Doctor of Philosophy
School of Geosciences
Dickson, Mark Edward, The development of rock coast morphology on Lord Howe Island, Australia, Doctor of Philosophy thesis, School of Geosciences, University of Wollongong, 2002. http://ro.uow.edu.au/theses/1987
Lord Howe Island, situated 600 k m east of Australia, provides a unique opportunity to study the development of rock coast landforms and the long-term planation of an oceanic island. The island is a remnant of a large shield volcano that was built by late-Miocene hotspot volcanism. Since this time, the island has gradually migrated northward into warmer seas, and marine planation, operating at a decreasing rate over time, has reduced the island to a vestige of the original volcano.
Lord Howe Island currently lies at the southern limit to coral growth. A fringing coral reef has developed on only the western side of the island, such that one side of the island is very effectively sheltered from wave attack, while the other side is exposed to waves of unlimited fetch. The contrast between landforms on each of these coastlines provides a clear illustration of the importance of wave erosion in rock coast evolution: the reef-protected coastline consists of depositional beaches, vegetated hillslopes, diminutive cliffs, and basalt shore platforms that are veneered with talus, whereas the exposed coastline includes precipitous basalt cliffs up to 800 m high that are plunging or have cliff-foot talus slopes, and cliffs 200 m or less high that have actively eroding shore platforms. Variation in rock resistance also exerts a clear control on coastal morphology at Lord Howe Island. Rock resistance was assessed through Schmidt Hammer testing as well as measurements taken on discontinuities within rocks, and wave characteristics were assessed with bathymetric data and data on the local wave climate.
At Lord Howe Island, talus slopes occur beneath cliffs where a critical size of cliff face has supplied sufficient talus to offset wave abrasion and removal of talus during the Holocene highstand. Plunging cliffs or shore platforms occur where source areas are smaller. In some areas, a critical depth of water prevents waves from breaking against the cliff face and plunging cliffs occur irrespective of rock resistance. Where water depths are shallower, however, plunging cliffs only exist if the resistance of rocks exceeds the assailing force of waves. In this respect, time is a crucial factor, as rock resistance is gradually degraded by weathering processes.
Type B shore platforms have developed along sections of coast where the assailing force of waves has exceeded rock resistance. There is a close positive correlation between the elevation of shore platforms and nearshore water depth, and this indicates the important role of breaking waves in shore platform formation at Lord Howe Island. Several other factors indicate that platforms at Lord Howe Island have been strongly influenced by wave erosion, particularly the fact that platforms on the exposed coastline are wider than platforms on the lagoonal coastline. However, weathering processes become more important as platform width increases, and platforms become lower and flatter as they become wider. This thesis proposes a four-stage model of Type B, microtidal shore platform development at Lord Howe Island. The model attempts to synthesise divergent hypotheses in the literature with a time-based perspective of platform evolution.