A conveyor belt of volcanoes
The Juan Fernández Islands are the visible peaks of a massive underwater volcanic chain, the Juan Fernández Ridge. This 800-kilometer-long ridge is the product of a stationary mantle plume, or hotspot, punching through the Nazca Plate as it moves eastward. The process acts like a conveyor belt: as the plate shifts, new volcanoes form over the hotspot, creating a chronological line of seamounts and islands.
This movement is theoretical; it has been measured. Radiometric dating of the volcanic rocks shows a clear age progression along the chain. The westernmost island, Alejandro Selkirk, is the youngest, at approximately 1.0 to 2.4 million years old. Robinson Crusoe Island is older, having formed between 3.8 and 4.2 million years ago. Santa Clara is older still, at 5.8 million years. The oldest part of the chain, the O'Higgins Guyot, is now a submerged, flat-topped seamount near the Chilean coast, with an age of around 8.4 million years. This age progression allows geologists to calculate the Nazca Plate's long-term velocity at approximately 81 millimeters per year. The islands are primarily built from basaltic lava flows, typical of oceanic hotspots.
An isolated laboratory for evolution
The geological isolation of the Juan Fernández Islands has made them a site of evolution. Their distance from mainland South America—about 670 kilometers—has limited the number of species that could colonize them, allowing unique life forms to develop. The archipelago is a UNESCO World Biosphere Reserve, recognized for its degree of endemism.
The flora is particularly distinct, with 137 endemic plant species. This represents one of the highest concentrations of unique plant species per square kilometer on the planet. Notable examples include the Luma de Juan Fernández (Nothomyrcia fernandeziana) and a whole genus of flowering plants, Robinsonia. The animal life is equally specialized. The Juan Fernández firecrown (Sephanoides fernandensis) is an endemic hummingbird, and the Juan Fernández fur seal (Arctocephalus philippii) is a mammal found nowhere else. This unique biodiversity is a consequence of the islands' volcanic birth and long-standing separation from other landmasses, showing how geology shapes biology over deep time.