Mount Hood, Oregon

• Mount Hood
  • Early History
  • Construction of the Cone
  • Glaciation
  • Postglacial Eruptive History
  • Historical Activity

• Select Road Logs
  • Panorama Point
  • Polallie Creek
  • Bennett Pass
  • White River
  • Old Maid Flow
  • Timberline

Mount Hood

Mount Hood, 3,426 meters (11,245 feet) high, is the fourth highest peak in the Cascades and the highest in Oregon. It was named after a British admiral and first described in 1792 by William Broughton, member of an expedition under command of Captain George Vancouver (Broughton, 1929). The first geologic reconnaissance primarily described the existing glaciers (Hague, 1871). ...

Map of Mount Hood area, showing localities mentioned in text. -- Modified from: Swanson, et.al., 1989, AGU Field Trip Guidebook T106

The late Pliocene Sandy Glacier volcano produced basalt, basaltic andesite, and minor andesite near the site of Mount Hood (Wise, 1968, 1969). More than 900 meters of this composite cone are exposed on the west flank of Mount Hood under Sandy and Reid Glaciers and contain the most mafic rocks known on Mount Hood. The youngest exposed flows of the Sandy Glacier volcano have a K-Ar whole-rock age of about 1.3 million years (Keith, et.al., 1985). Reversely magnetized andesite flows of approximately the same age (Keith, et.al., 1985) occur in inverted topography on Vista Ridge and apparently came from a vent now buried under the north flank of Mount Hood. Aeromagnetic data indicate a reversely magnetized body, perhaps the source of the flows, beneath the northern flank (Flanagan and Williams, 1982; Williams and Keith, 1982).

Construction of the Cone

The volcano comprises approximately 70 percent andesite flows and 30 percent clastic material (mostly concentrated high on the cone) (Wise, 1969). Flows near the summit dip away from a source higher than, and north of, the present summit. Most of the flows are less than 3 meters thick, but notable exceptions occur in Steele Cliff and Illumination Ridge, where lava apparently ponded to depths of over 100 meters. Eruptions were relatively nonexplosive, and significant tephra deposits were limited to the flanks and a small area east of the mountain, where they rarely total more than 1 meter thick.

Most of the cone-building flows are medium-K silicic andesites; a few others are mafic dacite. The cone-building flows include no basaltic andesite or basalt, in contrast to the older Sandy Glacier volcano. The cone-building flows are phyric, chiefly tow-pyroxene andesite with lesser olivine andesite; hornblende is a disequilibrium phase in about half of the flows. Neither Wise (1969) nor White (1980) recognized an overall change with time in major-or trace-element compositions.

After construction of most of the cone, relatively small flows erupted from satellite vents on Vista Ridge and The Pinnacle. A flow from The Pinnacle has a K-Ar whole-rock age of 0.15 +/- 0.02 million years (Keith, et.al., 1985). Wise (1969) interpreted flows erupted near Cloud Cap Inn to be young, but one sample yielded whole-rock K-Ar ages of 0.49 to 0.65 million years, similar to or even older than the age of the main cone (Keith et.al., 1985). The satellite flows including Cloud Cap are more mafic than the andesite forming the main cone (Wise, 1969; White, 1980), comprising medium-K mafic andesites or basaltic andesites.

Between about 0.05 and 0.1 million years (based on profiles of soil development), a sector collapse on the north or northeast side of the volcano formed a debris avalanche that traveled the length of Hood River, crossed the Columbia River, and moved 5 kilometers up the White Salmon River - a distance of at least 40 kilometers (Vallance, 1986). The avalanche deposits locally are more than 40 meters thick; much of Hood River town is built on them. No source for the avalanche is evident, and no avalanche deposits crop out within 10 kilometers of the volcano, because of a blanket of glacial outwash. This leaves as a mystery the exact point of origin of what is probably the largest single event to occur on Mount Hood.

The 5-kilometer-long Parkdale flow erupted in Upper Hood River Valley about 6,000 years ago. It chemically resembles the basaltic andesite from The Pinnacles (Wise, 1969).

The latest addition to the cone was a composite hornblende dacite dome, Crater Rock, just south of the summit. Wise (1969) considered the dome as coeval with the main stage of cone building, but Crandell (1980) and Cameron and Pringle (1986) interpreted it to have formed 200-300 years ago.

The summit of Mount Hood is about 1 kilometer south of the apex of a gravity high of at least 8 mGals. Williams and Keith (1982) interpret the high to reflect a dense intrusive body that fed Mount Hood and its forerunners.

