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

Historical Record

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.

The most recent eruptive period at Mount Hood, the Old Maid eruptive period, occurred at about the time that U. S. and European parties were exploring the Pacific Northwest coast in the late 18th century. The first of them to describe the mountain was British naval Lieutenant W. E. Broughton, the leader of a party sent up the Columbia River in October 1792 from Captain George Vancouver's expedition to bolster British claims to the northwest. He named the mountain for A. A. Hood, a famous British naval officer. Broughton reached as far upstream as the mouth of the Sandy River and noted a shallow bar extending across the Columbia, but his log reveals nothing about possible eruptive activity. Lewis and Clark visited the mouth of the Sandy River in November 1805 and April 1806, 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. They also describe a large bar between two distributary channels that forces the Columbia into a narrow channel against the north side of the valley.

Early settlers reported eruptive activity in 1859 and 1865 (summarized in Harris, 1988). Witnesses refer to fire, smoke, flying rocks, and voluminous steaming, which may well describe modest explosive eruptions from the cooling conduit and dome (Crater Rock) active decades earlier during the Old Maid eruptive period. Crandell (1980) thought that a scattering of pumice on the south and east flanks may have been produced by the 1859 or 1865 event. We have found no other deposits that can be tied unequivocally to either of these 19th century events.

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. 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. (Earthquake information from the USGS and University of Washington Geophysics Program)