Left: Three-dimensional map of the electrical resistivity of the Earth's crust and uppermost mantle in the southern Washington Cascades volcanic range that has resulted from this study. Red colors indicate the geologic materials are electrically conductive, and blue colors indicate they are electrically resistive. The major volcanoes are marked with black cone symbols: MSH - Mount Saint Helens, MA - Mount Adams, GR - Goat Rocks, MT - Mount Rainier. The thin, bright colored band that lies immediately to the west of Mount Saint Helens is interpreted to result from relatively permeable metamorphosed sediments, mainly of marine origin, that are wedged between relatively impermeable and electrically resistive crustal blocks. The feature marked SLB is the Spirit Lake Batholith, while underneath it the feature marked LCC is a Lower Crustal Conductor that is inferred to be a source of the partially molten material that trickles up from the mantle below and that is the source of the magmas that feed the Cascade volcanoes.
(Right) A cartoon sketch illustrating how the partially molten material from below may be diverted around the Spirit Lake Batholith as it rises toward the surface. Two zones of elevated seismic activity marked as MSZ and WRSZ are also seen to align along the electrically conductive feature seen in the resistivity image at the left.