Coast Range Arc

[1] One of the major events during the Coast Range Arc was about 85 million years ago when a huge rift developed near the center of the oceanic Farallon Plate.

[2] Some geologists believe some fundamental change in convection within the Earth's mantle caused the rifting event, while others believe the huge oceanic plate became mechanically unstable as it continued to subduct beneath the Pacific Northwest.

[2] Volcanism began to decline along the length of the arc about 60 million years ago during the early Paleogene period of the Cenozoic era as the rapid northern movement of the Kula Plate became parallel with the Pacific Northwest, creating a transform fault plate boundary similar to the Queen Charlotte Fault.

Cataclysmic eruptions at the British Columbia–Yukon border created a huge nested caldera called the Bennett Lake Volcanic Complex about 50 million years ago during the early Eocene period.

These eruptions were from vents along arcuate fracture systems associated with the caldera, which discharged about 850 cubic kilometres (200 cu mi) of pyroclastic material.

[2] What remains of the Coast Range Arc to this day are granitic intrusions, which were formed when magma intruded and cooled at depth beneath the volcanoes.

[2] Massive amounts of molten granite injected over this period, burning the old oceanic sediments into a glittering medium-grade metamorphic rock called schist.