Lonquimay reaches 2,865 metres in the La Araucania Region of Chile, a snow-capped stratovolcano shaped like a cone with its top sliced off. Its rock is mostly andesite, with streaks of basalt and dacite woven through, the record of countless eruptions stacked one atop another since late in the Pleistocene. It keeps powerful company. Tolhuaca volcano stands immediately to its northwest, and to the south rise Sierra Nevada and the restless giant Llaima. Around its base spreads the protected wilderness of Malalcahuello-Nalcas, a landscape of araucaria forest and old lava that softens the mountain's hard volcanic bones.

The 1988 eruption was, in geological terms, an act of construction. For three weeks beforehand the ground had trembled with rising magma, and when the flank vent finally opened on December 25 it began not to destroy the mountain but to add to it. Over thirteen months the eruption heaped up loose cinder and rock into an entirely new cone, growing it more than 200 metres high. Lava flowed from the vent and crept northeast down the slope, and as the months passed the flow slowed and the vent narrowed, yet by the end the andesite had still run more than ten kilometres from its source. The eruption rated a 3 on the Volcanic Explosivity Index, enough to loft ash into the troposphere and inflict serious local damage.

Scientists who have modelled the inner architecture of Lonquimay have reached a striking conclusion: the volcano has essentially grown as tall as it ever will. The plumbing beneath it now favours the flanks over the summit, which is why Navidad erupted from the side rather than the peak. Any future large eruption, the research suggests, will most likely break out low on the mountain again, building new cones across its slopes rather than raising the summit any higher. Lonquimay, in other words, has finished growing up and may now only grow outward, one flank eruption at a time.

For the people living below, the eruption was measured not in cubic kilometres of lava but in loss. The ash that drifted from Navidad carried a heavy load of fluorine, and as it settled across some 800 square kilometres of grassland it poisoned the pasture. Livestock that grazed the contaminated grass sickened and died; a veterinarian in the town of Lonquimay tallied 4,158 animals lost, among them more than two thousand goats and nearly fifteen hundred sheep, the herds on which mountain families depended. In July 1989 the authorities began evacuating thousands of residents from the town of Lonquimay and the nearby Bernardo Nanco area. Only one person died, an elderly man whose lungs gave out after long exposure to the ash, but the toxic fallout lingered in the soil and water for as long as two years, a slow tax exacted on the communities the mountain had chosen for neighbors.

Most of the time, Lonquimay does none of this. The snow gathers on its truncated cone through the long austral winter, and the araucaria forests of Malalcahuello-Nalcas stand undisturbed at its feet, their thousand-year-old crowns indifferent to a mountain that erupts only on the timescale of human generations. The pewen, sacred to the Mapuche, have weathered far more eruptions than any person ever will. It is worth remembering, looking up at that serene white peak, that geology measures patience differently. A mountain that built a new cone within living memory is not finished; it is merely resting between sentences.