To summarize, the Union Peak volcano was built by long-continued outpourings of fluid lava, chiefly from a central vent, but in the later stages from fissures on the flanks. These formed a broad, low shield. Explosive eruptions on the sides of the shield then formed several cinder cones, while other explosions from the central vent built a summit cone within the lava-walled crater. Finally, a viscous mass rose in the central conduit and congealed there as a rigid plug.
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Plate 4. Fig. 1. The Union Peak volcano, looking south from the Rim Road near the Watchman. The long, gentle slopes of the volcano are of lava; the summit pinnacle represents the filling of the central conduit. Between the gentle lava slopes and the central plug, and largely concealed by talus, are the eroded remnants of a tuff cone that formerly occupied the crater. Mount McLoughlin, another High Cascade cone, shows faintly in the distance. The foreground is part of Mount Mazama. |
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