Balls Pyramid and the efficacy of marine abrasion part of Vignettes:Vignette Collection
Balls Pyramid is a striking example of the penultimate stage in the marine planation of a volcanic island. It is a remarkable monolith of basalt that rises spectacularly to a height of 551 m (Fig. 1), as wave erosion has truncated thew top of a volcano, 25 km south of Lord Howe Island in the Tasman Sea (Fig. 2). There are numerous volcanoes that now form 'high islands' in the tropical Pacific Ocean. Almost all of these are surrounded by coral reefs which thrive in high wave energy settings in the tropics. Reefs attenuate the waves, which break on the reef crest, protecting the slopes of the volcanic islands from erosion. Balls Pyramid lies just south of the southernmost reef in the Pacific Ocean, which is on the western margin of Lord Howe Island. As a consequence wave energy can reach the flanks of Balls Pyramid which rise as steep 'plunging cliffs', like those around the eastern and southern margins of Lord Howe Island, where the island is unprotected by reefs. Balls Pyramid is composed of nearly horizontally-bedded lava flows, the remnants of a volcanic plug formed in a former vent of a volcano. Potassium-argon dating indicates it is similar, or slightly younger in age, than Mount Gower and Mount Lidgbird on Lord Howe Island which have been potassium-argon dated to 6.4 Ma. After their eruption as a shield volcano, the volcanic hillslopes have been truncated to form a broad submarine shelf. Balls Pyramid sits in the centre of this shelf, which forms a platform that is about 20 km from north to south, and averages 10 km wide (Fig. 3). It has an average water depth of around 50 m. A similar but slightly larger shelf surrounds Lord Howe Island, and the two platforms are separated by a canyon that exceeds 500 m in depth (Fig. 4). Ultimately wave erosion will completely truncate the volcanoes. There has been some controversy over the development of plunging cliffs, termed plunging because they continue below the water line into deep water. The efficacy of marine abrasion was recognised by Charles Darwin who visited St Helena in the south Atlantic Ocean and described the vertical cliffs around the margin and the planated shelf. He disputed the view, which was then widely held, that these plunging cliffs were abraded at wave base, the depth to which orbital velocities associated with wave motion are felt (equivalent to half of the wave length, the distance between successive wave crests). He realised that waves that break exert a greater force, and that it is necessary for large waves to reach the cliffs unimpaired to have maximum erosional capacity. We now understand that the level of the sea has varied over an amplitude of 100-120 m over the Quaternary ice ages, and it appears likely that much of the erosion of the shelf around Balls Pyramid occurred relatively soon after eruption of the volcano, and at times when the sea was lower than at present. The efficacy of planation would have decreased as broad platforms developed in front of the vertical cliffs and talus accumulated at their toe. Both Balls Pyramid and Lord Howe Island lie on the margin of the Lord Howe Rise, a remnant of continental crust in the Tasman Sea, and on the Australian plate which is moving northwards at around 6 cm per year. These truncated volcanoes will eventually move into more tropical waters. At present the shelves are covered by calcareous sediments derived largely from temperate organisms, but they are likely to become suitable for coral reef development, as is already possible on the western margin of Lord Howe Island. This limit to reef growth has been sensitive to past climate changes and is likely to respond to future changes in climate. Recent observations on the shelf around Lord Howe Island have revealed a fossil reef in mid shelf around almost the entire island, indicating past variation in the extent of reefs there. Balls Pyramid represents the penultimate stage in the planation of a volcanic island, and its position is at a critical point on the northward-migrating plate which is about to carry it into reef-forming seas. It is likely to be completely truncated by wave erosion in the future, unless a coral reef forms around it and protects its shoreline.
