A Philippine Island Detective Story – GWC Mag

Sometime in the past 50 years, a small island appeared 100 meters off the coast of Lumaniag Village in the Philippines, 77 kilometers southwest of Manila. The island consists of large coral boulders and mangroves sitting atop a gravel bar and helps shield the village from punishing waves and storms.

But accounts of when and how the island appeared remained fuzzy. So a team of researchers set out to uncover when exactly the island came to be and what events might have contributed to its growth. They collected field measurements, archival data, and interviews from village elders to reveal the island’s stormy history.

Collecting Clues

The island is 100 meters at its widest and runs parallel to the mainland coastline for about half a kilometer. Mangroves cover its landward side, and gravel and coral boulders litter the seaward side.

“Powerful waves brought large boulders to the area and made the island visible overnight.”

The base of the island is a gravel bar, which Lumaniag villagers agreed built up slowly, said Adonis Gallentes, lead researcher on the project and a marine science graduate student at the University of the Philippines Diliman. For a while it remained submerged, even at low tide, until “powerful waves brought large boulders to the area and made the island visible overnight,” Gallentes said the villagers reported.

But was it a tsunami or a storm? Was it one event or many?

During several visits to the island, the researchers cataloged 108 coral boulders that were at least 1 meter on the longest side and measured each boulder’s position, size, orientation, and density. They snorkeled to survey the surrounding reef, mapped the coastal profile, and collected bathymetry data of the seafloor. These measurements helped show that the boulders came from the reef southwest to northwest of the island. To move them, waves at least 1.8 meters high would have been needed.

Next, the researchers determined when the coral boulders were deposited on the nascent gravel bar. They drilled core samples from the boulders, and uranium-thorium (U-Th) radioisotope ages from 22 of them revealed that the corals were ripped from the reef and died in the 1960s to early 1970s.

To corroborate the U-Th ages, the researchers gathered historical records on earthquakes, tsunamis, and typhoons. There were no records of tsunamis taking place around the time the coral boulders died, but the team identified 11 typhoons with the right timings, strengths, and tracks that could have moved them.

“Everything suggests that the island was formed by storm events, not tsunamis, and that the most crucial events happened during the 1960s to early 1970s,” Gallentes said.

“This study demonstrates that a coast’s geomorphology can be greatly altered by typhoons,” and not only the strongest ones.

Finally, with a short list of typhoon candidates in hand, the team interviewed 11 Lumaniag villagers old enough to have lived through the storms. The villagers agreed that no tsunami-generating earthquakes took place during the time in question, but they recalled the destructive effects of Typhoons Dading (international name: Winnie) in 1964 and Welming (Emma) in 1967. In addition, one resident specifically mentioned that boulders were brought in by a typhoon in 1971 when her sister was 15 days old. Most likely, that would have been Typhoon Barang (Elaine) or Krising-Dadang (Faye-Gloria) in October of that year.

The team investigated these storms, looking at wind speed and path, and found that all four created waves strong enough to have transported the boulders. None of the storms was in the strongest category for typhoons, but they all had just the right track to help turn an unassuming gravel bar into a barrier island.

“This study demonstrates that a coast’s geomorphology can be greatly altered by typhoons,” and not only the strongest ones, the team wrote in a Geomorphology paper on the research. The researchers will present their work at AGU’s Annual Meeting 2023 on 12 December.

A Village’s Shield

“I find this research compelling and timely,” said Talea Mayo, a coastal ocean modeler at Emory University in Atlanta. She complimented the researchers’ “comprehensive” use of tools to unravel the island’s history. “The use of U-Th to narrow and identify the likely timeline of associated events, and then a combined approach to further narrow likely causes, is novel, with fascinating results.”

Studies like this one “may inform the development of nature-based approaches to the resilience of coastal communities,” said Mayo, who was not involved with the research.

Lumaniag’s shield could one day be overcome by a strong enough storm.

“At present, the nearby community is thankful for the existence of this new island,” Gallentes said. Strong storms have become more frequent in recent decades, he added, but the island “acts like a shield” by dissipating waves before they reach the mainland.

Human intervention strengthened that shield. Villagers planted a patch of mangroves on the island’s landward side in the 1990s, and more mangroves naturally colonized other areas of the island later. They have helped prevent subsequent large storms from sweeping the boulders away.

However, Lumaniag’s shield could one day be overcome by a strong enough storm. “When that happens,” Gallentes said, “the gravels and mangrove debris that will be carried by the waves can even be more destructive to the community.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

Citation: Cartier, K. M. S. (2023), A Philippine island detective story, Eos, 104, https://doi.org/10.1029/2023EO230447. Published on 11 December 2023.

Text © 2023. AGU. CC BY-NC-ND 3.0
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