How Volcanic Mountains Form in California

California's volcanic mountains (e.g., Mount Shasta, Lassen Peak, Medicine Lake Volcano) arise from subduction of the Pacific Plate beneath the North American Plate:

1. Subduction: Dense oceanic Pacific Plate sinks under continental North American Plate.
2. Mantle Melting: Water from subducting slab lowers mantle melting point, generating magma.
3. Magma Chambers: Molten rock pools underground, building pressure.
4. Uplift: Pressure domes the surface, forming volcanic edifices.
5. Eruptions: Magma breaches crust—lava flows, ash, pyroclasts.
6. Mountain Building: Repeated eruptions stack layers into stratovolcanoes or shields.
7. Erosion: Post-activity, water/ice/wind sculpt peaks, valleys, canyons.

The Cascade Range (Shasta, Lassen) and the Modoc Plateau (Medicine Lake) exemplify this process. California's volcanism is ongoing—Lassen erupted 1914–17; Long Valley Caldera shows unrest.