DESCRIPTION:
Mount Hood Volcano, Oregon

Hood85_mount_hood_from_timberline_ca1985.jpg
Mount Hood, Oregon, as seen from timberline.
USGS Photo taken ca.1985 by Lyn Topinka.
[medium size] ... [large size] ... [TIF Image, 24 M]

Mount Hood

Location: Oregon

Latitude: 45.374 N

Longitude: 121.694 W

Height: 3,426 Meters (11,239 Feet)

Type: Stratovolcano

Number of eruptions in past 200 years: 2 (?) ²

Latest Eruptions: 1865; 1859(?), 200-300 years ago; 1,500-1,800 years ago ³.

Nature or products of eruptions: Incandescent (?) gaseous plumes; small amount of pumice; mudflows; pyroclastic flows; dome extrusions ³.

Present thermal activity: Extensive fumaroles emitting steam and other gases; also hot ground on upper southwestern side.

Remarks: Occasional seismic swarms ²

Mount Hood, Oregon's highest peak, forms a prominent backdrop to the state's largest city, Portland. The eroded summit area consists of several andesitic or dacitic lava domes; Pleistocene collapses produced avalanches and lahars that traveled across the Columbia River to the north. The glacially eroded volcano has had at least four major eruptive periods during the past 15,000 years. The last three occurred within the past 1,800 years from vents high on the SW flank and produced deposits that were distributed primarily to the south and west along the Sandy and Zigzag rivers. The last eruptive period took place around 170-220 years ago, when dacitic lava domes, pyroclastic flows and mudflows were produced without major explosive eruptions. Minor 19th-century eruptions were witnessed from Portland.

Snow-clad Mount Hood dominates the Cascade skyline from the Portland metropolitan area to the wheat fields of Wasco and Sherman Counties. The mountain contributes valuable water, scenic, and recreational resources that help sustain the agricultural and tourist segments of the economies of surrounding cities and counties. Mount Hood is also one of the major volcanoes of the Cascade Range, having erupted repeatedly for hundreds of thousands of years, most recently during two episodes in the past 1,500 years. The last episode ended shortly before the arrival of Lewis and Clark in 1805. When Mount Hood erupts again, it will severely affect areas on its flanks and far downstream in the major river valleys that head on the volcano. Volcanic ash may fall on areas up to several hundred kilometers downwind.

Eruptive activity at Mount Hood during the past 30,000 years has been dominated by growth and collapse of lava domes. The last two episodes of eruptive activity occurred 1,500 and 200 years ago. Repeated collapse of lava domes extruded near the site of Crater Rock, Mount Hood's youngest lava dome, generated pyroclastic flows and lahars and built much of the broad smooth fan on the south and southwest flank of the volcano.

For the general public, Mount Hood is perhaps the most accessible and preeminent of Oregon's volcanoes, located only 75 kilometers east-southeast of Portland, Oregon. It is the highest peak in the state (3,426 meters [11,239 feet]) and one of the most often climbed peaks in the Pacific Northwest. In summer, Mount Hood's timberline wilderness is a pastoral garden for backpackers. In winter and spring the volcano's slopes host several downhill ski runs and cross-country tracks. U.S. Highway 26 crosses the south flank of Mount Hood, and Oregon Highway 35 meets it along the east side. Numerous paved or graded roads provide further access. A hiking trail encircles the volcano, much of which is protected within the Mount Hood Wilderness, part of the Mount Hood National Forest.

Mount Hood, 3,426 meters (11,239 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).

No major eruptive events have occurred at Mount Hood since systematic records began in the 1820's. Reports of steam and tephra emissions accompanied by red glows or "flames" from the area of post-glacial vent are known from 1859, 1865 (twice) and 1903 A.D. No tephra deposits have been correlated with these events, though the summit area is capped by stratified tephra and scattered pumice blocks and breadcrust bombs.

