NATURAL HAZARDS AND DISASTERS

LANDSLIDES IN THE LEEWARD ISLANDS






LEEWARD ISLANDS

The Leeward Islands are found north of the Windward Islands extending to just east of the Virgin Islands between latitudes 15° 45' N to 18° 35'N longitudes 61° 45'W 63° 20'W (Rand McNally, 1988). St. Christopher (St. Kitts)–Nevis and Antigua-Barbuda are former British colonies which are now independent countries. Anguilla maintains an associated state status with Britain. While Montserrat remains a British colony, St. Eustace, Saba, and St. Maarten (southeastern part of St. Martin) are colonies of the Netherlands. The remainder of St. Martin and St. Barthelemny are colonies of France. Guadeloupe is an overseas department of France.

These islands lie within the trade winds belt resulting in a subtropical climate. Islands with sufficient relief receive an adequate rainfall, but those with a more subdued topography tend to be dry to semi-arid. The main hurricane track passes through these islands.

Geology of the Leeward Islands

The Leeward Islands exhibit two geologically distinct belts (Fink and Fairbridge, 1975). The island of Guadeloupe marks the southern end of the two belts and embodies characteristics of both. Guadeloupe consists of two distinct parts. Basse Terre, the western half, is dominated by Soufriere, a 1,467-meter high active volcano. The north, the inner belt or arc of islands is also volcanic in origin. These include Monserrat, Nevis, St. Christopher, St. Eustace, and Saba. An active volcano also named Soufriere, like the volcano on Guadeloupe, is found on the island of Nevis. Andesitic Flows, Pyroclastic units, and volcanoclastics of recent to Eocene age dominate this belt. These volcanics are interbedded with Pliocene and Pleistocene limestones on some islands, notably on St. Eustace, St. Christopher, and Monserrat. The small cluster of volcanic islands between Guadeloupe and Dominica called the Iles d'Saints physically represents a connection of this volcanic belt to the Windward Islands.

The eastern half of Guadeloupe is called Grande Terre and is composed entirely of limestone overlying older andesitic and dacitic volcanics. Formiferal or oolitic limestone underlies the remaining islands in the outer belt or arc of islands including Guadeloupe's offshore islands of Marie Galante and Desirade, Antigua, Barbuda, St. Barthelemy, St Martin (St. Maartin), and Anguilla.

The inner volcanic islands tend to have higher, more rugged topography. Higher peaks include 1,467-meter Soufriere on the Basse Terre part of Guadeloupe and 1,156-meter Mt. Misery on St. Christopher. The limestone islands tend to be weathered to a topography limestone islands tend to be weathered to a topography consisting of low hills. The higher points on theses islands are represented by 70-meter high Crocus Hill on Anguilla and 402-meter high Boggy Peak on Antigua.

Landslides in the Leeward Islands

Published literature on landslide occurrence in the Leeward Islands could not be found. The physical characteristics of the islands and known landslide activity in the Windward Islands serves as a basis for inferred landslide conditions.

Small rockfall or rockslides along coastal escarpments are likely the only landslide activity present in the outer belt or limestone-dominated islands. Their bedrock and topography are not conducive to extensive landsliding. Minor failures may result from road construction on the steeper hillslopes on some islands. A brief reconnaissance on Antigua by the senior author in 1987 failed to identify any landslides outside the coastal cliff areas. Landsliding on the outer belt of the Leeward Islands seems to be infrequent, of limited size and areal extent, and of little economic significance.

Debris flows and slides are expected on the steeper slopes underlain by volcanic bedrock on the inner islands. These conditions are comparable to those associated with landslide activity on Dominica and St. Lucia. The Basse Terre part of Guadeloupe, Montserrat, St. Christopher, and Nevis should experience a moderate amount of rainfall-triggered debris flow and debris slide activity during the wet season. In 1987, the senior author observed some fresh-appearing debris flows during aerial overflight of Basse Terre and a large rockslide on the easternmost of the Iles d'Sants. Landsliding on the inner belt of the Leeward Islands is likely not unusual and may be a common occurrence. Small debris flows and slides could be locally significant on these islands. However, affected areas may be largely confined to undeveloped or little developed areas. Their effect on roads and agriculture probably represents a small, but significant impact to inhabitants.

Rand McNally, 1988. World atlas of nations. New York: Rand McNally

Fink, L.K., Jr. and R.W. Fairbridge, 1975. Leeward Islands. In R.W. Fairbridge (ed.), The Encyclopedia of World Regional Geology, Part 1: Western Hemisphere, p. 339-340. Stroudburg, Dowden, Hutchinson and Ross.

(Taken from: DeGraff, J.V., Bryce, R., Jibson, R.W., Mora, S., and Rogers, C.T. 1989. Landslides: Their extent and significance in the Caribbean. In E.E. Brabb and B.L. Harrod (eds), Landslides: Extent and Economic Significance. p. 51-80. Rotterdam: A.A. Balkema. )

" Transcribed by Nicholas DeGraff, University opf California, Santa Cruz"



BIBLIOGRAPHY
Publications on: Caribbean Landslides






first version: 19990724 benrafi,second version :19990809 maxrafi

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