Mount Jefferson (Oregon) - Geology and Geography

Geology and Geography

The average elevation of the terrain around Jefferson is 5,500 to 6,500 feet (1,700 to 2,000 m), meaning that Jefferson's cone still towers nearly 1 mile (1.6 km) above it. 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 reversal of the Earth's magnetic field 700,000 years ago, the 23.4-cubic-mile (97.5 km3) Minto Lavas created a large volcanic plateau formed from coalescing shield volcanoes. They were heavily eroded by glaciers before Jefferson started to grow.

Mount 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 (21 km3) of basaltic andesite lava flows that are called the Main Cone Lavas. These lavas form a mass of rock 5 to 40 feet (1.5 to 12 m) 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 that these volcanic eruptions probably occurred in a warm interglacial period. Glaciers did form directly on the Main Cone Lavas and cause erosion later.

The volcano then went dormant for an unknown period of time, after which somewhat less than 1 cubic mile (4.2 km3) 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 2 miles (3.2 km) 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 (3,700 m). Smaller versions of the ice age glaciers continue to erode Jefferson, with the largest of these being the Whitewater Glacier east of the saddle-like part of the summit ridge. Also east of the summit lies the Waldo Glacier, while to the west and northwest, the Milk Creek, Jefferson Park and Russell Glaciers persist. 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 pyroclastic flows. The most recent eruption was in 950 AD, with cinder cone formation.

Jefferson's peak is the highest point in both Linn County and Jefferson County, and a point on its north ridge at 9,000 feet (2,700 m) is the highest point of Marion County.