Summary of Landslides in US and British Virgin Islands
Introduction
Between latitudes 17° 40'N and 18° 50'N and longitudes 64° 75'W and 65° 10'W are a group of small islands and cays called the Virgin Islands (Weaver, 1975). These islands are divided politically into the British Virgin Islands to the north and the U.S. Virgin Islands to the south. The British Virgin Islands encompasses 153 square kilometers. Totola, Virgin Gorda, and Anegada are the larger islands. At 527 meters, Mt. Sage on Tortola is the highest point in the British Virgin Islands (Rand McNally, 1988). There are 344 square kilometers in the U.S. Virgin Islands. St. Thomas, St. John, and St. Croix account for most of this area. The highest point in the U.S. Virgin Islands is Crown Mountain on St. Thomas at an elevation of 474 meters. In general, the islands are rugged with steep slopes (Weaver, 1975). Anegada is an exception being low and flat. A relatively low and flat area called the "Valley" on the southern part of Virgin Gorda and a central lowland on St. Croix represent the other significant areas of gentler terrain. The islands are subtropical with rainfall varying between 100 and 1300 millimeters. While dense tropical vegetation can be found in some locations, cactus and thornbush typifying drier conditions is more common.
Geology of the Virgin Islands
Weaver (1975) notes all of the Virgin Islands with the exception of St. Croix rise from the Virgin Island Bank. The relationship of this bank to the insular shelf around Puerto Rico and other structural relationships indicates the Virgin Islands are the eastern end of the Greater Antilles. Submarine lava flows, spilitic and keartophyric breccias, andesitic pyroclastics, andesitic breccia tuffs, and volcanic sandstones account for the majority of exposed bedrock in the Virgin Islands (Weaver, 1975). These rocks are folded homoclinally at tangles averaging 40 degrees to the north in the U.S. Virgin Islands and steeper to overturned in the British. IN contrast, most of the bedrock exposed on Virgin Gorda is tonalite and granodiorite from a large batholitic intrusion. Tortola and St. John have less extensive exposures of this bedrock. Limestone is found exposed in many of the islands as blocks within primarily volcanic units and as Pleistocene accumulations. ON St. Croix, the Northside Range and the East End Range consist of Cretaceous tuffaceous and bolcanoclastic sediments. These rocks are both folded and faulted. A gabbro intrusion and a diorite intrusion are found in the Northside and East End ranges, respectively. A low graben filled with Tertiary sediments separates these two mountain ranges. The bedrock geology of Anegada is markedly different from the rest of the Virgin Islands. It consists of flat-lying limestone of Pleistocene age. It is assumed this limestone platform rests on a basement of older volcanics.
Landslides in the Virgin Islands
Almost no published literature on landslides is available for the Virgin Islands. A reconnaissance of landslide potential on St. Thomas (Brabb, 1984) provides some insight on landslide activity in these islands and their social and economic impact. Earthflows, debris slides, and individual boulders are recognized landslide types on St. Thomas. Debris flows are not documented or reported as occurring on this island. The largest landslide documented on ST. Thomas is 60 meters long and 60 meters wide. It was mapped in an area about 1.5 kilometers north of Charlotte Amalie in 1979.
On April 18, 1983, a storm drenched Dorothea Bay with nearly 400 millimeters in 14 hours. It illustrates the conditions leading to landslide activity which can be found on St. Thomas. In addition to extensive flooding, this storm event produced a number of landslides. Two earthflows developed in weathered colluvium. These are small features about 30 meters long and 30 meters wide. Very small debris slides occurred in colluvium exposed at the top of some road cuts. Boulders temporarily blocked several roads. One boulder which was 6 meters in maximum diameter traveled 10 meters downslope before stopping next to and above a house (Brabb, 1984).
Assuming the landslide activity on St. Thomas is representative of landsliding in the Virgin Islands, it appears landslide activity is limited in magnitude and areal extent. From an economic standpoint, it involves and additional cost to road maintenance that is minor compared to other storm damage to roads. The threat of fatalities or injuries exists for those areas with habitations downslope from large boulder accumulations. Based on present information, it appears eve this potential would only be realized under and unusually large storm event.
Brabb, E.E. 1984. Landslide potential on St. Thomas, Virgin Islands, p.97-102. U.S. Geological Survey Open –File Report 84-762.
Rand McNally 1988. World atlas of nations. New York: Rand McNally.
Weaver, J.D. 1975. Virgin Islands. In R.W. Fairbridge (ed.), The Encyclopedia of World Regional Geology, Part 1: Western Hemisphere, p. 654 – 655. Stroudsburg: Dowden, Huchinson & 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 of California, Santa Cruz"
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first version: 19990724
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