Aeromagnetic Data Reveal Dome Structure on Mount St HelensS. Polster (email: spolster@usgs.gov), C.A. Finn, and E. Anderson, U.S. Geological Survey, Denver, CO, USA

AbstractThree aeromagnetic studies of Mount St Helens were flown, in 1979, 1981, and 2007, that reveal basement structures (Finn and Williams, 1987), thermal structure, structure of the new dome and possibly altered layers. The recent survey found the new domes had lower than expected magnetization, suggesting the material is unconsolidated. These dataum, and the resulting model, help further constrain an electromagnetic survey in the northern part of the crater (Bedrosian et al., 2008), suggesting a little understood altered layer is larger than originally thought. The existence of this altered layer creates another hazard of lahars and landslides in the Mount St Helens region (Finn et al., 2007).

Introduction Mount St Helens, in southwestern Washington, is the youngest of the Cascade volcanoes. One of only two eruptions in the contiguous United States in the 20th century, the May 18th, 1980 eruption removed 2.76 km³ of material from the summit (Finn and Williams, 1987) and created a crater about 2 km wide and 500m deep. From June 1980 to October 1986, a dacitic dome grew in the crater to a height of 270m (Vallance et al., 2008). Following 18 years of quiet, the volcano became active again in 2004, in an eruption that has been characterized by the occurrence of more than seven steep-sided dacitic spines (Vallance et al., 2008). In this study, we examine the three surveys to use the data to determine the structures underlying the volcano and structures in and around the new dome.

Observations and ResultsAll the magnetic surveys show a positive residual magnetic anomaly on the south side of the volcano. Models suggest buried lava flows or buried intrusions, as observed at other Cascade volcanoes could be plausible sources for the plausible anomalies (Finn and Williams, 1987).Lower amplitude anomalies than those expected due to uniformly magnetized terrain characterize the 2007 domes and crater floor (fig. 3C).The lower than expected magnetizations over the new domes could be caused by rocks above their Curie temperature and/or that the dome is partially composed of dome talus that is randomly oriented (which would result in low total magnetization). Lower magnetizations than expected over the 19080-1986 and Pine Creek domes suggest that the domes are partially composed of talus.Comparison of our model with a geologic cross section (Fig. 6) and Time domain EM (TEM) model (Bedrosian et al., 2008) (Fig. 7) suggest in addition to unconsolidated jumbled material, altered rock could be included, making our model a combination of unconsolidated, randomly oriented material and alteration.High magnetizations are associated with the Pine Creek Dome (Fig. 5), indicating that domes of all ages are relatively magnetic. Therefore, the interface between magnetite poor and magnetite rich material in the preliminary model (Fig. 5) may indicate the top of domes of various ages.

Uniformly Magnetized TerrainFig. 3- 2007 Magnetic Anomaly Residual

ReferencesBedrosian, P.A., Burgess, M., Hotovec, A.,, 2008, Groundwater Hydrology within the crater of Mount St. Helens from Geophysical Constraints: American Geophysical Union Fall Meeting, San Francisco, California, Abstracts, V43E- 2191Dzurisin, D., Denlinger, R.P., Rosenbaum, J. G., Cooling rate and Thermal Structure Determined from Progressive Magnetization of the Dacite Dome at Mount St. Helens, Washington, J. Geophys. Res., 95, 2763-2780, 1990. Finn, C.A., Deszcz-Pan, M., Anderson, E.D., and John, J.D., Three-dimensional geophysical mapping of rock alteration and water content at Mount Adams, Washington: Implications for lahar hazards, J. Geophys. Res., 112, 2007.Finn, C.A., Williams, D., An Aeromagnetic Study of Mount St Helens, J. Geophys. Res., 92, 10194-10206, 1987.Hausback, B.P., Geologic Map of the Sasquatch Steps Area, North Flank of Mount St. Helens, Washington, U.S. Geol. Surv. Investigation Series I-2463, 2000.Vallance, J.W., Schneider, D.J., Schilling, S.P., 2008, Growth of the 2004-2006 Lava-Dome Complex at Mount St Helens, Washington, chap. 9 of Sherrod, D.R., Scott, W.E., and Stauffer, P.H., eds., A volcano rekindles: the renewed eruption of Mount St Helens, 2004-2006: U.S. Geological Survey Professional Paper 1750, 169-206.

