Despite its single fearsome reputation, this is not one volcano but several, fused into a northwest-trending massif between two lakes - Ranco to the north, Puyehue to the south. Geologists count four edifices: the collapsed Cordillera Nevada caldera, the ancient Mencheca volcano, the fissure vents of Cordón Caulle, and the Puyehue stratovolcano with its 2.4-kilometer crater. They sit where a cross-fracture meets the great Liquiñe-Ofqui Fault that runs the length of the southern Andes, a seam in the Earth that keeps the magma rising. The result is among the most varied volcanic terrain in the entire Southern Volcanic Zone - cinder cones, lava domes, calderas, and the widest range of volcanic rock types found anywhere in the region, steaming with hot springs and fumaroles.

The complex has been reshaping this land for roughly 300,000 years, and the rocks read like chapters. Around 300,000 years ago the old Mencheca volcano faded as the active front of Andean volcanism shifted west, kindling new vents. Some 100,000 years ago an immense eruption laid down the San Pablo ignimbrite, a sheet of welded ash that spread across the Chilean Central Valley nearly to the coast, leaving behind the Cordillera Nevada caldera. Through the long Llanquihue glaciation, when the Patagonian Ice Sheet buried these summits, lava still forced its way out, leaving glacial scratches on its own flows. The mountain and the ice fought for tens of thousands of years, and the mountain is still here.

Cordón Caulle is the restless heart of the complex, the only vent with a real historical record of eruptions - in 1921, 1929, and 1934. But its most haunting performance came in 1960. On May 22 that year, the largest earthquake ever measured, magnitude 9.5, ruptured the Chilean coast. Thirty-eight hours later, Cordón Caulle answered: a fissure 5.5 kilometers long split open, twenty-one vents firing along it, throwing ash eight kilometers high before settling into rivers of blocky lava that flowed for two months. The earthquake appeared to have shaken the volcano awake - a chilling demonstration of how the forces beneath the Andes are linked, one catastrophe triggering the next along the same deep faults.

The 2011 eruption made all of this vivid for a new generation. By the time it began, 3,500 people had been evacuated from the surrounding country, and ash soon lay thirty centimeters deep around Bariloche, just across the Argentine border. For days the column held at ten kilometers, feeding a plume that scientists tracked as it lapped the Southern Hemisphere and completed its first full circuit of the globe by mid-June. Down on the ground it was an ordeal - buried towns, ruined pasture, grounded airlines, and people scraping grey ash from their roofs and lungs. Seen from orbit it was something else: a single volcano writing its signature in a thin pale ribbon clear around the bottom of the world.