Cooling of an Igneous Dike 20 yr After Intrusion

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The 1975 Tolbachik, Kamchatka, Russia, eruption resulted in the formation of three basaltic cinder cones. During this eruption, shallow dike injection into a nearby cinder cone, Cone 1004, caused deformation and slumping of the cone. In 1995, temperatures at 2 m depth, electromagnetic anomalies, and geologic units were mapped on the slump block. These data reveal details about the cooling rates of a shallow dike in the slump block, including (1) measured temperatures as much as 475 °C, (2) temperatures greater than 200 °C along a 160-m-long and 30-m-wide zone that trends oblique to bedding, (3) a low resistivity zone at 80 m depth that may delimit a larger intrusion beneath the shallow dike, (4) a dry zone extending 15–20 m from the axis of the thermal anomaly, and (5) a condensation zone beyond this dry-out zone, characterized by steaming ground and high convective heat flux. Analytic models indicate that the surprisingly high temperatures persist because of the low thermal diffusivity of the scoria into which the dike is injected, a direct result of the scoria's high porosity. Two-phase nonisothermal flow models suggest that the observed patterns of dry-out and condensation are expected for small volume fractions of water in the scoria (≤10%). Thus, field observations from Tolbachik provide a graphic example of the roles of rock thermophysical properties and water in the unsaturated zone on rates of dike cooling.

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Geology, v. 25, issue 8, p. 711-714