Much of the ring plain surrounding Mt Taranaki consists of debris avalanches of differing ages. Debris avalanches move down the sides of volcanoes. Many extend from the mountain to beyond the present coastline. Māori and Europeans used these mounds as lookout posts, and to build fortifications on. These debris avalanches happened not only in the distant past – the massive Opua event, which swept the south-west flank, occurred comparatively recently: 7,000 years ago.