Bermuda is an isolated chain of islands in the North Atlantic Ocean, a thousand kilometers from the nearest landmass. Unlike islands formed along continental edges or active tectonic plate boundaries, Bermuda is an oceanic island sitting on an extinct volcanic seamount. The visible islands represent a thin cap of limestone built up over millions of years, resting upon a massive, submerged basaltic foundation. This structure details an unusual history of deep-sea volcanism followed by biological and climatic transformation.
The Deep Volcanic Origin
The history of Bermuda began deep beneath the ocean floor, where a mantle plume or “hotspot” delivered molten rock to the crust, far from any plate boundary. Around 45 to 35 million years ago, this volcanic activity built a huge shield volcano from successive flows of basaltic lava. The eruptions continued until the volcanic mountain, known as the Bermuda Pedestal, eventually broke the surface of the Atlantic Ocean, rising over 4,000 meters from the abyssal plain. The mountain may have towered 1,000 meters above the sea at its peak before the volcano became extinct approximately 30 million years ago. Today, the basaltic foundation is completely covered, with the top of the volcanic rock encountered in boreholes at an average depth of about 45 meters below sea level.
Subsidence and the Bermuda Platform
Once volcanic activity ceased, the immense mountain began to undergo a dual process of erosion and subsidence. The initial volcanic island was subject to relentless subaerial and marine erosion, with powerful ocean waves and weather rapidly wearing down the exposed basalt. This erosional process planed off the peak of the seamount, creating a flattened, submerged surface.
Simultaneously, the dense volcanic rock cooled, contracted, and the sheer weight of the structure caused it to slowly sink back into the ocean floor. This sinking is a common fate for oceanic volcanoes. The combined effects of erosion and subsidence created the extensive, shallow-water base known as the Bermuda Platform, which today covers about 665 square kilometers. This flat, submerged base provided the stable, shallow environment necessary for the island’s next phase of formation.
The Formation of the Limestone Cap
The visible islands of Bermuda formed atop this submerged platform through a process driven by marine life and global climate change. In the shallow, tropical waters, marine organisms like corals, algae, and mollusks thrived, producing vast quantities of calcium carbonate skeletons. When these organisms died, their remains broke down into fine carbonate sand.
The primary factor in creating the landmass was the fluctuation of global sea levels during the Pleistocene Epoch. As massive ice sheets grew and retreated, sea levels repeatedly dropped by over 100 meters, periodically exposing the entire Bermuda Platform. During these low-sea-level periods, strong winds picked up the dry carbonate sand and blew it inland, depositing it in massive, migrating dune systems.
Over time, rainwater percolated through these dunes, dissolving some calcium carbonate and then re-precipitating it between the sand grains. This natural cementation process hardened the sand dunes into a type of rock known as eolianite, or calcarenite. The islands of Bermuda are essentially the tops of these fossilized, cemented sand dunes, and their current topography is a direct result of this dune-building and subsequent dissolution. The famous pink sand beaches contain remnants of the red shells of the single-celled Homotrema rubrum.