacement times at Mount St. Helens and gives reasonable estimates for crease structures found on older silicic lava flows. We then used this model to calculate emplacement times of entire domes that consist of a single crease structure. By dividing the dome volume by the formation time, we are able to calculate extrusion rates for these older domes. These values, which include the first flow-rate estimates reported for rhyolitic lavas (0.03-106 m 3 /s), similar to average flow rates of 0.7-40 m 3 /s calculated for Mount St. Helens dacite lobes from detailed topographic maps.