Northwest Indians told early explorers about the fiery Mount St. Helens. In fact, an Indian name for the mountain, Louwala-Clough, means "smoking mountain". According to one legend, the mountain was once a beautiful maiden, "Loowit". When two sons of the Great Spirit "Sahale" fell in love with her, she could not choose between them. The two braves, Wyeast and Klickitat fought over her, burying villages and forests in the process. Sahale was furious. He smote the three lovers and erected a mighty mountain peak where each fell. Because Loowit was beautiful, her mountain (Mount St. Helens) was a beautiful, symmetrical cone of dazzling white. Wyeast (Mount Hood) lifts his head in pride, but Klickitat (Mount Adams) wept to see the beautiful maiden wrapped in snow, so he bends his head as he gazes on St. Helens.

Native American legends abound with descriptions of the brothers Wy'east (Hood) and Pahto (Adams) battling for the fair La-wa-la-clough (St. Helens). Behaviors attributed to Wy'east (as paraphrased from Harris' (1988) summary of Native American lore) include hurtling of hot rocks from gaping holes, sending forth streams of liquid fire, loss of formerly high summits, and choking of valleys with rocks. These are fair descriptions of Mount Hood's reconstructed activity over the past two millennia.

Mount Hood, 3,426 meters (11,239 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).

Hood River County was named after Hood River and Mount Hood which are both located within its boundaries. Mount Hood was named in 1792 after Lord Hood (Samuel) who, among other things, served in the British Navy during the American Revolutionary War.

Mount Hood is a Quaternary stratovolcano composed of lava flows, domes, and volcaniclastic deposits. The bulk of the volcano is andesite that ranges from 57-62 percent SiO2. Dacite (62-63 percent SiO2) is limited to the products of the last 15,000 years. Four eruptions during that time have spilled pyroclastic block-and-ash flows and lahars into the four river systems (Sandy, Salmon, Hood, and White Rivers) that drain the mountain and its 12 glaciers. Thus, the volcano poses a direct volcanic hazard to communities near the volcano and in downstream reaches.

The oldest rocks exposed in the Mount Hood area are flows of the middle Miocene Columbia River Basalt Group, a sequence of tholeiitic lava (erupted in eastern Oregon and Washington) that flooded through low points in the ancestral Cascade Range approximately 17 to 12 million years ago. During and after these eruptions, Columbia River Basalt was folded into a series of northeast-trending anticlines and synclines with structural relief of at least 600 meters. Folding diminished as middle and upper Miocene andesite lava and breccia accumulated in the synclines, probably culminating by approximately 10 million years. The Laurel Hill stock and related plutons of hypabyssal quartz diorite were emplaced approximately 8 to 9 million years ago.

Laurel Hill plutonic rocks underlie Government Camp and Multipor Ski area at approximately the 1,200 meter elevation on the south flank of Mount Hood. Pre-Hood bedrock is exposed at approximately the 1,250 to 1,500 meter elevation throughout the area; therefore Mount Hood has a constructional relief of only around 2,000 meters. The volume of Hood's cone is approximately 30 to 60 cubic kilometers, depending on the amount of relief on the pre-Hood surface. (Most of a cone's volume is in its lower parts, so the large range in the volume estimate arises from the uncertainty in guessing the amount of pre-Hood topographic relief.)

A pre-Mount Hood volcano, the Sandy Glacier volcano (basaltic andesite and andesite), is exposed on the west side of Mount Hood at the 1,650-meter elevation and is partly covered by Mount Hood rocks. The age of Sandy Glacier volcano is somewhat controversial because of two disparate K-Ar ages; approximately 3.2 million years and 1.3 million years. There is no field evidence to favor one age over the other. Basaltic andesite was erupted from vents at The Pinnacle and Cloud Cap on the north side of Mount Hood since 0.73 million years. These eruptive products are partly covered by and probably unrelated to the Mount Hood volcano.

