Geography of Bermuda - Formation and Geology

Formation and Geology

Seen from above the ocean's surface, the general characteristic or the islands is that of ridges, running from the North East to the South West, before turning towards the North. Together with the submerged, encircling reef, they form the edge of a volcanic caldera, to the south of which the islands lie. The islands enclose a smaller caldera, which forms the Great Sound, and another forms Harrington Sound. The volcano was formed on the fault line from which the Atlantic oceanic tectonic plates expand, pushing the Old and New Worlds away from each other. It has been theorised that the volcanoes were formed by the Bermuda hotspot, if that existed. A submarine volcanic mountain range, the Mid-Atlantic Ridge was formed atop this fault. Bermuda now sits far to the west of the ridge in foothills that once were the ridge.

This is because it is far older than other islands formed on the ridge, which lie much closer to the ridge (including Iceland, The Azores, Ascension Island, etc.). There are also two seamounts to the South-West of Bermuda, forming Argus Banks, and Challenger Banks. Neither of these breaks the surface of the ocean, but both support coral reefs and are popular fishing grounds. Although the volcanic base of Bermuda is igneous, it is covered by a limestone cap, formed by calcium-secreting marine organisms. This formed underwater, but, during ice ages, when sea levels were lower, the limestone broke down into sand. This blew into dunes in which the sand eventually fused back together to form an aeolian sandstone. Underwater, coral is present in the reefline which encompasses the archipelago. To the south, the reef line lies within a matter of yards from the shoreline. At its northward extremity, the reef lies fourteen miles (21 km) from the north shore.

Corals reefs, which protect the soft, limestone coastlines from wave-erosion, are protected by law. The island is thinly-covered by a red soil, high in iron, but low in magnesium. Yellowish leaf colour of plants in many areas of the island is a result of the low levels of magnesium, which is used in producing chlorophyll. Due to the lime content of the stone on which it sits, the soil is also very alkaline. In most parts of Bermuda, the soil is rarely more than a few inches deep, though, in some low-lying, inland areas it has collected quite deeply. At one point, the land mass of Bermuda was about 200 square miles (520 km2), large enough to allow the genetic diversity for many plants and animals that established themselves to develop into unique species and sub-species. Because the limestone cap is porous, rainwater quickly sinks through it and there are no permanent streams, or standing bodies of freshwater. A well sunk anywhere in Bermuda will find water as soon as it reaches sea level (the water table). Unfortunately, this water is brackish, since the rainwater filtering down from above mixes with sea water from below. Due to the high porosity of the limestone, there are few available freshwater lenses that are fully unmixed with salt water.

Despite the apparent lack of fresh groundwater, Bermuda is lush and green, a result of the consistent, high levels of rainfall. Bermuda's human inhabitants traditionally collect water from the roofs of their houses, and hillsides surfaced as catchments, the water being stored in tanks dug into the ground, often in the foundations of houses. In recent decades it was discovered that rain water, being less dense due to its lack of salinity, sits on top of the water table and mixes slowly. This lens can be siphoned off by wells and used for drinking. A private company, Watlington Waterworks, uses this method to obtain water which is provided to its customers by a system of mains. Most of the native and endemic flora and fauna arrived via natural dispersion from North America. Most notable of these was the juniper which evolved into the Bermuda cedar.