Schaefer, C.J. and Kattenhorn, S.A. (2003)


Thermal modeling of the cooling history of a basalt lava flow: Effect of flow shape and thermal perturbations induced by inflation fissures

Eos, Transactions of the American Geophysical Union, 84.

Thermal modeling of cooling basalt lava flows has typically been undertaken using 1-D analytical heat flow models for an infinite plane. In such models, flows are conceptualized as having a finite thickness, but are infinitely wide and infinitely long (i.e., sheet flows). These analytical models typically accounted only for conductive heat loss, or attempted to approximate the effect of a sudden convective heat loss by redefining the conduction boundary conditions at some point during the cooling history. Although such models have proven useful for the examination of sheet flows such as those of the Columbia River flood basalts, they are inadequate for considering the cooling history of low-volume flows having small (meters to a few 10s of meters) in-plane dimensions (i.e., small aspect ratios, or width/thickness). In such flows, cross-sectional flow shape exerts a strong control on the thermal evolution of the flow during cooling, and hence on the cooling fracture patterns that develop in response to thermal stresses. The advent of numerical thermal models has recently enabled other researchers to predict isotherm patterns in lava flows with in-plane lateral peripheries. We build on these numerical modeling efforts by examining the effect of variable flow shape on lava flow cooling history. We also explicitly model the effects of convective heat loss through inflation fissures that develop in response to inflation of the lava flow during extrusion. This choice of controlling factors is predicated by observations of flow shapes and fracture characteristics of low-volume basalt flows of the Eastern Snake River Plain (ESRP), Idaho.

We use the finite element code ABAQUS to model the thermal evolution of small aspect ratio flows, both with and without an inflation fissure. The program accounts for radiation of heat and convection at exposed boundaries, latent heat of crystallization, and conduction of heat into the underlying substrate. In models that do not include an inflation fissure, the results predict that the final portion of a lava flow to solidify occurs slightly below the center of the flow, in agreement with field observations and previous analytical and numerical model results. Using the assumption that cooling fractures grow approximately perpendicular to isotherms, predicted isotherm patterns can be reconciled with fracture characteristics in cooled ESRP flows. The incremental introduction of an inflation fissure during cooling results in a one-month shorter time to complete solidification and significantly perturbs the isotherm patterns, and hence cooling fracture characteristics, of the flow. Furthermore, the inflation fissure dictates the location of the final portion of the flow to solidify (the "lava core") and may even cause the flow interior to segment into multiple lava cores. Based on these model results and our field observations of inflation fissure geometries, we hypothesize that the lava cores ultimately undergo rapid convective cooling and intense fracturing in response to being pierced by an inflation fissure, resulting in the development of a highly fractured zone (or "entablature") slightly below the flow center.

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