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Vancouver Island is made up of three distinctly different fragments of the Earth's crust, called terranes. The largest of these, Wrangellia, consists of igneous and sedimentary rocks that formed far from their present position. Through the motions of the Earth's tectonic plates, Wrangellia collided with the ancient edge of North America about 100 million years ago. Following that collision, coal-bearing sedimentary rocks of the Nanaimo Group accumulated along the east coast of the Island and beneath what much later became the Strait of Georgia. A second collision occurred about 54 million years ago when sedimentary and volcanic rocks of the Pacific Rim Terrane were rammed beneath the southern and western edges of Wrangellia along the San Juan and Survey Mountain faults. A third collision occurred about 42 million years ago when a volcanic island, perhaps similar to modern Iceland, and belonging to the Crescent Terrane, was emplaced beside and beneath the Pacific Rim Terrane along the Leech River Fault. Associated sea-floor volcanic rocks form the Olympic Mountains across the Strait of Juan de Fuca. Following these two later collisions, sedimentary rocks of the Carmanah Group accumulated upon the Crescent and Pacific Rim terranes.

One of the most prominent faults on Vancouver Island is the Leech River Fault. Its surface trace, or fault-line, is coincident with a narrow, steep-sided valley extending from Sombrio Point on the Strait of Juan de Fuca to Esquimalt Lagoon. The surface, or plane, of the fault is inclined to the north and separates the Pacific Rim Terrane from the Crescent Terrane (b). Another less prominent but important fault is the San Juan Fault extending from near Port Renfrew to beyond Cobble Hill. For much of its length it separates the Pacific Rim Terrane from Wrangellia.

Mineral resources (a) have been important to the economic development of southern Vancouver Island. Its volcanic rocks have yielded copper from mines on Mount Sicker near Duncan and gold from several veins there and elsewhere. Gold has also been recovered from stream placer deposits at Leechtown. Sedimentary rocks in the Nanaimo area contain coal seams that were mined from 1852 until 1967, during which time 72 million tonnes were extracted. Non-metallic minerals including sand and gravel (aggregate) for concrete and road construction, limestone for cement, and clay for bricks have been and remain of major importance to the local economy. In many instances abandoned mines and quarries have been reclaimed for other purposes, the most well known being the conversion of an old limestone quarry into the world-famous Butchart Gardens (below).

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