Mount Hood itself is built of normally polarized andesite and is younger than 0.73 million years. Potassium-argon ages from three main-cone lava flows are chiefly around 0.4 to 0.6 million years, but rocks this young can be difficult to date. The Pleistocene volcanic history of Mount Hood is poorly understood because there has been no detailed geologic mapping of the main cone.

The summit area of Mount Hood comprises several andesite or dacite domes. Solfataric alteration has weakened these rocks and made them susceptible to potentially catastrophic slope failure. Pleistocene laharic deposits that are rich in altered Mount Hood andesite underlie terraces in the lower Hood River valley (35 kilometers north of the volcano) and are present on both sides of the Columbia River near the town of Hood River, approximately 40 kilometers north of the volcano. The lahar presumably began as a debris avalanche that incorporated large masses of preexisting rock on the flank of the volcano, inundated the Hood River Valley, and temporarily filled the Columbia River to a depth of 30 meters. A single radiocarbon date from wood in the lahar is greater than 38,000 years (beyond the limit of Carbon-14 dating when the sample was run). Old lahars are also exposed 35 kilometers west of Mount Hood in the Sandy River drainage. Weathering horizons as much as 7 meters thick indicate that the deposits west of Mount Hood are many tens of thousands of years old.

Volcanic hazard studies have illuminated Mount Hood's history of the last 15,000 years, in which four eruptive periods are recognized: Polallie (15,000 to 12,000 years ago), Timberline (1,800 to 1,400 years ago), Zigzag (600 to 400 years ago), and Old Maid (250 to 180 years ago).

• Deposits of Polallie age, which were formed chiefly by pyroclastic flows and debris flows, occur on all sides of Mount Hood, thus indicating a vent at or near the summit. Many Polallie deposits are restricted to ridge tops and valley sides, positions that were probably determined by the thickness of glacier ice in a give valley. Deposits along ridge tops accumulated when ice filled the adjacent valley, whereas deposits on valley sides were formed adjacent to glaciers after the glaciers had shrunk in size. Valley-floor deposits were laid down beyond the ends of the valley glaciers during or following both of the earlier stages. There are no radiocarbon ages from Polallie deposits. The 15,000 to 12,000 -year age is estimated from the degree of weathering and the inferred relation of Polallie deposits to glaciers on Mount Hood.

• The Timberline eruptive period, which occurred 1,800 to 1,400 years ago, produced between 0.7 and 1.1 cubic kilometers of pyroclastic flows and debris flows. The vents for Timberline and all subsequent eruptions were high on the southwest flank, perhaps at the vent now filled by the Crater Rock dome. Deposits of Timberline age are mostly restricted to the southwest flank of the volcano, where they form a broad volcaniclastic fan across a 45 degree sector. Some Timberline debris flows, however, extend down the Sandy River for 80 kilometers to its mouth at the Columbia River near Troutdale, Oregon.

• The Zigzag eruptive period occurred 600 to 400 years ago. Deposits of this age are of limited volume and have been found only in the valleys of the Sandy and Zigzag Rivers.

• The Old Maid eruptive period occurred 250 to 180 years ago. Rocks and unconsolidated deposits include a dome in Mount Hood's crater (Crater Rock), a pyroclastic flow, and many debris flows in the White and Sandy River valleys. Crater Rock is a dacite dome approximately 300 to 400 meters across its base and 170 meters high on its south side. Old Maid deposits represent approximately 0.15 cubic kilometers of dacitic magma. Old Maid eruptions (1760-1810 A.D.) occurred during a time when white men were exploring the Pacific Northwest. The Lewis and Clark expedition of 1804-1805 saw the Sandy River ^*** within a few years of the main Old Maid events, while the river was still clogged with eruptive debris. Their journal notations describe the Sandy as a shallow river choked with sediment and possessing a quicksand bottom. In contrast, the modern river is a deep, narrow, boulder-armored channel.

