Subduction and tectonic activity in the area began during the late cretaceous period, about million years ago. The area was previously an oceanic environment, consisting mainly of sedimentary and volcanic rocks. Extensive volcanic activity began to take place in the oligocene, about million years ago. However, mountain building in the area did not begin until the miocene, approximately million years ago. Plummer Mountain is located in the Cloudy Pass batholith, an intrusive formation that was formed approximately million years ago, during the early miocene. Glacier Peak, a stratovolcano that is south of Plummer Mountain, began forming in the mid-Pleistocene. Due to Glacier Peak's proximity to Plummer Mountain, volcanic ash is quite common in the area.