Ocean islands are supposed to sink. As they get older, the plate on which they sit cools, becomes denser, and sinks. Or the island’s volcano gets so massive that it weighs down the entire island. Building on the expectation of island subsidence, Charles Darwin even penned a well-known theory about how sinking islands influence the growth of coral reefs.
A handful of islands break this rule, most of them in the Atlantic Ocean. One particular island, Santa Maria, of the Azores Islands, located 1300 kilometers west of Portugal, slowly continues to rise above the sea, baffling scientists.
There are a few well-understood mechanisms for island uplift, said Ricardo Ramalho, a geologist at Portugal’s University of Lisbon. Take Hawaii, for example. Hawaii is made up of volcanic islands, all formed from magma rising from a hot spot under the Pacific plate. As the plate moves over the hot spot, magma rises through the ocean floor, and the resulting volcanism forms an island. The plate continues to move, so older lithosphere cools and grows denser as it moves away from the magma source, causing the island to sink. But as that weight presses down on one part of the lithosphere, the lithosphere upstream, toward newer islands, rebounds, causing uplift in these younger islands.
But the Azores island cluster sits nowhere near a hot spot. In fact, these islands are 480 kilometers away from even the Mid-Atlantic Ridge, where new lithosphere forms from magma bubbling up. And Santa Maria, the oldest and southeasternmost island of the Azores, shows clear signs of uplift. Above the current sea level, cliffs contain evidence of lava flows that had encountered ocean water, and one side of the island boasts a wide, high-elevation rock staircase, eroded away long ago by waves and uplifted above the sea.
So Ramalho and a team of scientists decided to find out why. They recently published their findings in the Geological Society of America Bulletin.
History in the Cliffs
To figure out why the island continues to uplift, the scientists first had to reconstruct its history, which is written in the cliffs.
The researchers specifically looked for places on the cliff sides that showed where sea level once reached. Because lava that cools in air forms different shapes than lava that cools in water, it’s relatively simple to tell where flowing lava struck cool ocean water, Ramalho said. On land, lava cools in shapes called pahoehoe and aa (pronounced ah-ah), but in the sea it forms structures called pillow basalts.
The team sailed all around the island, building a map of where they found transitions between the lava types, features that they call “uplift tracers.” By dating these marker lavas and comparing them to where sea level is now, the researchers could build a basic history of the island’s past.
The researchers found that Santa Maria Island first broke the ocean’s surface 6 million years ago and then, through shield volcanism, grew steadily for about a million years. Then, 5.3 million years ago, the island subsided underneath the waves, most likely because of its own heavy weight pressing down on the lithosphere, Ramalho said. Later, the island reemerged because of renewed volcanism, while subsidence continued. Then, at 3.5 million years ago, volcanism began to wane again; this is evident in the youngest volcanic rocks that are about the same age, Ramalho said. It was also at this point that uplift began, the team’s uplift markers showed.
Because the nearest island, São Miguel, is much younger than 3.5 million years, its weight on the lithosphere couldn’t have kicked off a teeter-totter-like rise in the lithosphere under Santa Maria, Ramalho said, which rules out that mechanism of uplift.
Another possible mechanism of uplift could be erosion, which would wear the island down and thus make it lighter and cause it to rise. But this mechanism is unlikely because eroded materials tend to accumulate at the base of the island edifice and slow down uplift. Sudden unloading of material is also unlikely, Ramalho said, because the uplift would be sudden, rather than long-lasting.
After ruling out those two possibilities, the researchers were left with a final mechanism. Because the beginning of uplift coincided with waning volcanism, the researchers suspect that the magmatic activity didn’t stop altogether, but rather shifted from surface volcanism to intrusive activity deep in the crust.
This means that the “quietly active” magma has been intruding for millions of years into the crust beneath the island, causing the crust to bulge, which pushes the island upward, Ramalho said.
To test the plausibility of this new hypothesis, the research team determined a rough rate of past magma production by calculating the volume of the island and dividing it by how long Santa Maria took to form. The researchers then calculated the approximate amount of magma that would have intruded into the crust beneath the island in order to explain the observed uplift, which started about the same time volcanism ended, 3.5 million years ago. This hypothetical rate at which magma entered the crust matched the known rate of island formation, which supported the idea that uplift was driven by a shift from surface volcanism to a deep, intrusive system, the researchers report in their 21 October paper.
The team’s explanation for Santa Maria Island’s uplift defies the conventional wisdom that “once an ocean island volcano has been carried away from its plume origin for several million years, it becomes extinct,” said Karen Harpp, a geologist at Colgate University in Hamilton, N.Y. Ramalho’s field observations and analysis of a possible mechanism of uplift at Santa Maria Island are “important support for similar observations [of uplift] elsewhere,” she added.
Indeed, studies of other island chains in the groups in the eastern Atlantic, such as the Cape Verde Archipelago and the Madeira Archipelago, indicate that deep volcanism might have persisted under them as well, Ramalho said. Other researchers also inferred that the same mechanism could be driving uplift in islands in the Canary Archipelago.
These new findings “might help us to understand how magmatic systems behave in the long term,” Ramalho said. “It raises questions about what is feeding [the magma] and what controls the transfer of magma from depth to surface.”
Perhaps this phenomenon “is more ubiquitous than previously thought,” he continued.
—JoAnna Wendel (@JoAnnaScience), Staff Writer