Schaefer, C.J. & Kattenhorn, S.A (2001)
The Effect of Cooling History on Fracture Patterns in Basalt Lava Flows: Insights From Field Observations, Eastern Snake River Plain, Idaho
We present field observations of fracture characteristics within basalt lava flows of the Eastern Snake River Plain (ESRP), Idaho, in an attempt to relate complex fracture geometries to flow morphology and cooling history. In so doing, we demonstrate that simple conduction cooling models, typically used to explain the development of column-bounding fractures (colonnade), are insufficient to account for the variety of other fracture types observed in ESRP flows. Flows typically contain four fracture types: (1) colonnade, (2) entablature, (3) horizontal fractures near flow tops, and (4) inflation fractures.
Colonnade fractures predominate in flows mapped in the vicinity of the Idaho National Engineering and Environmental Laboratory. Colonnade fracture spacing varies spatially within a flow and with respect to entablature and inflation fractures, where present. Following colonnade fractures in prevalence are horizontal fractures that segment upper colonnade columns and are typically coincident with vesicular layers near flow tops. Horizontal fractures are therefore absent in the lower colonnade due to the absence of vesicular layers. As a result, fracture densities are greatest in the upper portions of flows. Vesicular layers are also commonly associated with termination points and jogs along vertical fractures. Entablature fracture zones, positioned between the upper and lower colonnade approximately two-thirds of the flow thickness down from the top, are not present in all flows. Entablatures consist of arcuate horizontal fractures connecting vertical fractures that cross through the entablature. Inflation fractures, up to 4m wide, formed as the outer crust of flows were placed under tension due to inflation. They occur along flow boundaries, pressure ridges, tumuli, pressure plateaus, and plateau pits. Inflation fractures may be filled by lava from later flows, but in the most recent flows are typically filled with loess and are open at their upper extent. They display meter-scale growth increments, analogous to centimeter-scale growth increments along individual column bounding fractures. Curvature of inflation fracture increments indicates resolved shear stresses on the increments, implying inflation and deflation processes influenced fracture growth. In addition, inflation fractures introduce perturbations into the thermal evolution of a cooling lava flow, causing abrupt variations in fracture orientations and possibly contributing to rapid entablature cooling.
The evolution of all types of fractures depends on the emplacement history and resultant thermal evolution of a lava flow. For example, magma pulsing, whereby the flow receives multiple pulses of lava during the cooling history, typically results in the development of multiple vesicular layers near the flow top. In addition, the influx of new lava with each pulse results in a thermal spike during flow cooling, altering thermal stresses and fracture spacing.
Characterizing the relationships between vesicular layer intensity and the occurrence of horizontal fractures, vesicular layer interaction with colonnade fractures, entablature evolution and geometry, and the influence of inflation fractures on the thermal evolution of flows, will enable a better understanding of fluid migration in the subsurface. A potential application is the use of borehole core data, which are conventionally assumed to be limited by their one-dimensionality, to create improved two- and three-dimensional subsurface fracture distribution and fluid flow models.