Sea stacks form as a result of the constant crashing of oceanic waves against a cliff. Erosion creates sea caves out of the cliff, and arches develop over time. As the rock is worn down further, the arches become sea stacks.
Sea caves form when the strong energy of the waves creates cracks in the rock surface. Over time, the cracks become larger, eventually forming blowholes. The waves continue to erode the blowholes, and they become sea caves. If the cliff juts out into the ocean, the waves can break down both sides of the rock, turning sea caves into arches. If one half of the arch is softer than the other, the softer side breaks down faster, leaving one side to become a sea stack.
A sea stack located off the coast of Oregon called Jump-Off Joe was made of sandstone, eroding over the course of the 20th century. Other sea stacks in Bandon Beach, Ore. are composed of sandstone or chert, rocks that are relatively hard when measured on the Mohs scale. Waves erode rocks through corrosion and hydraulic action, but the waves can only quickly break down rock that is soft. The inner part of the headland or cliff that is harder takes a longer time to break down into sea caves or arches. The process continues until the waves push the coastal landscape further inland.