Formation of North America's Far Western Region by Accretion

The Pacific Coast region was built by accretion—collision and welding of microplates and terranes onto the North American Plate over millions of years.

Key Geologic Phases

1. Farallon Plate Subduction: Created volcanic arcs, island chains, spreading centers.
2. Oceanic Arc Collisions: Formed accreted terranes.
3. Terrane Accretion: Varied terranes (oceanic crust, volcanics, sediments, metamorphics) built the continental margin.
4. Continued Subduction & Magmatism: Formed Sierra Nevada, Cascades, Coast Mountains.
5. Transform Faults (e.g., San Andreas): Accommodated plate motion, shaped topography.
6. Uplift & Erosion: Created Klamath Mountains, Sierra Nevada, Coast Ranges; glaciers and rivers carved rugged terrain.

This complex history makes the region seismically active and geologically diverse.