File:Mt. Hood earthquake swarm (January 2021) (Cascade Range, Oregon, USA).jpg

English: This satellite image shows a swarm of 108 small earthquakes that occurred at Mt. Hood in Oregon from 17 January to 21 January 2021. The United States Geological Survey has characterized the quakes as tectonic in origin (= fault movements) rather than moving magma under the volcano. The largest event was magnitude 2.7. Hypocenters were between 2.7 and 6.9 kilometers deep.

Mt. Hood is a subduction zone stratovolcano in northwestern Oregon and is part of a north-south trending chain of volcanoes called the Cascade Range. Notable Cascade volcanoes include Mt. St. Helens, Mt. Rainier, Mt. Shasta, and Mt. Mazama (now Crater Lake Caldera).

Mt. Hood area volcanism started in the Middle Miocene (8 to 10 million years ago), just after Columbia River Flood Basalt volcanism. Miocene and Pliocene andesites and basalts cap topographic ridges in the Mt. Hood area. During the Late Pliocene (sensu traditio), at 3 to 1.3 million years ago, the Sandy Glacier Volcano occupied the site of the current Mt. Hood. It has been mostly buried by the modern Mt. Hood volcanic cone. Mt. Hood itself is less than 730,000 years old - all of its rocks have modern magnetic signatures. Lavas and other eruptive materials are high-silica andesites and low-silica dacites. 70% of the Mt. Hood cone consists of lava flows, while the remaining 30% is volcaniclcastic deposits. The modern cone is less than 300,000 years old. Near the summit of Mt. Hood are lava flows younger than 200,000 years old.

Unlike nearby Mt. St. Helens, Mt. Hood is much older and has had little explosive activity. Most of Mt. Hood's volcanism has consisted of andesite lava flows and dome-building. Few explosive events have occurred here - few tephra deposits have a Mt. Hood source.

Mt. Hood rocks are often porphyritic two-pyroxene andesites, plus a little olivine. There's been little chemical variation in Mt. Hood lavas through time. Because of this, individual lava flows are difficult to date based on lithology - they're all the same. Much of Mt. Hood itself is hydrothermally-altered rocks.

Names are assigned to the various eruptive phases in Mt. Hood's history. The Polallie eruptive phase occurred from 12 to 25 thousand years ago. The Timberline eruptive phase occurred ~1500 years ago. The Old Maid eruptive phase occurred over 200 years ago, often dated to 1780-1801 A.D. Dome building occurred from 1781 to 1793. During that time, relatively small eruptions occurred periodically.

Because Mt. Hood is a snow-clad volcano, activity results in melting of snow and mobilization of loose materials - this generates lahars (volcanic mudflows). Rainstorms could also mobilize loose debris in the area. Mt. Hood lahars have probably formed by both mechanisms.