Bermuda's geography: a limestone cap on a drowned volcano

How a volcano became Bermuda's limestone

The foundation is basalt. The Bermuda seamount rose from the Atlantic floor by episodic eruptions of lava and, judging from the volcanic sediment around it, stood as much as 1,000 metres above sea level as an island. Its height above the ocean floor is given as more than 4,000 metres by Bermuda Geology and more than 4,300 metres by Britannica. It is one of four seamounts, Bermuda, Challenger, Bowditch and Argus, in a chain about 100 kilometres long running from southwest to northeast, on the crest of a swell called the Bermuda Rise.

The dating comes from boreholes. Louis Pirsson drilled the first one through the limestone near Gibbs Hill Lighthouse in 1912 and met undisturbed volcanic rock only at 171 metres below sea level. The Deep Drill project of 1972 on St George's Island reached volcanic rock at 26 metres and cored down to 793 metres, and that core gave a potassium-argon age of 33.5 million years for intrusive magma. Earth magazine places the formation of the seamounts between 45 and 35 million years ago; Bermuda Geology puts the last major eruption about 30 million years ago.

Since then the waves have planed the volcano down to a flat-topped stump. Across the central and western parishes its surface lies about 45 metres below sea level, and at the east end between 23 and 32 metres, where volcanic sand and pebbles at Whalebone Bay and Stokes Point show the last of the old island survived longest. On top sits a cap of marine limestone that Britannica gives as about 60 metres thick.

The part above water was made by wind. During warm interglacial periods, waves broke the reef and algal limestone into sand; when ice ages lowered the sea, that sand blew into dunes and hardened into rock. The layers are separated by red fossil soils, which the Wikipedia account of the geology attributes to Saharan dust settling during glacial stages. Older ridges inland are rounder and lower than the younger ones along the outer coast.

Why the seamount has not sunk the way oceanic volcanoes usually do is still argued. Bermuda Geology notes that volcanic rock lies about 45 metres down here against more than 1,000 metres beneath some other dormant islands, and reports a 2026 proposal by Frazer and Park that a buoyant layer of lighter magma, about 20 kilometres thick, was injected beneath the islands and holds them up.

How many islands does Bermuda have

The answer depends on who counted and what they counted as an island. The main islands are joined by bridges and a causeway and read as one piece of land, and the rest are islets and rocks in the sounds and harbours.

Britannica describes the main group as a fishhook, about 40 kilometres from end to end and on average under 1.6 kilometres wide. Main Island, also called Bermuda, is about 22.5 kilometres long. Eight larger inhabited islands are connected by bridges. The territory's record on this site gives the area as 53 km², and the coastline is about 103 kilometres.

Where Bermuda sits in the open Atlantic

The nearest land outside the territory is Cape Hatteras in North Carolina, about 1,035 kilometres to the west-northwest in the Wikipedia figure and 1,050 kilometres in Britannica's. Cape Sable Island in Nova Scotia is 1,236 kilometres to the north, Halifax 1,350 kilometres, and Miami about 1,770 kilometres to the southwest. Britannica stresses that Bermuda is neither geologically nor spatially part of the West Indies, which lie more than 1,300 kilometres to the south and southwest.

The islands sit on the southeastern rim of their own platform. Earth magazine describes the reef running about 10 kilometres out to the northwest of the land, while off the southeast coast a few kilometres of shallows end in a steep drop. The sea around them is the western Sargasso Sea, and the Gulf Stream passes upwind. The United Nations gives the exclusive economic zone as 450,370 km², more than eight thousand times the land area.

Why every Bermudian roof collects rain

The limestone holds no surface water. There are no rivers, no freshwater lakes and, Britannica notes, no problem with standing water either, because the thin fertile soil and the rock beneath it drain almost at once. Mean annual rainfall is about 1,450 millimetres, fairly evenly spread through the year, and dry spells in April and May are when the water supply is most exposed.

The answer is on the houses. The law requires each household to collect the rainwater piped down from its roof, and most dwellings have at least one tank built into the foundations. UNESCO's description of St George's explains the look that follows: the soft local limestone is used for walls and roofs alike, both are whitewashed, and the stepped pitch of the roofs channels rain through gutters into cisterns. Wells and seawater distillation plants make up the rest.

Underground there are two main aquifers, the Langton and the Brighton, and four freshwater lenses floating on the salt water beneath. The Central Lens under Main Island covers about 7.2 km² and is more than 10 metres thick.

