Lord Howe Island, crescent-shaped, 11 km long and 0.5 km to 2.8 km wide (1 455 hectares), is located 702 km north-east of Sydney in the South Pacific Ocean. It is known for its spectacular beauty, rare flora and fauna, lagoon and coral reef, and twin volcanic peaks. It was declared a World Heritage site in 1982. It is part of the largely submerged Lord Howe Rise, a volcanic undersea ridge 160 km - 300 km wide and 18 km - 29 km thick, separating the Tasman and New Caledonian Basins. Lord Howe Island is the eroded remnant of a large shield volcano on the western edge of the Rise. Extensive erosion has removed most of the original structure, leaving the Island itself, together with the spectacular 500 m spire of Balls Pyramid, 23 km to the south. Perhaps only 2.5% of the original island remains above sea level.
There were two main periods of volcanic activity. The first episode, 6.9 million years ago, erupted tholeiitic flows (and associated dykes) of the North Ridge Basalt (underlying most of the northern half of the Island), which built up most of the volcanic shield, and the Boat Harbour Breccia which formed in the throat of the volcano. The original eruptive centre was near Mount Lidgbird to the south, and there were over 30 individual lava flows 1 m - 10 m thick. The original volcano was probably several tens of kilometres in diameter. Ball's Pyramid is the erosional remnant of a smaller shield volcano, which may have been around 6 km in diameter at sea level.
The second volcanic episode was around 6.4 million years ago, when there was a large-scale collapse of the summit in the Mount Lidgbird area, resulting in a giant caldera (perhaps 900m deep and 5 km by 2 km) which was quickly filled by lavas of the Mount Lidgbird Basalt. This is a 560 m succession of alkali olivine basalts and hawaiites which form the twin peaks of Mount Gower (875 m) and Mount Lidgebird (777 m) which now dominate the southern half of the Island with their vertical cliffs. Prominent columnar jointing can be seen in the basalt.
There are Pleistocene and Holocene sediments in the flat-lying areas, mainly cross-bedded calcarenite, with soils, lagoon deposits and alluvium. The calcarenites (Neds Beach Calcarenite) have yielded bones (and occasionally eggs) of an extinct giant horned turtle, Meiolania platyceps. The Island has the most southern true coral reef in the world.
Copyright © Australian Museum, 2004