Glaciation

Modern glacier termini are at about 2,100 meters, but in the last major alpine glaciation (Fraser, about 29-10 thousand years ago) glaciers reached the 700-800 meter level. During this time, ice spread 15 kilometers from the summit area (Crandell, 1980).

Lacustrine siltstone from near-terminus periglacial lakes plaster valley walls just upstream form the mouth of Polallie Creek on the east side of the mountain. Highway 35 crosses White River near the maximum extent of Fraser ice, and the left-lateral moraine is prominent just upstream from the bridge. The full extent of the Fraser-age glaciers has not been accurately mapped.

Glacier retreat released large volumes of outwash, some of which filled the ancestral Hood River Valley near Parkdale, forming the flat surfaces of Upper Hood River Valley and Dee Flat. Outwash also formed a debris fan in the upper East Fork Hood River.

Evidence of older glaciation is seen in roadcuts on the southeast side of the volcano and in rolling morainal landscape near Brightwood west of the volcano. The deposits are not dated but may be coeval with the Hayden Creek Drift near Mount Rainier (Crandell, 1968), probably about 0.14 million years ago (Colman and Pierce, 1981).

Simplified Map - Glaciers of Mount Hood, Oregon -- Modified from: Swanson, et.al., 1989, AGU Field Trip Guidebook T106

Postglacial Eruptive History

1. Polallie (15-12 thousand years ago [Crandell, 1980]),
2. Timberline (1,400 to 1,800 years B.P. (before present) [Crandell, 1980; Cameron and Pringle, 1986]),
3. Zigzag (400 to 600 years B.P. [Cameron and Pringle, 1986]), and
4. Old Maid (170 to 220 years B.P. [Cameron and Pringle, 1987]).

Work in progress will determine the details of these episodes in order to assess hazards of a future eruption.

The Polallie eruptive period occurred during the final stage of the Fraser Glaciation (Crandell, 1980). Lahars, thin tephras, and pyroclastic flows intertongue with late Fraser-age outwash in Upper Hood River Valley. Elsewhere, Polallie deposits mantle ridge crests and valley walls but not valley floors. Probably glacial ice still occupied valley floors at the time of the eruptions. No radiometric ages have been obtained for the Polallie.

The Timberline eruptive period broke the apparent 10-thousand-year-long post-Polallie quiescence. The vent shifted from its summit location during Polallie time to the high southwest flank. Erupted material except airfall tephra was consequently confined to the Sandy, Salmon, and Zigzag River drainages, where it formed the broad, gently-sloping debris fan that dominates the southwest flank of the volcano. Pyroclastic flows dated at 1,440 +/- 155 years B.P. (Cameron and Pringle, 1986) moved at least 8 kilometers down the Zigzag River, and lahars reached the mouth of the Sandy River more than 80 kilometers from the volcano. Small debris fans formed in the canyons of the upper Salmon and Sandy Rivers. An upper age for the Timberline of 1,830 +/- 50 years B.P. was obtained from a pyroclastic-flow deposit near Zigzag (Cameron and Pringle, unpub. data).

The Zigzag eruptive period was apparently minor, feeding several lahars and related floods into the Zigzag River and one pyroclastic flow into the Sandy River. The pyroclastic flow has an age of 455 +/- 130 years old (Cameron and Pringle, 1986).

The Old Maid eruptive period apparently began with emplacement of the Crater Rock hornblende dacite dome high on the north flank of the cone. Reconnaissance works shows that the dome comprises three lobes of markedly differing internal structure (Cameron and Pringle, 1986) but does not clarify how each lobe relates to the others. Numerous lahars probably fed by avalanches from the dome and accompanying snowmelt entered the Sandy, Zigzag, Salmon, and White Rivers; a pyroclastic flow traveled from the Crater Rock area at least 9 kilometers along the White River. One lahar extends 65 kilometers along the White River, and the sandy run-out deposit from another is identifiable 80 kilometers from the mountain. At least 60 kilometers of lahar and fluvial deposits partly fill the upper White River canyon near Timberline Lodge. A terrace made of a lahar overlain by reworked eruptive debris is more than 13 meters thick on the lower Sandy River, 60 kilometers from the mountain. Dendrochronologic dating of some of these events (Cameron and Pringle, 1987) indicates that the Sandy River lahar occurred in the mid-1790s, the pyroclastic flow in the upper White River about 1800, and the lahar that traveled 80 kilometers down the White River between 1800 and 1810.