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Pine Creek pyroclastic flows

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Photo by Daniel DzurisinBellingham

Vancouver Island

Mt RainierStudy Area

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Columbia RiverMt Hood

Mt Jefferson

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Resistivity (ohm/m)

Bedrosian et al., 2008

Lower than expected magnetization

Modeling of Magnetic DataVolcanic rocks are highly magnetic, so the expression of volcanic edifices dominates the magnetic signatures. The 1979 and 1981 models, along with rock properties of the 1986 dome suggest that much of Mount St Helens is uniformly magnetized between 4.1 and 4.9 A/m at the Earth’s current magnetic field (Finn and Williams, 1987 and Dzurisin et al. 1990). Removal the magnetic effects of uniformly magnetized terrain from the observed magnetic data reveal anomalies related to basement structures, compositional changes, hydrological interactions, and temperature variations. The magnetic effects of uniformly magnetized terrain were calculated for all surveys using the magnetic inclination of 69.0° and declination of 20.6° and a magnetic intensity of 4.2 A/m. In order to remove these effects, magnetic anomalies due to uniformly magnetized terrain (Fig. 1-3B) were subtracted from the observed anomalies (Fig 1-3A) to create the residual anomalies (Fig. 1-3C). Residual anomalies can indicate variations in magnetization, increased temperatures (above Curie temperature), random magnetic vectors, among other scenarios. All maps were then reduced to the pole to place anomalies directly over their sources.

Fig. 1- 1979Magnetic Anomaly Uniformly Magnetized Terrain Residual

Flight Lines

Shaded DEM overlain by color shaded-relief aeromagnetic data

Shaded DEM overlain by color shaded-relief image of magnetic anomaly due to uniformly magnetized terrain

Shaded DEM and color shaded-relief residual anomalies. Location of model profile A-A’ (Fig. 4).

Fig. 2-1981 Magnetic Anomaly Uniformly Magnetized Terrain Residual

Shaded DEM overlain by color shaded-relief aeromagnetic data Shaded DEM overlain by color shaded-relief image of magnetic anomaly due to uniformly magnetized terrain

Shaded DEM overlain by color shaded-relief image of residual anomalies

Flight Lines flown in 1981

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LIDAR image in 2007 overlain by color shaded-relief image of aeromagnetic data extrapolated to 200m

LIDAR overlain by color shaded-relief image of magnetic anomaly due to uniformly magnetized terrain

LIDAR overlain by color shaded-relief image of residual anomalies. Included are the profile paths of B-B’ of Fig. 5 profile C-C’ of Fig. 6 and Profile D-D’

from Fig. 7.

Buried valley

Buried ridge/cone

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R1- 1980 debris avalanche layersC1- ‘wet’ 1980 debris avalanche: perched water tableC2- altered (?) Pine creek, Castle creek memberR2- Unknown layer, boundaries not well constrained

2004 Dome

Electromagnetic cross-section along C-C’ surveyed in 2007. The R2 layer is not well constrained.

Magnetite-poor layers

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Goat Rocks pyroclastic deposits

Fig. 6

1980 debris avalanche

Cross section of northern half of Mount St Helens craters (D-D’ left) from Hausback 2000). The Goat Rocks eruptive period dates from 1800-1857AD, the Castle Creek period is 2,200- 1,700 BP, and the Pine Creek period is 3,000- 2,500 BP (Hausback, 2000).

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The regions in the models with intensity of 0 A/m correlate with locations of jumbled, unconsolidated material and underlying altered material; near the new domes, this jumbled material is about 300m thick (Fig. 5) and possibly talus associated with the collapse deformation and/or decoupling of spines; north of the domes this could be lahar material (Hausback, 2000).

Fig. 4

2004-2007 Dome

1980-1986 Dome

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Pine Creek Dome

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Approximate Pine Creek Dome

Model, using the 1979 and 1981 surveys, of the subsurface