  *** NOTE: dates misleading, while Lewis and Clark began their expedition in 1804, they were in the Washington/Oregon area in 1805 and 1806. They passed the Sandy River in November 1805, and during their return trip, in April 1806 they explored the lower 6 miles. For more information about Lewis and Clark and the Cascade Range Volcanoes check out our pages: The Volcanoes of Lewis and Clark.

The eruptive history of Mount Hood prior to about 15,000 years ago is chiefly recorded by the volcanic rocks that form the volcano. These rocks were described by Wise (1969), who divided them into four main groups. From oldest to youngest these are olivine andesite lava flows that form the base of the volcano, long flows of pyroxene andesite that extend down former canyons that headed on the volcano, flows of pyroxene andesite that form the part of the volcano above about 1,800 meters (5,900 feet), and a remnant of a round dome of dacite that forms Crater Rock (Wise, 1969). The bulk of the volcano is made up of andesite in which the silicon dioxide (SiO2) content ranges from about 57 to 61 percent. Rocks at Mount Hood that were described by Wise (1969) as dacites contain 62-63 percent SiO2 and are limited to the products of the geologically most recent eruptions, including the deposits of the Polallie, Timberline, and Old Maid eruptive periods ... These dacites generally contain the ferromagnesian minerals hypersthene or hornblende, or both, and may include small amounts of augite. At some other volcanoes the presence or absence of these and other minerals has been used to distinguish volcanic deposits of different ages, but this was not found to be possible during the present (1980) study of Mount Hood.

Several volcanic vents on the lower flanks of Mount Hood or beyond its base have erupted olivine andesite lavas which are chemically similar to one another, but which do not seem to be genetically related to Mount Hood (Wise, 1969, p.994, 999-1000). One vent is at The Pinnacle on the north flank of the volcano, another is near Cloud Cap Inn on the northeast flank, and a third lies in the valley of the Middle Fork Hood River 11.5 kilometers northeast of the summit of Mount Hood. Wise believed that the lava flows from the vent at The Pinnacle were erupted before the Fraser Glaciation, but regarded those from the vent near Cloud Cap Inn as of post-Fraser age. Lava flows from The Pinnacle are locally overlain by glacial deposits of Fraser age. Soil profiles on deposits that overlie the lavas from the vent at Cloud Cap Inn indicate that these lavas, too, are pre-Fraser and probably are older than the Hayden Creek Drift.

The olivine andesite lava flow in the Middle Fork Hood River valley extends down the valley floor about 6 kilometers from a vent south-southwest of Parkdale (The length of the lava flow was mistakenly given as 6 miles by Wise (1969)). The flow is about 60 meters thick (Wise, 1969) and as much as 1.2 kilometers wide. Charcoal from a soil beneath the lava flow had a radiocarbon age of 6,890 +/- 130 years (Harris, 1973, p.66-67; this report, table1).

The age of the main edifice of Mount Hood is poorly known. All tested lava flows have normal magnetic polarity, so the cone is probably younger then 0.73 million years. Whole-rock K-Ar ages for two flows and one dike of the main cone range from 0.6 to 0.3 million years (Keith et.al., 1985); the dated flows are from stratigraphically low parts of the section in the upper Zigzag River canyon.

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 two-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.

Mount Hood has had at least four eruptive periods in and after late Fraser time, in order of decreasing age:

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 Pollallie.

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."

In 1907, a U.S. Geological Survey topographer described dense steaming around Crater Rock accompanied by nighttime glow. Mild fumarolic activity has continued throughout this century, mostly in areas around Crater Rock.

