Geologically the canyon is carved into the tholeiitic and post-shield alkalic lavas of the canyon basalt. The lavas of the canyon provide evidence for massive faulting and collapse in the early history of the island. The west side of the canyon is all thin, west-dipping lavas of the Napali Member, while the east side is very thick, flat-lying lavas of the Olokele and Makaweli Members. The two sides are separated by an enormous fault along which a large part of the island moved downwards in a big collapse.

The canyon has a unique geologic history—it was formed not only by the steady process of erosion, but also by a catastrophic collapse of the volcano that created Kaua'i.

Like the other Hawaiian islands, Kaua'i is the top of an enormous volcano rising from the ocean floor. With lava flows dated to about 5 million years ago, Kauaii is the oldest of the large Hawaiian islands. Roughly 4 million years ago, while Kaua?i was still erupting almost continuously, a portion of the island collapsed. This collapse formed a depression, which then filled with lava flows.

In the time since, rainwater from the slopes of Mount Wai'ale'ale have eroded Waimea Canyon along one edge of the collapse. The cliffs on the west side of the canyon are composed of thin lava flows that ran down the surface of the Kaua'i volcano. On the other side of the canyon, the cliff walls are built from thick lava flows that pooled in the depression. Over time, the exposed basalt has weathered from its original black to bright red.

Waimea Canyon State Park encompasses 1,866 acres (7.5 km²) and is a popular tourist attraction on the island. It provides a wilderness area with numerous hiking trails. It can be accessed from Waimea on Hawai?i state road 550, which is 18 miles long and leads up to Koke'e State Park. The island of Niihau, only a short distance west of Kauai at that point, can be clearly seen from the highway.[5]