The post-glacial products are dominantly mafic dacite and silicic medium-K andesite (Wise, 1969; White, 1980; Crandell, 1980), generally more silicic than the cone-building flows. White (1980, p.5) claims that, within the post-glacial sequence, "a general trend can be seen in which rocks from the younger units are slightly richer in SiO2 and poorer in MgO, CaO, and Fe2O3."

Historical Activity

Present thermal activity is in fumarole fields near Crater Rock, at the apex of a semi-circular zone of fumaroles and hydrothermally-altered, heated ground. In summer 1987, maximum ground temperatures were near 85 degrees C and maximum fumarole temperatures were about 92 degrees C (Cameron, 1988), slightly above the boiling point of water at 3100 m. Many of the fumaroles are actively precipitating crystalline sulfur. Comparison of modern and historical photographs shows that the amount of perpetually snow-free ground surrounding the fumarole fields has been increasing since last century. Until the 1980 eruption of Mount St. Helens, the only volcanically related human fatality in the Cascades occurred in the thermal area at Mount Hood in 1934, when a climber exploring ice caves in Coalman Glacier melted by fumaroles suffocated in the oxygen-poor gas.

Jokulhlaups (glacial-outburst floods) have been recorded from the Zigzag, Ladd, Coe, and White River Glaciers. In 1922, a dark debris flow issued from a crevasse high on Zigzag Glacier and moved 650 meters over the ice before entering another crevasse; this event initiated a scare that Mount Hood was erupting (Conway, 1921). The Ladd Glacier jokulhlaup in 1961 destroyed sections of the road around the west side of the mountain and partly undermined a tower of a major powerline (Birch, 1961). The Coe Glacier outburst occurred around 1963, causing a section of trail to be abandoned and the "round-the-mountain" trail to be rerouted farther from the glacier. Jokulhlaups from White River Glacier were reported in 1926, 1931, 1946, 1949, 1959, and 1968; the Highway 35 bridge over the White River was destroyed during each episode. The more frequent outbursts from White River Glacier may be due in part to an increase in size of the fumarole field at the head of the glacier at Crater Rock (Cameron, 1988).

A rainfall-induced debris flow on Polallie Creek on Christmas 1980 killed one and destroyed an 8-kilometer section of Highway 35. The flow started as a moderate slope failure of only 3800 cubic meters but rapidly bulked up and deposited over 76,000 cubic meters of debris at the mouth of Polallie Creek (Gallino and Pierson, 1984). The debris dammed the East Fork Hood River, creating a temporary lake; the dam breached, and flooding destroyed the highway.

Felt earthquakes on Mount Hood occur every 2 years on the average. Seismic monitoring, in effect since 1977 (Weaver et.al., 1982), indicates a generalized concentration of earthquakes just south of the summit area and 2-7 kilometers below sea level. A seismic swarm in July 1980, during which nearly 60 earthquakes (mostly 5-6 km deep with a maximum bodywave magnitude of 2.8) recorded in a 5-day period (Rite and Iyer, 1981), prompted development of an emergency response plan to coordinate local authorities in the event of future eruption.

Geodetic surveillance of the volcano was initiated in 1980, and 30 EDM lines and several tilt stations were resurveyed in 1983 and 1984 (Chadwick et.al., 1985; Cascades Volcano Observatory, unpub.data). Observed changes are within the range of expected error.

• Trip Mileage 0.0, at first 4-way stop south of toll bridge in Hood River, Oregon. Follow Highway 35 south over railroad tracks.

• Panorama Point

  Trip mileage 3.7 - STOP 59: from Highway 35: ... Panorama Point. From right to left: Underwood Mountain volcano on west side of White Salmon River; Columbia Gorge, with Hood River valley in foreground capped near shopping center by deposits of Missoula floods; Pleistocene Mount Defiance volcano across terraced Hood River Valley; main part of Hood River Valley; Middle Mountain separating Upper Valley from the main valley; Mount Hood; and Hood River fault zone bounding the east side of the valley. Several cones of olivine basalt occur in Hood River Valley south of here. The orchards grow on the Parkdale soils (Harris, 1973), probably ash-cloud deposits (perhaps rain-flushed as suggested by accretionary lapilli) associated with pyroclastic flows erupted from Mount Hood during late Polallie time about 12,000 years ago (Crandell and Rubin, 1977; Crandell, 1980).

• Polallie Creek

  Trip mileage 25.2 - STOP 61: from Highway 25: ... Mouth of Polallie Creek. Observe deposits and effects of the Christmas 1980 debris flow, whose volume is more than 20 times that of the initial slumped material owing to sediment-bulking during transport (Gallino and Pierson, 1984). The debris flow, generated by intense rainfall, rushed down Polallie Creek, which cut into volcaniclastic deposits of Polallie age that contributed to the bulking. The debris flow dammed the river for 12-18 minutes; once the dam breached, a flood surged downstream, destroying 13 kilometers of road and causing $13-million of damage.

