A small number of islands are rising out of the ocean when our theories of geological processes suggest they should be sinking beneath the waves. The most dramatic example of this is Santa Maria in the Azores. A new paper provides an explanation for its growth, and possibly that of other islands gaining ground against the odds.

Oceanic islands are pushed up by volcanic activity beneath. Eventually, the volcanism stops and islands start to sink from a combination of erosion and changes to the continental crust on which they rest. Rising sea levels mean that, relative to the water line, islands worldwide are sinking much faster today.

Although there are a small number of exceptions around the world, most of these are for reasons we have understood for a while, such as bulges appearing in the seafloor beneath the island in response to things happening elsewhere. In the Geology Society of America Bulletin, a team led by the University of Bristol's Dr Ricardo Ramalho explain Santa Maria's anomalous growth to magma intrusions causing a thickening of the crust beneath it.

Ramalho previously attracted attention with the discovery that the Cape Verde Islands, far to Santa Maria's south, have been struck by tsunamis the size of skyscrapers. His work on Santa Maria is much less dramatic, but using Argon/Argon dating he was able to measure the island as being 6 million years old, younger than previous estimates but still the oldest island in the Azores.

After an initial period of growth, followed by subsidence, Ramalho found Santa Maria was partially or totally submerged from 5.3-4.1 million years ago. It started rising again 3.5 million years ago.

Santa Maria's uplift is particularly surprising because surrounding islands are sinking. After considering, and ruling out, a number of other possible explanations for this anomalous rise, Ramalho was left with one possibility. “Magma, instead of rising to the surface, started to accumulate below the island, slowly contributing to jack the volcano upwards,” Ramalho explained in a statement. 

The finding leaves unanswered the question of what could be causing this accumulation of magma, which is particularly unexpected given the relative youth of the crust on which Santa Maria stands. Ramalho is keen to study other locations where similar processes may be occurring.

The rate of growth is unlikely to allow Santa Maria to outrun sea level rise over the next few decades, but with a maximum elevation of 580 meters (1,900 feet), the island's 5,500 inhabitants have plenty of places to retreat to if their towns get washed away, and the process should be a little slower than for people at similar elevation elsewhere in the world.