Earthquakes occur sporadically at Mount Hood, typically as short-lived swarms of small events (less than or equal to magnitude 3.5) that locate chiefly on the south flank and below the summit at depths of less than 11 kilometers (information from USGS and University of Washington Geophysics Program). One to several swarms per year have been recorded since the seismic system was upgraded in 1980. A typical swarm occurred in summer 1980; a magnitude-2.9 event was followed by seven aftershocks ranging in magnitude from 1.6 to 2.8. A swarm in February 1990 had 30 earthquakes, all smaller than Magnitude 1.3. Later that year, a magnitude 3.5 earthquake was followed by 12 aftershocks. Focal mechanism for the Magnitude 3.5 event shows dominantly dip slip with slight component of lateral slip (nodal planes N30E,43SE, minor left-lateral; and N30W,60SW, minor right-lateral). Recently, on the morning of April 7, 1996, events of magnitude 3.0 and 2.4 occurred at a depth of 7 kilometers below the summit.

No major eruptive events have occurred at Mount Hood since systematic records began in the 1820's. Reports of steam and tephra emissions accompanied by red glows or "flames" from the area of post-glacial vent are known from 1859, 1865 (twice) and 1903 A.D. No tephra deposits have been correlated with these events, though the summit area is capped by stratified tephra and scattered pumice blocks and breadcrust bombs.

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.

Jökulhlaups (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 jökulhlaup 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. Jökulhlaups 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.

Mount Hood's crater contains a dome of hypersthene-hornblende dacite called Crater Rock. The dome is about 300-400 meters across at its base and about 170 meters high on its south side. According to Wise (1969), the solid rock of the dome is surrounded by a zone of brecciated rock of the same composition. ...

Crater Rock is believed to be a remnant of a dacite dome that was formed 200-300 years ago. This age assignment is based on the ages of the pyroclastic-flow deposits and mudflows of the White and Sandy River valleys ..., which are inferred to have been formed as the dome was being extruded. No large-volume source of dacitic rock debris other than the dome exists at the heads of these two valleys. Such a relatively young age assignment is consistent with the extensive fumarolic activity that persists today at and adjacent to Crater Rock. Although Wise (1966, 1969) believed Crater Rock to be the source of the broad fan of rock debris on the south side of the volcano (Timberline deposits of this report), that debris evidently originated at another dacite dome at the same location.

... As the dome was extruded, it probably was never significantly larger and higher than it is today, because rockfalls and avalanches probably lowered it as quickly as it was formed.

Nestled in the crater of Oregon's majestic Mount Hood volcano is Crater Rock, a prominent feature known to thousands of skiers, climbers, and tourists who journey each year to the famous Timberline Lodge located high on the volcano's south flank. Crater Rock stands about 100 meters above the sloping crater floor and warm fumaroles along its base emit sulfur gases and a faint steam plume that is sometimes visible from the lodge. What most visitors do not know, however, is that Crater Rock is a volcanic lava dome only 200 years old.

Lava domes are mounds that form when thick, pasty lava is erupted slowly and piles up over a volcanic vent. Crater Rock sits atop the vent and conduit through which molten rock traveled from deep below Mount Hood to the surface. During the past 2,000 years, growth and destruction of earlier lava domes at the site of Crater Rock produced hundreds of pyroclastic flows -- avalanches of hot volcanic rock, gas, and air moving at hurricane speed -- that swept down the volcano's steep southwest flank as far as 11 kilometers. The strikingly smooth, sloping surface on which the lodge and ski area are built, as well as the nearby community of Government Camp and an important highway across the Cascades, was created by these pyroclastic flows.

In 1907, a U.S. Geological Survey topographer described dense steaming around Crater Rock accompanied by nighttime glow. Mild fumarolic activity has continued throughout this century, mostly in areas around Crater Rock.

A regional seismic network operated jointly by the U.S. Geological Survey and the Geophysics Program at the University of Washington detects and locates earthquakes around Mount Hood. As many as 50 earthquakes have been recorded in one year, but the events are seldom strong enough to be felt by people on the volcano. Most occur in swarms of several events that are located below the summit area or the south flank at depths of less than 10 kilometers (6 miles). An increase in this level of earthquake activity would be noticed quickly. At monitored volcanoes similar to Mount Hood, a notable increase in seismicity has occurred days to months before the onset of eruptions.

