Three types of structural geologic evidence have been used to determine the geometry of conduits that supplied magma to rhyolite flows and domes. For the Medicine Lake Highland Volcano, vent alignments, ground-crack geometry, and the distribution of flow banding, surface fracture patterns, and remelted zones on dome surfaces help us to conclude that dikes rather than pipe-like conduits fed these 1,100-yr-old extrusions. Solidified remains of the tops of some of these dikes may be as shallow as 30 m below the surface. Conduit geometry is an important factor in understanding and forecasting silicic eruptions and in siting geothermal wells.

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