REPORT:
Geologic History of Mount Hood Volcano, Oregon --
A Field-Trip Guidebook

STOP 1 - Panorama Point overlook of Cascade Range, Mount Hood, and Hood River Valley

On a clear day, the view of Mount Hood is excellent. Below and east of the summit lies Cooper Spur, a remnant of the broad fan of pyroclastic-flow and lahar deposits of the Polallie eruptive period that originated from near-summit lava domes. Ridges radiating outward from the lower flanks are underlain chiefly by andesite and some basaltic andesite lava flows. The steep rugged north face contrasts markedly with the smooth fan-aproned south face that will be visible from later stops. The contrast results from Holocene eruptive activity, primarily lava-dome growth and collapse, that has been restricted to the south face.

The Hood River Valley is an incompletely understood structural depression extending north into Washington and southward toward Mount Hood. The valley's east margin is a series of anastomosing normal-slip faults that displace the Columbia River Basalt Group by about 550 meters in the area of Panorama Point. Panorama Point itself is a promontory of the Wanapum Basalt Formation, but the hills to the east in the Hood River escarpment are underlain by the Grande Ronde Basalt, a stratigraphically lower formation (also in CRBG) displaced upward by the faults.

The valley extends north a few kilometers into Washington, although an early Pleistocene volcano, Underwood Mountain, fills much of it there. A lava from Underwood Mountain has a K-Ar age of 0.85+/-0.02 Ma (K-Ar, whole rock; Hammond and Korosec, 1983).

The west side of the valley slopes gently upward toward the Cascade Range summit. Pliocene and Pleistocene lava flows blanket the CRBG on most of the visible upland surfaces. The valley floor is mantled by middle Pleistocene alluvial deposits of the Hood River, including at least one lahar deposit derived from Mount Hood (Stop 2). The lahar probably originated from a large debris avalanche on the upper flanks of either the present or an ancestral cone. Missoula flood deposits form a late Pleistocene capping of sand and silt as thick as 30 meters in some parts of Hood River valley.

Mount Hood lies 40 kilometers south-southwest of Panorama Point. Much closer (14 kilometers) and nearly in line with it is Middle Mountain, which is underlain by lava of the CRBG. The field trip route passes through the gap east of Middle Mountain, whereas the Hood River wraps around its west side. Cinder cones of lastest Pliocene or early Pleistocene age dot the surface between here and there: Van Horn Butte, Lenz Butte, and several cones at Booth Hill. Associated lava flows are exposed locally. A lava from Booth Hill (Mile 71.3) has a K-Ar age of 2.07+/-).60 Ma (whole-rock; Conrey and others, 1996a). Behind Middle Mountain but beyond this view is the Upper Hood River Valley. The geomorphic and structural setting of the two valleys is similar, but structural relief is more than 600 meters in the Upper Hood River Valley.

Middle Mountain is enigmatic in its structural setting. Faults in the area are mapped as steep to vertical normal faults. But Middle Mountain may have been part of a pre-Pliocene northeast-trending fold system. These folds, many of which have reverse faults on one or both limbs, are better preserved to the east on the Columbia Plain. Pliocene or Quaternary extensional faults have been superimposed on an older faulted and folded middle Miocene terrane (Swanson and other, 1981; Korosec, 1987).