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Bermuda - Geography

Bermuda is located in the North Atlantic Ocean, near the western edge of the Sargasso Sea, roughly 580 nautical miles (1070 km, 670 mi) east-southeast of Cape Hatteras on the Outer Banks of North Carolina and roughly 590 nautical miles (1100 km, 690 mi) southeast of Martha's Vineyard. The island lies due east of Fripp Island, South Carolina. It has 103 km (64 mi) of coastline. There are two incorporated municipalities in Bermuda: the City of Hamilton and the Town of St George. Bermuda is divided into various "parishes," in which there are some localities called "villages," such as Flatts Village, Tucker's Town and Somerset Village.

Present day Bermuda is a low-lying oceanic archipelago comprising a fishhook-shaped chain of four main islands which are surrounded by hundreds of islets, giving a total land mass of 5,370 hectares. Gently rolling hills rise to a maximum elevation of 79 m. Bermuda lines the southeastern edge of a large lagoon formed by the truncated top of one of three extinct volcanic peaks forming the Bermuda Seamount which rises about 4,000 m from the seabed. Presenting a total shoreline of 290 km, the islands also enclose several large inshore basins.

Although usually referred to in the singular, the territory consists of approximately 138 islands, with a total area of 53.3 square kilometers (20.6 sq mi). The largest island, main Island, is sometimes itself called Bermuda. Compiling a list of the islands is often complicated, as many have more than one name (as does the entire archipelago, which has also been known historically as La Garza, Virgineola, and the Isle of Devils). Despite its small land mass, there has been a tendency for place names to be repeated; there are, for example, two islands named Long Island, and St George's Town is located on St George's Island within St George's Parish (each known as St George's).

The surface soil is a curious kind of red earth, which is also found in ochre-like strata throughout the limestone. It is generally mixed with vegetable matter and coral sand. There is a total want of streams and wells of fresh water, and the inhabitants are dependent on the rain, which they collect and preserve in tanks.

Bermuda currently supports what is considered one of the “healthiest” coral reef systems of the Wider Caribbean Region. Coral cover- or coral density- varies among the different reef types, ranging from 22% to 70%. This creates habitats for an array of reef fish and invertebrates. As noted in the Biodiversity Strategy and Action Plan, Bermuda’s reefs are of global importance, being the northernmost reef system due to its proximity to the Gulf Stream. The northerly latitude of Bermuda’s reefs has benefited the health of the reefs by mitigating certain climate change impacts, such as increased “bleaching” events.

The islands of Bermuda sit atop a long-extinct, mid-ocean volcanic seamount located 1,000 kilometers (600 miles) east of the Carolinas in that part of the North Atlantic Ocean known as the Sargasso Sea. The oceanic crust surrounding the base of the volcano is estimated to have formed 123- to 124-million years ago (mya) at the Mid-Atlantic Ridge, the submarine mountain range running along the axis of the Atlantic Ocean. Extending toward the ocean’s surface are four northeast-to-southwest trending volcanic peaks, including the emergent Bermuda Pedestal and the submerged Challenger, Argus, and Bowditch seamounts. The islands of Bermuda are located along the southeast margin of the largest peak, the Bermuda Pedestal.

Several theories have been proposed as to the origin of the Bermuda volcanoes. Initially, scientists believed that a large, underwater volcanic eruption along the Mid-Atlantic Ridge approximately 110 mya established a series of volcanic peaks that moved westward with the North American tectonic plate (as the Atlantic Ocean widened by plate tectonics and seafloor spreading). However, this interpretation is not well supported by the much younger age of volcanic rocks recovered from deep drill cores in Bermuda.

It is now generally accepted that the four peaks comprising the Bermuda seamounts were formed by extensive volcanic activity occurring approximately 33 mya. Why the eruptions occurred at this time and place is still being debated by geologists. One theory hypothesizes that the volcanism occurred over a hot spot or plume in the Earth’s crust. A newer theory proposes that the volcanic activity occurred due to a worldwide reorganization of the planet’s tectonic plates that occurred due to the closing of the Tethys Ocean (when Arabia collided with Eurasia during the Cenozoic Era, 35 to 40 mya). Based on the current area of the Bermuda Pedestal and the maximum elevation of other mid-ocean volcanoes, the elevation of the original Bermuda volcano would have been about 1,000 meters (3.280 feet). Using typical shoreline erosion rates, it would have taken from 3- to 10-million years to reduce the island to sea level.

As the original volcanic peaks of Bermuda were eroded below sea level by wave action, coral reefs formed in the shallows and limestone from calcareous algae and corals began depositing during the early Pleistocene (about 1.8 mya). Today, a thin limestone cap — about 15 to 100 m (49 to 328 ft) thick — completely covers the underlying volcanic rock. During the Pleistocene, alternating glacial and interglacial periods, with associated changes in sea-level, resulted in the deposition of a series of limestone layers predominantly composed of carbonate eolianites (remnant sand dunes that were converted to rock), with a few marine deposits and terra-rossa paleosols (red fossil soils composed of wind blown dust from the Sahara Desert in North Africa).

The carbonate eolianites and marine limestones would have formed during interglacial periods of higher sea levels when the platform was submerged, while the paleosols formed during times of glaciation and low sea-levels. Cycles of cementation, erosion, and re-cementation occurred due the fluctuations in sea level. Six to nine interglacial stands are represented in the rock layers that vary in their age and structure.

The surface of Bermuda is characterized by karst terrain, a landform where the principle weathering process is solution rather than erosion. Karstic areas tend to have exposed bedrock with only a thin layer of soil, few surface streams, and subterranean drainage. The surface karst in Bermuda consists of sinkholes and irregularly shaped pits with complex dissolution channels.

More than 150 inland limestone caves are known in and near Bermuda. These caves are believed to have formed during glacial periods when sea levels were 100 to 130 m (328 to 427 ft) lower. During these periods, the entire top of the Bermuda Pedestal was exposed so that the land mass was much larger and the fresh groundwater lens was much more extensive than that of present day Bermuda. As rainwater soaked through the soil, it picked up carbon dioxide to form a weak solution of carbonic acid. This acidic groundwater acted to dissolve the limestone, forming lateral voids. Over time, the dissolution process enlarged these voids to form liner conduits, caves, and underground lakes. Dripping water created massive stalactites and stalagmites in all parts of Bermuda caves.

As post-glacial sea levels rose, the air-filled voids within the caves became flooded with sea water and cave formation ceased. Bermuda caves are today characterized by fissure entrances and large breakdown chambers that formed by collapse of roof rock when formerly flooded portions were exposed to air as glacial sea levels fell.