A caldera, which means cauldron in Spanish, refers to a hollow volcanic feature formed by the collapse of a volcano’s peak. While a caldera may only be viewed as an aftermath of a volcanic eruption, it has become the topic of widespread research—primarily to determine the possibility for these structures to cause natural disasters like landslides and the potential amount of minerals in ores within the caldera.
Dr. Jenny Anne Barretto, a Filipino marine geologist and geophysicist based in New Zealand, discovered the world’s largest caldera, which they named the Apolaki—after a mythological Filipino god of the sun—located at Benham Rise, a large undersea region east of Northern Luzon. Barretto, along with co-authors Dr. Ray Wood and Dr. John Milsom, published their findings last October 20 in the journal article Benham Rise unveiled: Morphology and structure of an Eocene large igneous province in the West Philippine Basin.
The LaSallian takes this opportunity to revisit one of the significant contributions of a Filipino scientist in 2019.
Deep into the rise
The research aims to describe the morphological features of the Benham Rise, including the Apolaki caldera, and to present their implications despite the limited geological and geophysical data. Barretto remarks, “There are these scientific data available, which have not been analyzed in detail. Curiosity for the unknown drives us—scientists—to conduct research.”
In 2009, the Philippines filed a proposal to the United Nations Commission on the Limits of the Continental Shelf (UN-CLCS), arguing that Benham Rise is an extension of the country’s continental shelf, the submerged edge of a landmass. The claim was approved three years later. “The Philippines had to show that Benham Rise is morphologically connected to the Luzon landmass,” explains Barretto in an interview with The LaSallian. Last 2017, the region was renamed as the Philippine Rise by President Rodrigo Duterte through Executive Order No. 25.
The Philippine Rise is found and delimited within the West Philippine Basin, a sunken surface of oceanic crust at the location referred by its namesake, since the late Eocene epoch some 56 million to 33 million years ago. Dr. Renato Solidum Jr., the Philippine Institute of Volcanology and Seismology Officer in Charge, describes it as an ocean island basalt—a type of rock derived from underwater volcanic activity. “It is a big volcano, which is why it is not surprising that there are many volcanic centers,” Solidum adds, referring to the large and long-living edifices on the volcano.
The National Mapping and Resource Information Authority (NAMRIA), a Philippine government agency mandated to be the country’s map-making and distributing center, conducted a multibeam bathymetry survey—a process of measuring the depth of a body of water such as oceans, seas, and lakes by transmitting and receiving sound pulses from the seafloor—in 2007, two years before it would be used to back the country’s claim on the territory. The agency also included rock samples and their chemical composition characteristics as additional support.
“Other coastal [regions] making submissions to the UN-CLCS also use the same kind of data. Depending on how complicated the case is, a coastal [region] may require more data gathering as supporting evidence. However, in all cases, bathymetry data takes priority,” Barretto expounds.
With the multibeam bathymetry, a large area such as the Philippine Rise can be surveyed faster and more accurately as it uses transducers that convert electrical signals to acoustic pulses, and vice versa.
Through a magnifying glass
With the research’s objective to present the features of the Philippine Rise given the limited data, Barretto and her research team subsequently made use of NAMRIA’s data to describe the Apolaki caldera.
With a rim diameter measuring 150 km, Apolaki is currently the world’s largest known caldera, surpassing the previous record holder, Yellowstone, which measures only 60 km across. Their study also reveals that the caldera was formed by “hotspot-related magmatism interacting with a spreading ridge”—the upward magma movement at the ocean floor where the new crust was forming, leading to the joining and eventual separating of Benham Rise with surrounding geologic structures.
Apolaki is one of few calderas in the Philippines, a list that includes the Corregidor in Bataan, Mt. Pinatubo in Zambales, and the Taal Caldera in Batangas. The Corregidor’s most featured characteristics include its 4-km base diameter and 170-m elevation. On the other hand, Mt. Pinatubo’s caldera is most notable for its formation on the summit that formed after it erupted in 1991. Lastly, the highlight of the 25-km wide Taal caldera—apart from its small size in contrast to Apolaki—is in its formation that can be traced several centuries back.
With regard to the possibility of volcanic activity, Solidum explains that due to water pressure, undersea volcanoes are less likely to erupt. “Unlike if the volcano is on land, the atmospheric pressure is much less than the power of the volcano to explode,” he adds. Solidum also assures the public that the Apolaki is no longer active; its last eruption supposedly occurred millions of years ago, so there is little reason to worry about it erupting again.
Nevertheless, Barretto necessitates further research on Apolaki and the Philippine Rise as a whole, not only for the processes behind the formation of these geologic features, but also for the possible impact these could have on local and regional environments.
One of their recommendations is to back up the existence of Apolaki and its related structures through a seismic reflection survey, a method similar to bathymetry that utilizes a seismograph to detect and record seismic activity through a receiver. “Dredging for rock samples and eventual drilling at key locations within the caldera and platform base [are] essential to determine magma composition and ages,” she adds, as these data gathering methods would provide a more accurate reconstruction of the rise’s formation and information on its possible mineral resources.
For now, Barretto says that just discovering something like Apolaki is already a significant breakthrough, “Apolaki may well be the world’s largest caldera. The short-lived but immense magmatic pulses associated with its formation must have altered the chemistry and physics of the waters and atmosphere in this part of the Pacific.”
