From Wood and Kienle (1990): "Capital Mountain is a relatively small andesitic shield volcano with a roughly circular summit caldera 4 km in diameter. The shield consists chiefly of lava flows and subordinate volcaniclastic rocks that dip 3 to 25 degrees away from the summit area. The caldera, apparently of non-explosive origin, is filled with thick, flat-lying flows. Talus, flow breccias, and pillow lavas occur locally between the caldera wall and intracaldera flows. A prominent andesite plug, 100 m high, marks the general center of an area of post-caldera-fill activity and is the locus of a spectacular radial dike swarm. Shield and intracaldera lavas are chiefly hypersthene andesite, but shield lavas range in composition from basalt to dacite. Dikes are also chiefly andesite; one prominent rhyolite dike originating from a small rhyolite laccolith extends almost completely across the volcano."
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