Faculty Advisor

Valentine, Mike

Area of Study

Science and Mathematics

Publication Date

Summer 2014


The Summit Creek Basalts are a sequence of steeply dipping subaerial late Eocene basaltic flows located southeast of Washington’s Mount Rainier. Despite previous petrologic and paleomagnetic investigations, the origins of these basalts are poorly understood. It is uncertain whether they erupted in situ or were transported to their present location by tectonic processes. It is possible that these lavas were derived from the same magma source as a sequence of flows in the Crescent Formation on the Olympic Peninsula, as both erupted between 45 and 50 million years ago and have similar chemical compositions. A new paleomagnetic analysis of the orientation of the magnetic field recorded in the Summit Creek Basalts was conducted to provide evidence for any rotation or north-south motion that has occurred since the flows erupted. Over a dozen flows were sampled along Highway 12 and Carleton Ridge in May and June of 2014. The basalt cores were analyzed with a Molspin spinner magnetometer using standard alternating field demagnetization and thermal demagnetization. These methods determined the thermal remnant magnetization of the flows and clues to the types of magnetic minerals in the basalt. Structural features were also mapped to provide further insight into the stresses that shaped the unit. It was found that the flows had been rotated an average of 45 degrees counterclockwise, tilted, and deformed by tectonic stresses, which indicate that they have been moved from their original location.


University of Puget Sound