Cooling rate and thermal structure determined from progressive magnetization of the dacite dome at Mount St. Helens, Washington

Journal Article · Sat Mar 10 00:00:00 EST 1990 · Journal of Geophysical Research; (United States)

DOI:https://doi.org/10.1029/JB095iB03p02763· OSTI ID:5009915

Dzurisin, D ^[1]; Denlinger, R P ^[2]; Rosenbaum, J G ^[3]

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1. Geological Survey, Vancouver, WA (United States)
2. Univ. of Washington, Seattle (United States)
3. Geological Survey, Denver, CO (United States)

The study 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 {plus minus} 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 magetization 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. The rate of internal convective heat loss through fractures varies with rainfall, snowmelt, and large-scale fracturing during subsequent eruptive episodes. In accordance with a model for solidification of the 1959 lava lake at Kilauea Iki, Hawaii, the authors picture the dome's magnetized carapace as being a two-phase, porous, convective zone separated from the nonmagnetized core of the dome by a thin, single-phase conductive zone. As a consequence of the heat balance between the conductive and convective zones, the blocking-temperature isotherm migrates inward at a relatively constant rate. If the dome remains inactive, the time scale for its complete magnetization is estimated to be 18-36 years, a forecast which can be refined by shallow drilling into the dome and by continuing studies of its growing magnetic anomaly.

• OSTI ID: 5009915
• Journal Information: Journal of Geophysical Research; (United States), Vol. 95:B3; ISSN 0148-0227
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
MT ST HELENS
MAGNETIC SURVEYS
VOLCANIC ROCKS
COOLING
CONVECTION
GEOLOGIC FRACTURES
GEOLOGIC MODELS
GEOMORPHOLOGY
HEAT LOSSES
MAGNETIC PROPERTIES
MAGNETIZATION
PETROGRAPHY
ROCK MECHANICS
SOLIDIFICATION
TEMPERATURE DISTRIBUTION
THERMAL CONDUCTION
TIME DEPENDENCE
WASHINGTON
CASCADE MOUNTAINS
DEVELOPED COUNTRIES
ENERGY LOSSES
ENERGY TRANSFER
FEDERAL REGION X
GEOLOGIC STRUCTURES
GEOLOGY
GEOPHYSICAL SURVEYS
HEAT TRANSFER
IGNEOUS ROCKS
LOSSES
MASS TRANSFER
MECHANICS
MOUNTAINS
NORTH AMERICA
PHASE TRANSFORMATIONS
PHYSICAL PROPERTIES
ROCKS
SURVEYS
USA
580000* - Geosciences