The island of Mo'orea is of volcanic origin and once had a broad shield volcano shape, much like the geologically newer islands of Hawaii. Its gentle outer slopes dropped abruptly into a huge summit caldera that included a vast plug of very hard basalt.

The island is now so eroded away that the northern slopes are completely gone, and the other three sides rise to a ragged half-circle of spiry green peaks. The caldera has been eroded down to form the Opunohu Valley, Cooks Bay, and Opunohu Bay. Mount Rotui represents the hard plug that was once the core of the volcano, and stands alone, isolated in a great ring of mountains.