Based on my research, I'd say it becomes a Sea arch before becoming a Sea stack due to the breaking waves erode, or wear away, the rock at sea level bit by bit, forming arches. Softer rock falls away more quicklythan harder rock.
Explanation:
A sea stack is a pillar of rock that is cut off from the headland or cliff. When the waves erode the base of the sea arch, they widen it. Eventually, the roof is unable to support itself and it collapses, the former tip of the headland cut off as a sea stack.
