Basaltic island evolution close to the limits of reef growth: Norfolk Island, Southwest Pacific

Oceanic islands at mid to high latitudes are usually encircled by sea cliffs and other rock coast landforms. In contrast, islands in low latitudes are usually fringed by coral reefs that dissipate wave energy. Norfolk Island is a volcanic island (~3 Ma) near the centre of an extensive marine shelf, close to the southern limit to reef growth in the Pacific Ocean. This study describes the cliffs that encircle Norfolk Island and the geomorphology of offshore sea stacks, hanging valleys, boulder beaches, shore platforms and offshore platforms, using a combination of remote-sensing and field survey. Much of the steep basaltic coast is deeply weathered and landslides are common. Hanging valleys imply that marine erosion rates generally outpace long-term fluvial and other subaerial erosion rates. Talus slopes produced by cliff collapse are rare, suggesting that cliff-erosion debris is being removed faster than it is generated. The only section of the coastline where this is not the case is the southern coast, where aeolianite dunes and small patch reefs have developed within sandy embayments that are protected by a calcarenite rim. Contemporary coastal landforms indicate highly effective marine erosion typical of a reef-less eroding remnant volcano. However, the shelf surrounding Norfolk Island appears too wide to have been eroded entirely within the 2–3 million years since it was formed, implying that there may be other volcanic, tectonic or geophysical processes contributing to the long-term evolution of Norfolk Island. • Norfolk Island comprises a rocky, predominantly erosional, coastline. • Marine erosion outpaces subaerial erosion in all but the southern coastline segment. • Norfolk Island is more similar to reef-less volcanoes than reef-encircled volcanoes. • The anomalously-wide marine shelf implies more complex evolution than adjacent hotspot chains.