The island consists of the eroded remnant of a tuff ring, erupted about half a million years ago. It is considerably younger than, and compositionally distinct from, the nearby onshore Alexandra Volcanics (Mount Karioi and Mount Pirongia) and Okete Volcanics. It is located on the eastern edge of the North Taranaki Graben, rising above sea level from a base about deep. In heavy swells the island can be washed over, so that only about has vegetation and that limited to Prasiola (algae), Tortula (moss) and Xanthoria, and Xanthoparmelia lichens.