History
Mt. Jefferson was named in honor of US President Thomas Jefferson by the Lewis and Clark Expedition (this was the only High Cascade mountain they named). The expedition, who were sponsored by President Jefferson, first saw the peak from the mouth of the Williamette River.
One of the most complete studies of the volcano was complied in a large report written in the 1920s by former professor of geology at the University of Oregon, Edwin T. Hodge. Since then there has been little work done on a comprehensive study of the mountain itself (probably due to its remoteness). The most notable exception was a 1974 study of the volcano's glacial and volcanic history carried out by Kenneth G. Sutton.
Geology and geography
The average elevation of the terrain around Jefferson is 5500 to 6500 feet (1,676 to 1,981 meters), meaning that Jefferson's cone still towers nearly a mile above. Previous studies estimated that the cone is made of roughly equal amounts of tephra and lava, but Sutton's 1974 study found half as much tephra as expected. The remainder of the material thought to be tephra was in fact broken lava rock.
Sometime before the last magnetic reversal of the Earth's magnetic field 700,000 years ago, the Minto Lavas (23.4 cubic miles total volume) created a large volcanic plateau formed from coalescing shield volcanoes that were heavily eroded by glaciers before Jefferson started to grow.
Mt. Jefferson started life as a highly explosive vent, which in turn built a tephra-rich cone (this same cone now forms the volcano's core). Much of this structure was subsequently buried under about 5 cubic miles of basaltic andesite lava flows that are called the Main Cone Lavas. These lavas form a mass of rock 5 to 40 feet (1.52 to 12.19 meters) thick near the top of the old cone and become progressively thicker further down the mountain.
The lack of lahar (volcanic mudflow) or avalanche deposits associated with the original cone and the Main Cone Lavas indicates these these volcanic eruptions probably occurred in a warm interglacial period. Glaciers did directly form on the Main Cone Lavas and did cause erosion later, however.
The volcano then went dormant for an unknown period of time, after which somewhat less than 1 cubic mile of andesitic lava flows erupted, forming a second stage cone on top of the original (these flows were thicker and had a higher silica content than the Main Cone Lavas). At the same time, two large cone complexes formed on Jefferson's flanks. The North Complex formed near where Jefferson Park now lies and another complex erupted two miles south of the mountain's current summit (forming the reddish-brown dacite now seen at Goat's Peak). In addition, more silica-rich andesite lava flowed from fissures of the volcano's south flank.
A long period of dormancy returned to Jefferson. During that time a succession of large glaciers removed most of the second stage lava along with at least a third of the original cone. Erosion was more severe on Jefferson's western flank and as a result solidified lava flows on the volcano's eastern slope now compose the summit. Jefferson's height before this intense period of glacial erosion is estimated at 12,000 feet (3,657.6 meters). Smaller versions of the ice age glaciers continue to erode Jefferson. Two of the largest, Whitewater Glacier east of the saddle-like part of the summit ridge and Milk Creek Glacier on the west side, threaten to collapse the low ridge that separates them.
Sometime between 40,000 and 140,000 years ago during a glacial maximum, Jefferson changed its behavior from quiet eruptions of lava to violent explosions of pumice-rich tephra and pryoclastic flows. No evidence for later eruptions from the volcano itself has been found, leading researchers to speculate that Jefferson may be extinct. However, it should be noted that Jefferson has awoke from very long periods of inactivity at least twice before.
There have been, however, eruptions in the Jefferson area since Jefferson last erupted.
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