Could the height of an island be maintained for tens of millions of years without the heat of a mantle plume?” Geological data beneath the Bermuda island has revealed an unusual structure that contradicts the presently accepted theory about the formation of islands in the ocean. By analyzing the echoes from distant earthquakes, scientists mapped the crust-mantle border and detected an excessive layer of underplated rock with a thickness of 20 kilometers and twice as thick than for other islands in the ocean.
Bermuda occupies an elevation above the seafloor called a bathymetric swell, characterized by a similar topographic feature to other volcanic hotspots such as Hawai‘i and the Galápagos Islands. In the traditional models, topographic swells result from the buoyancy of rising mantle plumes deep in Earth’s core. However, there has been no evidence from seismic tomography and geochemical data to indicate an active plume under Bermuda, with its volcanoes having become inactive for more than 30 million years. According to conventional heat models, this topographic swell should have collapsed by now.
This data came from an ongoing seismographic station established on the island. Waves from earthquakes thousands of kilometers away were picked up by the station. As the waves traveled through the layers under the Earth, they were reflected back and refracted. This created echoes that were measured by the scientists. A vertical section of the layers of the Earth’s rocks was thus obtained. The oceanic layer of the Earth and the boundary of the Mohorovičić Discontinuity, the “Moho” boundary between the Earth’s crust and mantle, were shown by the data. But there is an enormous layer of rocks hitherto unknown, underlying the Earth’s crust. This is composed of cooled magma that is slightly less dense than the mantle, giving the whole area a long-term buoyant effect.
Underplating is a non-unique process and has occurred in other volcanic settings, including the volcanic environment of Hawai‘i, where magmatic sills and dikes underlie a distinctively different chemical lower crust. While underplates are known to exist, Bermuda’s is unique because of its magnitude, and it may have formed by a different process. Geochemistry of an underplate is relatively rare and was found in a deep 1972 drill hole, indicating it was composed of transition zone material at 250-400 miles depth, and possibly also of pre-Pangaea breakup subducted slabs. This was volatile-rich, including carbon dioxide and water, and its presence caused the melting point depression and the resultant magmas that moved under the crust 35-30 million years ago when Bermuda was volcanic.
The increasing recognition of the role of underplating in maintaining swells is a new development in geodynamics. Theoretical work suggests that the input of low-density material can diminish or eliminate the need for a high thermal anomaly, consistent with the high heat flow anomalies found over swells such as Hawaii. By contrast, in the case of Bermuda, the uplifting force is practically entirely chemical, as opposed to thermal. The underplate is thought to extend laterally up to 50–100 kilometers beneath the island, forming a saucer-shaped topography.
Such information demands sophisticated imaging of seismograms. For example, Reverse Time Migration Full Waveform Inversion, a technique applied to mid-manthe geometries beneath Hawai‘i only last year, is capable of imaging strong impedance contrasts, while traditional tomographic imaging smooths them out. At Bermuda, the ability to locate the layers thanks to teleseismic reflections helped to compensate for the lack of station control. These approaches also reflect the general interest of seismologists to combine anisotropy research, experiments in mineral physics, and mantle convection simulations in interpreting subsurface anomalies.
This discovery also has implications regarding the plume versus non-plume origin of ocean islands. Although the plume model is successful in describing hotspot volcanism, an increasing number of localities are found to experience uplift supported by compositional buoyancy due to underplating or the presence of light mantle domains. This process can occur over geological timescales even when volcanoes are inactive. The extensive underplate found at Bermuda is not a phenomenon isolated by comparison to a particular region but is a reflection of certain complex processes occurring within the Earth’s mantle, which cannot be easily described by a simple paradigm.
The existence of Bermuda’s thick underplate shows how ancient volcanic actions can be impressed upon a permanent form and how contemporary technology can detect it. Further such layers like Bermuda might be uncovered as more information becomes available through better resolution.
