Unique Agates on Panamanian Island Reveal Underwater Volcanic Activity | Smithsonian Voices

Sea conditions in the Pearl Islands can be dangerous. Information on the geology of eastern Panama, including Pearl and Darien Islands, is sparse due to the logistical challenges posed by its remote location, tropical rock weathering, and dense tropical vegetation.
Courtesy of Stewart Redwood

In the archipelago of Las Perlas (Islands of Pearls), in the Gulf of Panama, large quantities of agates have been found during archaeological excavations of a pre-ceramic settlement on the island of Pedro Gonzalez. Evidence shows that early human settlers put these large agates to good use by fashioning them into tools. But how did these agates form and why are they so big?

Consulting geologist Stewart Redwood, David Buchs, research associate at the Smithsonian Tropical Research Institute (STRI), and Davey Cavell, PhD student in geochemistry at the University of Birmingham, published an article in Geological magazine online in December 2021 and in print in the May 2022 edition, entitled Underwater volcanic activity and formation of giant tonsils along the island arc of Panama as a precursor to 6,000-year-old agate mining on Pedro González Islandon their multidisciplinary study of the geological setting of the island in which the agates were formed, and what it reveals about the evolution and closure of the Isthmus of Panama.

“The geology of the island was not what we expected,” Redwood says. “Volcanic and sedimentary rocks show formation characteristics near an oceanic island within a volcanic arc.”

The project began in 2009 when STRI archaeologist Richard Cooke was studying the presence of a pre-ceramic human settlement on the island’s Don Bernardo beach. Cooke found evidence of the exploitation of dolphins and other sea and land animals using agate tools. The settlement dates back 6,000 years, making it the oldest known human settlement on the Pearl Islands.

At the time, Cooke invited his longtime collaborator Redwood to offer his opinion on the source of the agate tonsils on the island. Tonsils are blisters in volcanic rock formations filled with other minerals such as quartz, calcite, chalcedony or agate. Tonsils are usually very small, so the giant agates found on Pedro Gonzalez are a unique geological curiosity as there are no other known examples anywhere else in the world.

“My preliminary observations were that the source of the agates was a lava flow,” says Redwood. “Subsequent work showed that the distribution occurs in a well-defined area that indicates a plume of hot, silica-laden water rising from the depths.”

Giant agate tonsils have been found in various locations on Pedro Gonzalez Island.
Courtesy of Stewart Redwood

These agates were completely filled, with only a capillary gap where the last band of silica rings met, making them more brittle and therefore easier for old flint cutters to break.
Courtesy of Georges A. Pearson

Agate is a semi-precious, fibrous, quartz-like silica mineral. It forms mainly due to volcanic activity: bubbles of steam and gas are trapped in the lava which cools and creates cavities in the rock. Hot water carrying silica (a major component of rocks and the earth’s crust) enters these cavities and coagulates into silica gel, which crystallizes into agate.

“Agate is an interesting mineral and looks beautiful both in pebble form and cut into thin slices under a microscope,” says Redwood. “It grows by self-organizing crystallization, rather than external changes in temperature and pressure, and it matures over time.”

Redwood points out that agate is ideal for making cutting and drilling tools due to its hardness and sharpness when broken. “These giant tonsils are just the size you need to fit in your hand, and they’re very abundant on some parts of the island,” he adds.

Archaeologist and former STRI fellow Georges A. Pearson, who collaborated with Cooke in a 2020 paper on the preceramic occupation of Las Perlas, studied whole agate nodules as well as tools and pieces that were probably used for flint cutting (making sharp stone tools by chipping and shaping).

“Agates were heat-treated by pre-Columbian inhabitants,” says Pearson. “The nodules were slowly ‘cooked’ over long hours, even days, to make them easier to scale and shape. This work was mostly done by women, who were responsible for making and maintaining food processing tools, such as corn and yucca graters.

Redwood was later invited by archaeologist Juan Guillermo Martin of Universidad del Norte, Barranquilla, Colombia, to conduct further geological investigations during the rescue archeology program in 2015 and 2016, during the dry season of Panama from December to April. Buchs, a volcanologist and sedimentologist, joined him on the second trip, supported by a grant from the National Geographic Society.

Cavell, who is a researcher working on a project in the Darien, contributed analyzes of volcanic rock and agate samples and microscopic studies to his doctoral project, while Redwood and Buchs produced a geological map of the shoreline. from the island, by boat.

“The shore has excellent rock exposure, compared to almost none on the island’s interior,” says Redwood. “We used a local pangaan open boat with an outboard motor, and we had a panguero (boat operator).

Their findings may help determine when the Earth’s plates collided with the eastern Panamanian island arc and volcanic activity ceased. Island arcs are chains of volcanic islands produced when one crustal plate replaces another in a subduction zone, causing the subducted plate to melt deep within the Earth’s crust; this molten rock rises like an underwater volcano and forms islands.


An oblique satellite image of Pedro Gonzalez Island, which shows areas where agates have been found, and a map of the Isthmus of Panama, highlighting the location of the Las Perlas Archipelago in the east from Panama.

Courtesy of Stewart Redwood

Preliminary age dating shows that [the island arc of the Pearl Islands]
dates back about 20 million years.

“This is the first time that a relatively old island arc has been recognized in the Pearl Islands, which may stretch from the Panama Canal to Darien Province,” Redwood says. “Preliminary dating shows it to be about 20 million years old, from the Oligocene to the early Miocene.”

Information on the geology of eastern Panama, including Pearl and Darien Islands, is sparse due to the logistical challenges posed by its remote location, tropical rock weathering, and dense tropical vegetation. However, discovering the extent and age of this eastern Panama volcanic arc would be essential in the debate over the formation and closure of the isthmus by collision with South America.

Redwood indicates that the next publication on the coastal geomorphology of the Pearl Islands is being prepared and that they are planning the next field study to complete the geological mapping of the other islands of the archipelago.

“We have already mapped more than half of the islands,” he says. “This will allow us to produce a geological map of the islands and to better define the extent and nature of the large underwater volcano of which we see part on Pedro Gonzalez, and whether there is further evidence that it actually emerged. to form a volcano. island.”

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