Atoll

Hoa are one of the most conspicuous geomorphic features of atolls in the (central) South Pacific. The term hoa is a Polynesian word, now commonly adopted in the literature on atolls, and is used to describe the shallow channels separating individual motu. Hoa5 are considered to start from the lagoon side of an atoll and extend across the reef towards the oceanside. They are not to be confused with reef passes (gaps in the reef) known as ava in Polynesia. The characteristics of hoa vary considerably, even on the same atoll, and in consequence several attempts have been made at their classification into groups. The early work of Chevalier (1972) on the French Polynesian atolls is a benchmark in which the following six types of hoa were identified:

• Open on the lagoon side only - the most common type,

• Functional hoa that are open to the ocean and allow the exchange of sea-water between the lagoon and ocean, either continually or perhaps only at higher stages of the tide or during storms,

• Open at the ocean side only, with the lagoon exit blocked with sediments or conglomerate,

• Open at the ocean side only, and closed by conglomerate at the lagoon exit,

• Blocked at both ends and enclosing a hypersaline channel,

• Dry hoa, either blocked and vegetated, or distinctly emerged above modern sea level. Emerged hoa are probably Holocene channels, abandoned by sea-level fall, and were termed palaeohoa by Stoddart and Fosberg (1994).

Over the last few decades, increasing attention has been directed towards the role of tropical cyclones as causative agents in hoa formation on Pacific atolls. Observations suggest that cyclones have several effects. Wave erosion may widen and deepen already active hoa. Unusual currents created by storm surge through the channels may enhance this process. New hoa may be eroded, either completely or perhaps only part way across a motu. Sometimes non-functional hoa (dry types or those open at one end only) may be cut through by large waves and thus reactivated into functional channels, for example at Rangiroa, Tikehau and Matavia atolls in the northwestern Tuamotu archipelago (Bourrouilh-Le Jan and Talandier 1985). Deltas or outwash fans may form at the mouths of hoa and extend into the atoll lagoon.

5 The word hoa is used in both singular and plural forms.

Conversely, sediment deposition and the building of storm embankments may block ocean-side hoa exits. On Taiaro atoll (4.5°S, 172.2°W), Salvat et al. (1977) noted that non-functional hoa were associated with storm ridges 2 m high, and for Reao atoll (18.5°S, 13.4°W) in the Tuamotus, Pirazzoli et al. (1987) suggest that hoa on northern side of the atoll were blocked by a cyclone in 1903.

On Orona atoll (4.5°S, 172.2°W) in the southern Phoenix Islands of Kiribati, Stoddart and Fosberg (1994) became convinced that palaeohoa were cut by tropical cyclones during the Holocene at a time of high sea-level stand, by the sea overtopping the motu and eroding channels. The presence of large, basally eroded storm blocks sitting on the dry floor of the palaeo-hoa provided neat evidence for this idea on Orona. In addition, a wider view reveals that hoa are common on atolls within the cyclone belts, but rare on atolls beyond them. From this observation, Stoddart and Fosberg (1994) proposed that the existence of palaeohoa on the margins of modern cyclone-affected areas might be a useful clue for interpreting the spatial extension of Holocene palaeocyclone activity.

0 0

Post a comment