The islands are at latitude 18°25’ north and longitude 64°40’ west, roughly the same latitude as Mumbai, Honolulu, and Mexico City. They lie at the confluence of the Atlantic Ocean and the Caribbean Sea. The Atlantic Ocean lies to the north and east of the islands, while the Caribbean is to the south and west.
The Virgin Islands comprise more than 90 individual islands, many of them nothing more than uninhabited rocks surrounded by sea. They have a combined coastline of 167 miles and a combined landmass of 193 square miles, about twice the size of the Vatican City.
With the exception of Anegada, which is a coral island, the Virgins are volcanic. They emerged from the Caribbean Sea some 65 million years ago as a result of alternating periods of undersea mountain-building, followed by periods of uplift and periods of explosive volcanism. The highest point in the islands is Sage Mountain (1,709 feet above sea level) on Tortola.
Up until the Pleistocene era, about 100,000 years ago, the British Virgin Islands, St. Thomas, and St. John, plus their related satellite cays, were joined with Puerto Rico to form a single landmass. When the sea level rose, all but the uppermost mountains and highest valleys were submerged by water, and the islands we know today were formed.
St. Croix, divided from St. Thomas by a two-mile-deep trench, was always separate from the rest of the Virgin Islands, however. As a result, unique plant and animal species can be found there.
While the Virgin Islands form a single geographical unit, they are divided into two distinct territories with separate histories, economies, and administrations. The U.S. Virgin Islands, the more westward of the Virgins, comprise St. Thomas, St. Croix, and St. John. The British Virgin Islands comprise Tortola, Virgin Gorda, Jost Van Dyke, Anegada, and dozens of smaller islands and cays. The primary cities are Charlotte Amalie on St. Thomas, Christiansted on St. Croix, and Road Town on Tortola.
The boundary between the U.S. and British islands winds between St. John and Tortola and between Hans Lollick (U.S.) and the Tobagos (U.K.). The territories are separated by as little as one mile of water in places.
The Virgin Islands lie within an active earthquake zone registering some 900 measurable quakes each year. Most are minor—so weak you don’t feel them—but occasionally there are more significant events, usually marked by a loud rumbling noise and shaking.
The possibility exists for the islands to experience a major earthquake. The most significant earthquake in modern history took place in 1867, causing tsunamis that inundated the cities of Charlotte Amalie, Frederiksted, Christiansted, and Road Town. Massive seagoing ships at anchor were deposited well inland.
The islands’ seismicity comes from the fact that they lie just south of the boundary of the North American and Caribbean plates, where there is gradual subduction and displacement. The Puerto Rico Seismic Network at the University of Puerto Rico (http://redsismica.uprm.edu/english) and the Seismic Research Unit at the University of the West Indies (www.uwiseismic.com) monitor the islands’ seismic activity.
© Susanna Henighan Potter from Moon Virgin Islands, 4th edition