Mount Hood, 3,426 meters (11,239 feet) high, is the fourth highest peak in the Cascades and the highest in Oregon. ... Twelve glaciers and named snowfields cover approximately 80 percent of the cone above the 2,100-meter level and contain about 0.35 cubic kilometers of ice. Most of the glaciers have remained roughly constant in size over the last few decades, after retreating from a neo-glacial maximum early in the 18th century. 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.

[Map,12K,GIF]
Simplified Map - Glaciers of Mount Hood, Oregon
-- Modified from: Swanson, et.al., 1989, AGU Field Trip Guidebook T106

Mount Hood volcano is situated at the crest of the Cascade Range 75 kilometers east-southeast of Portland, Oregon, and 35 kilometers south of the Columbia River. ...

Much of the south slope of Mount Hood is a broad, relatively smooth fan that slopes down to the area around Government Camp and is bounded on the east by the White and Salmon River valleys. The White River flows southward and southeastward through a virtually uninhabited region for many tens of kilometers and joins the Deschutes River, a tributary of the Columbia. The Salmon River takes a long circuitous course southwesterly, then northwesterly, through uninhabited country and finally joins the Sandy River. The west side of the fan is bounded by the Zigzag River, which, with its tributaries, drains the southwest slope of Mount Hood. The Zigzag River flows westward to join the Sandy River near the community of Zigzag. The Sandy heads on the west side of the volcano and flows westerly and northwesterly to its confluence with the Columbia River at Troutdale. The north and east sides of the volcano are drained by tributaries of the Hood River, which joins the Columbia at the city of Hood River. The largest concentration of population near Mount Hood is situated along the floors of the Zigzag and Sandy river valleys. These valley floors will be endangered by future eruptions that produce floods and mudflows on the west and south slopes of Mount Hood.

Present thermal activity at Mount Hood 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, slightly above the boiling point of water at 3,100 meters. 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. ... Jökulhlaups (glacial-outburst floods) have been recorded from the Zigzag, Ladd, Coe, and White River Glaciers. ... 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. ...

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 is believed to be a remnant of a dacite dome that was formed 200-300 years ago. This age assignment is based on the ages of the pyroclastic-flow deposits and mudflows of the White and Sandy River valleys ..., which are inferred to have been formed as the dome was being extruded. No large-volume source of dacitic rock debris other than the dome exists at the heads of these two valleys. Such a relatively young age assignment is consistent with the extensive fumarolic activity that persists today at and adjacent to Crater Rock.

In 1907, a U.S. Geological Survey topographer described dense steaming around Crater Rock accompanied by nighttime glow. Mild fumarolic activity has continued throughout this century, mostly in areas around Crater Rock.

In addition to locating regional earthquakes, the Pacific Northwest Seismograph Network (PNSN), in cooperation with the Cascades Volcano Observatory, is also responsible for monitoring seismic activity at volcanoes in the Pacific Northwest. The PNSN currently operates seismometers on or near Mount Adams, Mount Rainier, Mount St. Helens, Mount Hood, Mount Baker, Three Sisters, and Crater Lake.

Between 1980 and 1984, the U.S. Geological Survey's David A. Johnston Cascades Volcano Observatory (CVO) established baseline geodetic networks at Mount Baker, Mount Rainer, and Mount St. Helens in Washington, Mount Hood and Crater Lake in Oregon, and Mount Shasta and Lassen Peak in California. To this list of potentially active volcanoes, CVO extended its monitoring program in 1985 to include Newberry and South Sister volcanoes in central Oregon. The Newberry and South Sister networks were re-measured in 1986 and will be measured periodically in future years. Improvements since 1984 in the recording of endpoint and flightline temperatures resulted in better overall data than obtained previously. The improvements included: calibration of all the sensors and precision thermistors, installation of a new recording system for flightline data, and recording of endpoint temperatures 6 meters above ground level. The data collected in 1985 and 1986 indicate little or no apparent deformation at either volcano between surveys.

