COOLING RATE AND THERMAL STRUCTURE DETERMINED FROM PROGRESSIVE MAGNETIZATION OF THE DACITE DOME AT MOUNT-ST-HELENS, WASHINGTON

Our study of a magnetic anomaly associated with the recently active dacite dome at Mount St. Helens suggests that the dome consists of a hot, nonmagnetized core surrounded by a cool, magnetized carapace and flanking talus. Temporal changes in the magnetic anomaly indicate that the magnetized carapace thickened at an average rate of 0.03 ± 0.01 m/d from 1984 to 1986. Petrographic and rock magnetic properties of dome samples indicate that the dominant process responsible for these changes is magnetization of extensively oxidized rock at progressively deeper levels within the dome as the rock cools through its blocking temperature, rather than subsequent changes in magnetization caused by further oxidation. Newly extruded material cools rapidly for a short period as heat is conducted outward in response to convective heat loss from its surface. The cooling rate gradually declines for several weeks, and thereafter the material cools at a relatively constant rate by convective heat loss from its interior along fractures that propagate inward. -from Authors