• Bennett Pass

  Trip mileage 35.2 - from Highway 35: ... Roadcuts expose three tills, the youngest of which is of Fraser age (Crandell, 1980). The ages of the older tills are not known.

• White River

  Trip mileage 37.2 - from Highway 35: ... Cross White River. Terrace upstream is of Old Maid age (Crandell, 1980), about 1760-1810 A.D. (Cameron and Pringle, 1987). Outburst floods from White River Glacier destroyed bridges here 5 times in this century, most recently in 1959 and 1968.

• Old Maid Flow

  Trip mileage 37.3 - from Highway 35: ... Turn right into parking area and then right on gravel road.

  Trip mileage 37.8: ... Turn right onto top of levee. If road is bad, continue straight and park wherever possible.

  Trip mileage 38.0 - STOP 62: ... Walk to stream cuts exposing dacitic lithic pyroclastic flow (as thick as 10 meters) of Old Maid age capped locally by at least two thin lahars (Crandell, 1980; Cameron and Pringle, 1987). The pyroclastic flow contains thoroughly charred wood and radially-jointed blocks that magnetic measurements indicate were above the Curie point when deposited (Crandell, 1980). The charred wood has a Carbon-14 age of 260 +/- 150 years (Crandell, 1980). The pyroclastic flow traveled down the easternmost drainage of the White River, probably from near Crater Rock, and entered the main drainage at a high angle, running about 75 meters up the west valley wall; Cameron and Pringle (1987) calculated a minimum velocity of 135 kilometers/hour from this relation. A Fraser-age lateral moraine stands above the left bank of the river.

• Timberline

  Trip mileage 45.0 - from Highway 26: ... Turn right toward Timberline Lodge. For the next 2.6 miles the road passes through small cuts of Fraser-age till and cone-building andesite.

  Trip mileage 50.5 - STOP 65: ... Turn left into bus parking lot just before Timberline Lodge. Restrooms in building nearest the lot. Walk back down road 100 meters to overflow parking lot and then up dirt road and cross country to Timberline (Skyline) trail contouring on hill slope. Follow trail east across small creek (headwaters of Salmon River). Stop on crest of next ridge at Ghost Forest Overlook. From here one can look down White River and see a buried forest engulfed by fluvial and cold-lahar deposits of Old Maid age that form a flat-topped terrace. A Carbon-14 age of 250 +/- 150 years B.P. dates one of the buried stumps (Crandell, 1980). The Old Maid deposits formed a fill whose surface is about 100 meters above the White River. Upstream is White River Glacier, source of outburst floods that periodically destroy the bridge on Highway 35. The main thermal area of the volcano (Devil's Kitchen) can be seen between the head of White River Glacier and the summit ridge; temperatures as high as 92 degrees C, which is above boiling at 3,171 meters, have been measured (Nehring, et.al., 1981; Cameron and Pringle, 1988). Crater Rock, remnant of a three-lobed hornblende dacite dome (Cameron and Pringle, 1986), can be seen left (west) of Devils' Kitchen. Crater Rock was probably emplaced at least in part in Old Maid time (Crandell, 1980; Cameron and Pringle, 1986), although Wise (1968, 1969) ascribed it to what today is called the Timberline eruptive period. Trim lines from the neo-glacial maximum (about 1740 A.D.) can be seen below Steel Cliff, to right of White River Glacier.

  Many trees near Timberline Lodge, including those at this locality (the ghost forest), were killed around 1795-1810, probably near 1800 (Lawrence, 1948). The trees were killed neither by beetles (no galleries exist below the remaining bark) nor by forest fire (no charring can be found on the dead trees). The ghost forest occurs only within about 500 meters of the west side of the White River canyon and within 2 kilometers of the east side (Cameron and Pringle, 1987). The trees were probably killed by a hot ash cloud from the dacitic lithic pyroclastic flow at STOP 62 (Cameron and Pringle, 1987).

  Walk to south end of lower paved parking area near bus parking lot. Several layers of Timberline-age tephra in a 35-centimeters-thick section overlie the weathered top of a Polallie-age lahar and underlie young soil and possibly an Old Maid-age tephra. The Timberline-age tephra here comprise two or three light-colored ash layers sandwiched in dark andesitic or muddy ash. Note the surrounding ghost forest, near its western limit.