The Cocos (Keeling) Islands, and what coral reefs can tell us about changes of sea level part of Vignettes:Vignette Collection
The Cocos (Keeling) Islands in the eastern Indian Ocean are the only atoll that Charles Darwin visited during the voyage of the Beagle.Darwin had developed a theory that gradual subsidence of volcanic islands in the tropical Pacific Ocean would lead to the progressive formation of fringing reef, barrier reef, and ultimately a coral atoll. His short visit to Cocos in 1836 provided him with the opportunity to look for support for his theory, and became central to his theory of coral reef development. His fellow sailors convinced him that undercutting of coconut trees and erosion of the shoreline (Fig. 1) were 'tolerably conclusive evidence' to substantiate his view. Darwin's theory of coral reef development was enthusiastically accepted on his return to England, and his first major scientific paper and his 1842 book focused on coral reefs (Darwin, 1842). The Cocos (Keeling) Islands had been settled over a decade previously by John Clunies-Ross, a Scottish entrepreneur. Clunies-Ross was absent from the atoll at the time of Darwin's visit, but in contrast to the glowing reviews following the publication of Darwin's book, Clunies-Ross wrote a scathing and vitriolic critique of the book. Mistakenly attributed to the Antarctic explorer James C. Ross, the review appeared in a Dutch journal, 13 years after publication of Darwin's book. Clunies-Ross was evidently concerned by the insinuation that the islands of which he had recently taken possession might be about to disappear below the sea. He wrote that 'a moderably attentive investigation of the Cocos islets affords ample reasons for believing that they have stood up to the present time above the level of the ocean during hundreds if not thousands of years' (Ross, 1855, p.9). The reef islands that occur around the rim of the atoll are low-lying and sandy, generally 3-4 metres above modern sea level. The islands sit on a conglomerate platform, composed of cemented coral boulders and other calcareous skeletal material. Radiocarbon dating of corals from within the conglomerate indicates that the platform on which the islands sit is composed of corals with ages of between 4000 and 3000 radiocarbon years BP (Woodroffe et al., 1990; 1994), and that the islands have accumulated above the conglomerate during the past 3000 years (Woodroffe et al., 1999). Our detailed surveys have also revealed fossil corals that are in growth position (Figs. 2 and 3) but which are above the highest elevation to which modern corals can grow (Fig. 4). In particular, flat-topped corals, called microatolls, record this upper growth level (Smithers and Woodroffe, 2001). These corals are unable to grow upwards any further because they are exposed at the lowest tides, so the corals grow laterally, living only around their perimeter (and looking like a minature coral atoll). Radiocarbon ages on fossil microatolls indicate that the sea was 50-80 cm higher than present about 3000 years ago, and that the level of the sea has since fallen relative to Cocos (Fig. 5). This is in contrast to what might be expected based on Darwin's subsidence theory, which would imply that the atoll was gradually sinking and that the level of the sea should have risen with respect to the atoll over recent millennia. Darwin's theory has been broadly substantiated by geological observations, including deep drilling on Pacific atolls whose volcanic basement sits at hundreds to thousands of metres below sea level. His theory applies over millions of years and explains the structure of corals reefs. However, there have also been substantial variations of sea level associated with Quaternary ice ages of about 100 m amplitude that have been superimposed on the longer-term pattern of subsidence. The fossil corals that are above present growth limits reflect these millennial-scale sea-level variations, and appear to result from complex isostatic response of the earth to the redistribution of water from melt of polar ice to the ocean volume. There are many parallels between Darwin's interpretation of short-term erosion as an indicator of longer-term processes and the present debate about the possible impact of sea-level rise and the probability that islands on the rims of atolls will disappear in response to global warming and sea-level rise. There appears to be frequent pre-supposition that any sign of erosion foreshadows the ultimate demise of reef islands. Observations of shoreline erosion are frequently interpreted as evidence for the vulnerability of these islands and their imminent disappearance below the sea, despite continued production of sediment on adjacent reefs and evidence for accumulation of sediment elsewhere around islands. Many processes are operating, but at different time scales. THe erosion shown in Figure 1 is a local phenomenon that contrasts with deposition on shorelines elsewhere on the atoll. The modern microatolls in Figure 4 have been growing for about 100 years and have experienced only small changes in sea level over that time. The islands have accumulated over several thousand years, as Clunies-Ross inferred, whereas gradual subsidence proposed by Darwin occurs at an imperceptible rate, but explains the overal structure and geological evolution of coral atolls over millions of years.