The Sargents Ridge cone of Mount Shasta

The oldest visible cone of Mount Shasta

Photograph by R. Christiansen on 26 July 1976

Sargents Ridge extends southward from the heights of Mount Shasta, on the right in the adjacent view. As we climbed this ridge and examined its surroundings, we found dozens of lava flows and fragmental layers. These materials erupted from a now extinct vent to form a large stratocone volcano sometime between about 100,000 and 200,000 years ago. Note in this view how layers on the right side of the ridge slope down to the right while layers on the left side slope to the left, beneath the present summit of Mount Shasta. These dipping layers show us that the old eruptive vent was located just east of Sargents Ridge. This view Sargents Ridge (skyline) is toward the northwest across upper Mud Creek. The prominent rock feature below the ridge is Thumb Rock (see next image); Konwakiton Glacier is in upper right.

Rapid growth marks history of Sargents Ridge cone

Photograph by R. Christiansen on 4 August 1975

The Sargents Ridge stratocone grew as numerous eruptions followed one another from the summit vent. We find scant evidence that erosion scarred the surface of the cone as the layers were added one by one. Geologically speaking, the cone was built in a short period of time–perhaps only a few thousand years. Our observations show that the layers were added by different types of activity including: lava flows erupted from the old summit vent; small intrusions of magma injected into the growing cone; fragmental debris produced by the breakup of viscous flowing lava volcanic ash and broken rock fragments ejected in explosive eruptions; and debris flows generated through melting of some of the volcano's snow cover by hot lavas and ejecta. This close view of Thumb Rock is toward the northeast along Sargents Ridge. Note layers of volcanic rocks exposed in Thumb Rock.

Slow erosion carves away Sargents Ridge cone

Photograph by R. Christiansen on 3 July 1975

Rapid growth of the stratocone was followed by a much longer period of erosion by glaciers and streams. Two or three ice ages have since covered the cone to its base with glaciers, grinding away much of the original edifice. In this aerial view, looking northwards, we see the strata that make up the Sargents Ridge cone exposed to view by deep erosional excavation in the headwaters of Mud Creek. Even now, long after the Sargents Ridge cone has ceased to be active, the Konwakiton Glacier, near the center in this view, continues to carve away more of the old stratocone.

Occasional Eruptions on the Eroded Volcano

Photograph by R. Christiansen on 3 July 1975

As erosion scarred deeply into the Sargents Ridge cone, infrequent eruptions continued to vent at the summit and on the flanks. In the foreground of this aerial view, we see the old crater rim of the Sargents Ridge cone. The prominent feature rising just beyond it is a lava dome, a solidified mass of highly viscous lava that piled up in an erosional breach of the old crater rim; a spine of the solidified viscous lava rose from its surface and a lava flow spread southward down the slope away from it. Similar volcanic domes, as well as cinder cones and lava flows, also erupted on the north, west, and south flanks of the old cone, out to its base. Aerial view of the Konwakiton lava dome along upper Sargents Ridge. View is toward the southeast.

Volcanic debris apron grows beyond base of Mount Shasta

Photograph by R. Christiansen on 21 July 1976

During both rapid growth and slow degradation of the Sargents Ridge cone, its flanks gradually accumulated volcanic debris. Our examination of this debris shows it to be of two kinds: deposits from explosive eruptions that spread high-speed flows of hot ash, gas, and other volcanic material down the slopes, some reaching many kilometers from the volcano; and deposits from snowmelt caused by eruptions of hot debris or by periods of climatic warming, local avalanching during earthquakes, or other noneruptive phenomena. Catastrophic floods and debris flows carried this material down streams that drain the volcanic slopes and onto the surrounding broad debris fans. Thus, we found that the areas at the volcano's base and extending across the lowlands far beyond it can be buried by debris, both during and after a time of frequent eruptions and cone building. This cliff along Ash Creek shows deposits from both the Sargents Ridge cone and the younger Misery Hill cone. The orange color represents the top of a soil horizon above the Sargents Ridge debris apron.

Back to Ancestral Mount Shasta			On to Misery Hill cone
Back to Cones of Mount Shasta volcano

References