73-11 Insights into the Early Construction of the Yana Volcanic Center, CA Using Physical Volcanology of Eruptive Units

Session: Using Volcanic Deposits to Help Us Understand Volcanic and Magmatic Processes (Posters)

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Abstract:

Lava flows are a common eruptive feature at stratovolcanoes and pose a significant hazard to active volcanic regions. Yet, lava flows are also invaluable in reconstructing ancient volcanic systems, especially compound volcanoes with multiple central vents. The Yana Volcanic Center (YVC) is a deeply incised, Pliocene-age stratovolcano located 30 km south of Lassen Peak. Glacial erosion has exhumed eruptive units (breccia and lava flows) that were emplaced early in its construction, as well as numerous magmatic intrusions, including dikes, sills, and stocks. Complex orientations of eruptive units indicate that several angular unconformities may be present, suggesting that the YVC is composed of multiple overlapping volcanoes. Our study focuses on a 0.25 km² basaltic andesite lava outcrop located along one of these angular unconformities and stratigraphically low within the YVC. A better understanding of this flow could provide information about the early history of this volcanic center.

The lava exposure lies along a steep slope with 73 m of vertical relief and features a prominent basal and upper colonnade and a middle entablature tier of columnar joints. The orientations of the colonnade joints vary from sub-vertical to sub-horizontal and were carefully mapped to understand how joint orientations varied spatially. We found that the columns are oriented nearly horizontal along the northern and southern contacts of the lava flow, while the joints in the central portions are nearly sub-vertical. Because columnar joints propagate orthogonal to cooling surfaces, this pattern suggests that the lava flow was emplaced in a southwest-northeast-trending paleovalley with relatively steep-sided walls.

We used the dip of the entablature tier to infer that the lava flowed in a southwest direction. This is similar to the dip of an overlying dacite flow and both lavas were likely emplaced along the slope of a volcano with a central vent located northeast of the outcrop, near the center of the YVC. In contrast, the dips of a thick collection of eruptive units located only 300 m away indicate that their central vent was located to the northwest, away from the center of the YVC. Our findings suggest that early in the YVC’s history, migrating volcanism resulted in multiple long-lived central vents identifiable by angular unconformities.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10191

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