Aeromagnetic Data Reveal Dome Structure on Mount St Helens S. Polster ( C.A. Finn, and E. Anderson, U.S. Geological Survey, Denver, CO, USA Abstract Three 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 Results  All 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 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 Terrain Fig Magnetic AnomalyResidual References Bedrosian, 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 Dzurisin, 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, , 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, Finn, C.A., Williams, D., An Aeromagnetic Study of Mount St Helens, J. Geophys. Res., 92, , 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, Vallance, J.W., Schneider, D.J., Schilling, S.P., 2008, Growth of the 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, : U.S. Geological Survey Professional Paper 1750, ABC CBA ABC A A’ A Fig. 5 BB’ Rampart Finn and Williams, 1987 Fig. 7 C D D’ R1 C1 C2 R2 C’ B B’ D D’ Castle Creek basalts and andesites Pine Creek pyroclastic flows Photo by Daniel Dzurisin Bellingham Vancouver Island Mt Rainier Study Area Mt. Adams Columbia River Mt Hood Mt Jefferson Three Sisters Resistivity (ohm/m) Bedrosian et al., 2008 Lower than expected magnetization Modeling of Magnetic Data Volcanic 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 Magnetic AnomalyUniformly Magnetized TerrainResidual 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 Magnetic Anomaly Uniformly Magnetized TerrainResidual 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 C 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 Calculated Legend (C-conductive, R-resistive) R debris avalanche layers C1- ‘wet’ 1980 debris avalanche: perched water table C2- altered (?) Pine creek, Castle creek member R2- Unknown layer, boundaries not well constrained 2004 Dome Electromagnetic cross-section along C-C’ surveyed in The R2 layer is not well constrained. Magnetite- poor layers ½ km ¼0¼ Goat Rocks pyroclastic deposits Fig 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 AD, the Castle Creek period is 2,200- 1,700 BP, and the Pine Creek period is 3,000- 2,500 BP (Hausback, 2000). B B’ Flight Lines CC’ Hausback, 2000  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 Dome Dome Pine Creek Dome R1 C1 C2 R2 Approximate Pine Creek Dome Model, using the 1979 and 1981 surveys, of the subsurface