The Birth of a Cascades Volcano: Defining Magma Storage, Evolution, and Crustal Influence for Early Silicic Volcanism in the Goat Rocks Volcanic Cluster, Cascade Range, Washington

Author

• Hailey Finch, Central Washington UniversityFollow

Document Type

Thesis

Date of Degree Completion

Spring 2026

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Dr. Hannah Shamloo

Second Committee Member

Dr. Kellie Wall

Third Committee Member

Dr. Chris Mattinson

Fourth Committee Member

Dr. Jeff Tepper

Abstract

Goat Rocks volcanic cluster (~3.1–0.1 Ma) is an extinct Cascade arc center in central Washington state where significant erosion by glaciation and fluvial processes has exposed its earliest deposits and local basement rocks. This volcanic cluster provides a unique opportunity to study the earliest activity of a Cascade arc system and how magmas evolved prior to eruption. Lava and tephra deposits of Miriam Creek (MC; ~3.5 Ma) and Devils Horns (DH; ~3.2 Ma) define the earliest silicic Pliocene volcanism in the Goat Rocks area. Underlying basement rocks include metasedimentary and intrusive units from the Rimrock Lake inlier (RRL; ~154 Ma), Eocene basalts (EOC; ~55–42 Ma), and volcaniclastic units from the Ohanapecosh Formation (OHP; ~32–26 Ma). Bulk-rock geochemistry indicates a strong subduction signature (enrichment in LILEs and depletion in HFSEs) for MC, DH, and basement rocks, as well as expected trends for differentiation. Bulk rock and zircon geochemistry establish that MC and DH  have different source magmas, residence time estimates indicate DH was likely already stored in the crust while MC erupted, and both units underwent shallow fractional crystallization. Zircon U-Pb ages from MC samples contain inherited zircon kernel density estimate age peaks at approximately 10-12 Ma, 25 Ma, and 1400 Ma, and a cluster of roughly continuous age peaks from approximately 60-240 Ma, while DH samples contain inherited age peaks at approximately 80-100 Ma and 1650 Ma. In contrast, age peaks from sampled basement are well-constrained and do not contain detrital populations. However, regional detrital zircon studies provide correlative age populations in the Western Mélange Belt (a unit often correlated with RRL). These detailed geochemical, mineral, and geochronologic studies indicate that MC and DH were sourced from subduction zone magmas with discrete crustal reservoirs, resided for different lengths of time, evolved during storage, and incorporated crustal material with detrital zircon populations – likely an unsampled unit of RRL. This study provides insight into how the earliest magmas in a Cascades center evolved, and could illustrate how currently active Cascades centers such as neighboring Mount Rainier or Mount St. Helens may have begun their lifetime.

Recommended Citation

Finch, Hailey, "The Birth of a Cascades Volcano: Defining Magma Storage, Evolution, and Crustal Influence for Early Silicic Volcanism in the Goat Rocks Volcanic Cluster, Cascade Range, Washington" (2026). All Master's Theses. 2330.
https://digitalcommons.cwu.edu/etd/2330