Mount Hood, which has been active for at least 500,000 years, occupies a long-lived focus of volcanic activity that has produced ancestral Hood-like volcanoes for the past 1.5 million years. Much of the Mount Hood edifice is formed of lava flows, but eruptive activity during the past 30,000 years has been dominated by growth and collapse of near-summit lava domes to produce broad fans of pyroclastic flow deposits. Similar deposits were probably formed in Mount Hood's past but were largely eroded, especially during ice ages, and are poorly represented in the geologic record. The last two periods of eruptive activity occurred about 1,500 years ago and in the late 18th century. In addition to Mount Hood, other volcanoes scattered through the nearby area have erupted during the past 500,000 years. In contrast to the long-lived activity at Hood, each of these regional volcanoes was active for a relatively short period of time. The youngest such volcano is the 7-km-long Parkdale lava flow whose vent lies about 12 km north-northeast of the summit of Mount Hood.
500 ka to 100 ka – Construction of much of Mount Hood edifice, partial destruction, and eruption of regional volcanoes
The broad flanks of Mount Hood cover an area almost 20 km (12.4 mi) in diameter and are formed of numerous sequences of lava flows that were emplaced between about 500,000 to 100,000 years ago. Each sequence comprises multiple flows tens of meters to locally more than 100 meters thick. Some sequences cover broad sectors; others are relatively narrow and were probably buttressed by glaciers significantly larger than those we see today. Well-known landmarks around the volcano such as Gnarl Ridge, Lamberson Butte, Yocum Ridge, Illumination Rock, and the high rock cuts along U.S. 26 east of Government Camp are formed of lava flows from this time period. While the Mount Hood edifice was growing, regional volcanoes of chiefly basalt and basaltic andesite lava flows formed. Mount Defiance (about 500,000 years old) on the south side of the Columbia River gorge and Lost Lake Butte northwest of Mount Hood are two examples of shield volcanoes. In addition one such shield volcano, the 425,000- year-old Cloud Cap volcano, erupted on the north flank of Hood from a vent only 5 kilometers or less from Hood’s summit. A cinder cone with flanking lava flows formed The Pinnacle on Hood’s north flank about 130,000 years ago.
At roughly 100,000 years ago, a large debris avalanche removed the summit and north flank of Mount Hood and generated a large lahar that flowed down the Hood River valley to the Columbia. It temporarily filled the Columbia valley and flowed several kilometers up the White Salmon River valley on the north side of the Columbia.
100 to 30 ka - Rebuilding the edifice
Lava flows younger than 100,000 years old are found on most flanks of Mount Hood. They fill the scar left by the Hood River lahar of about 100,000 years ago, which must have extended from today’s Eliot Branch westward to beyond Coe Branch. Lava flows about 40,000 to 50,000 years old that form Langille Crags and Stranahan Ridge and extend almost to Laurence Lake are the younger part of this sequence of scar-filling lava flows. Although overridden by extensive glaciers of the last ice age, many of the lava flows younger than 100,000 years display well-preserved lava-flow features such as levees and flow lobes. Much of Mt. Hood Meadows Ski Resort lies on a sequence of lava flows that are about 55,000 years old. Paradise Park, a popular destination on the Timberline Trail, is underlain by lava flows about 50,000 years old. North flank Barrett Spur and Cathedral Ridge form narrow ridges of lava flows that were likely buttressed by flanking extensive glaciers. By about 30,000 years ago, Mount Hood was probably hundreds of meters lower than and perhaps lacked the jutting summit of today.
30 to 12 ka – Polallie Eruptive Period
Eruptions of the Polallie eruptive period accompanied and immediately postdated the last major Pleistocene glaciation in the region. Extrusion of summit lava domes built much of the summit region we see today. Steel Cliff and the high cliff that forms the Eliot Glacier headwall are two of the youngest. Older domes, many of which have been altered by hot fluids to white, yellow, orange, and red rock masses, form the rest of the summit region. Dome extrusion produced numerous pyroclastic flows leading to development of lahars and deposition of tephra. Sequences of lahars of this age are found downstream along the Sandy, Hood, and White River valleys. Fans of pyroclastic-flow and lahar deposits formed all around the volcano. The remnants of such fans include (1) the bouldery ridge that rises from above Mount Hood Meadows to Steel Cliff and (2) the broad surface extending from Cooper Spur, below and to the east of the summit, which is deeply incised by Polallie Creek. Upper Zigzag Canyon exposes a spectacular section of deposits of the Polallie eruptive period that are capped by Mississippi Head, a short lava flow that was extruded late in Polallie time.
∼1.5 ka - Timberline Eruptive Period
Following an apparently dormant period between about 12,000 and 1,500 years ago, the Timberline eruptive period produced lava domes with accompanying pyroclastic-flow and lahar deposits during two major pulses, each lasting a number of decades. Early in the Timberline eruptive period, a large mass of older summit rocks avalanched down the southwest flank and formed a lahar that traveled the Zigzag and lower Sandy River valleys to the Columbia. The scar of this landslide forms the breached summit crater in which lava domes grew during the Timberline and later Old Maid eruptive periods. The farthest traveled pyroclastic flow reached about 12 km from the lava dome. Much of the broad fan of pyroclastic-flow and lahar deposits that underlies Timberline Lodge and Government Camp was constructed during this eruptive period. A section of U.S. 26 and several of the Villages at Mount Hood (Zigzag, Welches, Wemme, and Wildwood) are built on lahar deposits of Timberline age. In addition deposits of Timberline age built much of the delta of the Sandy River, which juts northward and displaces the Columbia River.
∼1781 C.E. - Old Maid Eruptive Period
The Old Maid eruptive period lasted for a decade or more following a dormant period of more than 1,000 years. It is the youngest major eruptive period at Mount Hood. Pyroclastic flows and lahars were generated by repeated collapse of a lava domeextruded near the site of present-day Crater Rock, which is the surviving remnant of that dome. Deposits of the pyroclastic flows and lahars are distributed between the Sandy River and White River drainages.
When Lewis and Clark visited the mouth of the Sandy River in November 1805 and April 1806, they noted its similarity to the braided Platte River of the High Plains, and named it the Quicksand River. Their description is unlike the present gravel-bed river and suggests that the river was responding to an excessive sediment load imposed by volcaniclastic deposits emplaced during Old Maid eruptions.
Early settlers reported eruptive activity in 1859 and 1865. Witnesses referred to fire, smoke, flying rocks, and voluminous steaming, which may describe modest explosive eruptions from the cooling conduit and dome(Crater Rock) that was active decades earlier during the Old Maid eruptive period. No deposits have been found that can be tied unequivocally to either of these 19th century events.