Rain dissolves limestone, and Bermuda is riddled with it. Earth magazine counts more than 150 caves and many sinkholes. The Walsingham Formation around Castle Harbour, the oldest limestone on the islands, forms the main cave district, and much of the inshore water is drowned karst. All the known caves were above sea level at the last glacial maximum about 18,000 years ago; some cave-dwelling crustaceans are thought to be far older than that, which suggests deeper caves still exist offshore.

How the Gulf Stream tempers the climate

Bermuda lies on the boundary between a humid subtropical and a tropical rainforest climate in the Köppen scheme, and the ocean keeps it mild in both directions. Britannica gives an average daytime high of 30 °C in August, the warmest month, and an average night-time low of 14 °C in February, the coldest. The sea averages 22.8 °C over the year, from 18.6 °C in February to 28.2 °C in August. Humidity delays the seasons, so the coldest weeks fall in February and March.

The records sit at the edges. The highest temperature, 34 °C, came in August 1989, when Hurricane Dean pushed hot air north. The lowest in the modern series is 43.3 °F, on 26 February 1993, although the Royal Engineers' nineteenth-century register shows 34.0 °F in Hamilton on 26 February 1880. Snow melts before it reaches the ground, but hail falls in winter, and on 26 February 1986 it lay thickly enough to cover parts of the island.

The Bermuda-Azores High governs the pattern. In spring and summer its ridge lies just south of the islands, bringing southwesterly winds and keeping fronts away; in autumn and winter it weakens and fronts from North America bring gales and gusty showers. On hot, still summer days a line of cloud known locally as Morgan's Cloud can build along the length of the island and has produced waterspouts in the eastern parishes. The sources disagree on the wettest month: October in the main Wikipedia article, August in its geography article.

When hurricanes reach the islands

The season runs from June to November. Storms that reach Bermuda have usually recurved northeast into the westerlies and begun to weaken, and a target 22.5 kilometres long is small, so a direct hit at hurricane strength is uncommon. The reef breaks the swell before it reaches the shore.

The record of direct hits is still long. Hurricane Emily in 1987 was the first in three decades. Fabian, on 5 September 2003, struck as a Category 3 storm with winds over 190 km/h and did heavy damage. Gonzalo, a Category 2, hit on 18 October 2014, and Nicole, a Category 3, on 14 October 2016. Paulette made a direct hit in 2020, and Ernesto crossed the island as a weakening Category 1 storm on 17 August 2024. A storm in 1995, Hurricane Felix, delayed the independence referendum by a day, a story told in the history of Bermuda.

What happened to the Bermuda cedar forest

When the first ships arrived, the islands were covered by forests of Bermuda juniper, the endemic tree known locally as cedar, with mangroves along the shore. The Wikipedia geography article estimates several million trees before settlement. Shipbuilding stripped large areas by the 1830s; the forest came back after the trade died, and by 1900 the islands were densely covered again.

The second loss was faster. In the 1940s two accidentally introduced scale insects began killing the junipers, and over the following decade roughly 8 million trees died. Cars were legalised in 1948 and the population, which reached 60,000 by the 1980s, spread out over the same ground. Casuarina and Brazilian pepper, planted or escaped to fill the gap, outgrew the slow cedar, which now survives mainly in managed land such as gardens and golf courses. Forest covered about 1,000 hectares in 2020, about 20% of the land, unchanged since 1990.

The native wildlife was always thin. Five species of bat are the only indigenous mammals. The Bermuda rock skink was long thought the only native land vertebrate other than birds, until DNA work showed the diamondback terrapin predates people too. The cahow, or Bermuda petrel, was believed extinct before the end of the 17th century until David Wingate, Robert Cushman Murphy and Louis Mowbray found nesting pairs on islets in Castle Harbour in 1951. Castle Island itself, the site of a fort, is now a refuge for the skink and the cahow. Counts of the plants differ: the main Wikipedia article gives 165 native vascular species out of about 1,000, fifteen of them endemic, and the geography article 14 endemic.

How the parishes divide the land

The grid is four centuries old. The company surveyor laid out one public territory at St George's and eight tribes, later called parishes, each cut into shareholders' lots divided by narrow tribe roads running from shore to shore. For three centuries most travel went by boat, and the roads mainly gave each lot access to the water. Today there are nine parishes, from St George's in the east to Sandys in the west, and two incorporated municipalities, the City of Hamilton and the Town of St George.

Land use in 2012 was recorded as 14.8% arable, with the remaining 85.2% split between developed land, 55% of it, and rural or open space, 45%. The islands carry 447 kilometres of paved roads, 225 kilometres of them public, and a former railway line that is now a walking trail. The airport stands on St David's Island, on land that was largely reclaimed from the sea for the wartime US airfield.