In addition to its primary responsibility of monitoring active Mount St. Helens, the David A. Johnson Cascades Volcano Observatory (CVO) has been charged with obtaining baseline geodetic and geochemical information at each of the other potentially active Cascade volcanoes. Dry tilt and/or trilateration networks were established during 1975-82 at Mount Baker, Mount St. Helens, Mount Hood, Mount Shasta, Lassen Peak, Crater Lake, and Long Valley caldera; coverage was extended during September 1982 to include Mount Rainier.

The dramatic reawakening of Mount St. Helens in March 1980 focused increased attention on the possibility of future eruptions elsewhere in the Cascade Range. Mount Baker had stirred briefly only 5 year earlier, prompting the installation of dry tilt (1975) and trilateration (1981) networks there to monitor possible ground deformation associated with increased thermal activity. A trilateration network was established on Mount Hood in 1980; tilt and trilateration networks were installed at Mount Shasta, Lassen Peak, and Crater Lake during 1981 and remeasured with null results in 1982. Dry tilt stations were likewise installed at Long Valley caldera during summer 1982, in response to increased seismicity and ground deformation there since 1978. This program of geodetic surveillance was extended to Mount Rainier during September 1982, to supplement continuous seismic monitoring there by the U.S.Geological Survey and The University of Washington.

Welcome to the Mt. Hood National Forest. Located twenty miles east of the city of Portland and the northern Willamette River valley, the Mt. Hood extends south from the strikingly beautiful Columbia River Gorge across more than sixty miles of forested mountains, lakes and streams to Olallie Scenic Area, a high lake basin under the slopes of Mt. Jefferson. Our many visitors enjoy fishing, camping, boating and hiking in the summer, hunting in the fall, and skiing and other snow sports in the winter. Berry-picking and mushroom collection are popular, and for many area residents, a trip in December to cut the family's Christmas tree is a long standing tradition. The Cascade Range Forest reserve was established in 1893, and divided into several National Forests in 1908, when the northern portion was merged with the Bull Run Reserve (city watershed) and named Oregon National Forest. The name was changed again to Mt. Hood National Forest in 1924. Some popular destinations that offer rewarding visits are Timberline Lodge, built in 1937 high on Mt. Hood, Lost Lake, Trillium Lake, Timothy Lake, Rock Creek Reservoir and portions of the Old Oregon Trail, including Barlow Road. There are 189,200 acres of designated wilderness in Wilderness Areas on the Forest. The largest is the Mt. Hood Wilderness, which includes the mountain's peak and upper slopes. Others are Badger Creek, Salmon-Huckleberry, Hatfield, and Bull-of-the-Woods. Olallie Scenic Area is a lightly-roaded lake basin that provides a primitive recreational experience.

Crater Rock

• Nestled in the crater of Oregon's majestic Mount Hood volcano is Crater Rock, a prominent feature known to thousands of skiers, climbers, and tourists who journey each year to the famous Timberline Lodge located high on the volcano's south flank. Crater Rock stands about 100 meters above the sloping crater floor and warm fumaroles along its base emit sulfur gases and a faint steam plume that is sometimes visible from the lodge. What most visitors do not know, however, is that Crater Rock is a volcanic lava dome only 200 years old. [See Crater Rock above] -- Excerpt from: Brantley and Scott, 1993, The Danger of Collapsing Lava Domes: Lessons for Mount Hood, Oregon: IN: Earthquakes & Volcanoes, v.24, n.6

• For MORE Crater Rock Information see Crater Rock section above

Devil's Kitchen

• 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. -- Swanson, et.al., 1989

Timberline Lodge

• The hotel is a National Historic Landmark, built as a Works Progress Administration (WPA) project under the Roosevelt Administration, during the Great Depression and opened in 1937. -- Excerpt from: Timberline Lodge FAQ, Timberline Lodge Website, 2002

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