DESCRIPTION:
Diamond Peak Shield Volcano, Oregon
Diamond Peak Vicinity

DiamondPeak05_aerial_diamond_peak_crescent_lake_12-10-05.jpg
Aerial view, Diamond Peak and Crescent Lake, Oregon, as seen from the west.
USGS Photograph taken on December 10, 2005, by Mike Doukas.
[medium size] ... [large size]

[Map,25K,InlineGIF]
Map, Central Oregon High Cascades
-- Modified from: Scott and Gardner, 1990

Diamond Peak (8,744 feet), the dominant landform in the Willamette Pass area, is a basaltic andesite shield approximately 15 cubic kilometers in volume. Like other shields in the area, it has a central pyroclastic cone (locally palagonitized but mostly fresh basaltic andesite cinders and glassy scoria) that is surrounded and surmounted by lava flows. Volcaniclastic rocks such as lahars and pyroclastic flows are unknown. Diamond Peak began erupting from a vent near its northern summit. A second vent later opened near the southern summit, piggy-backing its lava and tephra over the previously erupted volcanic rocks. This vent migration likely involved only a small interval of time. Diamond Peak is probably less than 100,000 years old, but is certainly older than the last glaciation, which ended approximately 11,000 years ago.

At Diamond Peak the eruptions probably became slightly more siliceous with time, though always in the range of olivine-bearing basaltic andesite. Early erupted flows (53-55 percent SiO2) filled the valley of Pioneer Gulch on the southwest side of the volcano. Most lava in the shield contains 55-58 percent SiO2. Other nearby volcanoes that are probably similar in age to Diamond Peak include Crater Butte (basaltic andesite) and Redtop Mountain (basalt), situated 4 kilometers southeast and 10 kilometers to the east, respectively. Emigrant Butte (basaltic andesite), located approximately 9 kilometers south of Diamond Peak, is more deeply gutted by glaciation.

Substantially older than Diamond Peak is Mount Yoran, whose summit is a deeply eroded neck poking out from Diamond Peak's north slope. A sample of Mount Yoran's basaltic andesite lava, which is normally polarized, has K-Ar whole-rock ages of 0.52, 0.50, and 0.33; a weighted average of these ages is approximately 0.40 million years. Lakeview Mountain, another basaltic andesite shield, probably erupted sometime in the interval 0.25-0.73 million years ago.

South of Summit Lake, lava from Cowhorn and Sawtooth Mountain shields interfinger along the range crest. These normally polarized central vents and surrounding cinder cones are chiefly basaltic andesite, but include lesser basalt and minor andesite. Both shields have been so deeply gutted by glaciation that their central conduit-filling plugs form the actual summits. In each case, the surrounding pyroclastic rocks are preserved only in the gulleys that separate the plug from the flanking aprons of lava flows. Sawtooth Mountain lava has a K-Ar whole rock age of 0.56 and 0.52 million years.

Oregon Highway 58 crosses the Cascade Range crest at Willamette Pass; a narrow graded road crosses near Emigrant Butte. The Pacific Crest Trail passes from Willamette Pass to Diamond Peak and south to Cowhorn Mountain. Maps of the Deschutes and Willamette National Forests are excellent guides to trails and roads.

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