Plutons in the PNW and SWBC

April 24 – October 13, 2025

This year, I visited several iconic outcrops of plutons in the region. In the Pacific Northwest and Southwest British Columbia, these igneous bodies are largely part of the Coast Plutonic Complex and related intrusive bodies that formed during Mesozoic subduction and terrane accretion along the western margin of North America. As island arcs and oceanic fragments were accreted to the continent, repeated pulses of magma were emplaced at mid to lower crustal depths, producing extensive granitic to dioritic plutons. Subsequent uplift, faulting, and intense glacial erosion have exposed these intrusive bodies at the surface, where their massive structure, jointing, and extreme resistance to erosion strongly influence modern topography.

Simplified large-scaled map of the North American Cordillera (adapted from Coney, 1980; Whitney et al., 2013). The yellow squares represent the field sites, the southern part of the Coast Plutonic Complex (Stawamus Chief, BC) and the Index Batholith (Index, WA)

The most recent site I visited this year in October was the Mount Index in Washington, accessed via the Lake Serene trail, which provides an exceptional view of the exposed granite of the Index Batholith. Index granite is well known for the building of the Smith Tower in downtown Seattle and the state capitol. From the lake basin, 1000m high rock walls rise vertically, showing massive, coarse grained intrusive textures with prominent joint sets that control cliff geometry and rockfall patterns. The over steepened headwalls above Lake Serene mark a glacially carved cirque, where older ice exploited existing joints and fractures within the pluton. These exposures highlight how intrusive fabrics, fracture spacing, and glacial erosion together shape this distinct formation.

The Index Massif, with the Main, Middle and North Peak. The Middle peak is regarded as the hardest summit to reach in Washington

Evidence of physical weathering in the rock, about 400m below Lake Serene near Bridal Veil Falls

A closeup of the textures in the upper third of the exposed rock, although the composition seems to be slightly different with more darker, mafic material – gabbro?

Me standing on “Lunch Rock” at Lake Serene. This bedrock is very smooth and well polished from glacial activity

Further back this year, I took a quick day hike up the backside of the famous Stawamus Chiefs in Squamish. Composed of three peaks and the smaller Slhanay on the northeast side, this is a massive granitic monolith belonging to the Coast Plutonic Complex. After the pluton was exhumated, glacial erosion dominated the last few million years which produced the steep walls of the chief, and also contributed in carving out the Howe Sound as a fjord. Polished and striated surfaces are common features on the rock here, while the deep gullies separating the summits formed along vertical joint sets that facilitated block failure and mass wasting after exposure.

From the second peak, looking at the Mount Garibaldi massif – Atwell Peak. Town of Squamish below

Deep gully separating the summits

Mafic dyke that cuts through the lighter granitic rock, this dyke also continues across the highway on the Malamute, a smaller granite monolith west of the Chief

Aerial view of the Stawamus Chief Peaks monolith, with the Squamish Estuary below and Sky Pilot Massif in the background. This photo was taken from a separate trip on the west side of the Squamish River in September

References:

Schematic map of the three belts of metamorphic core complexes in the North American Cordillera (adapted from Coney, 1980; Whitney et al., 2013). Access: https://www.researchgate.net/figure/Schematic-map-of-the-three-belts-of-metamorphic-core-complex-in-the-North-American_fig1_342082048.