Twenty-one point nine million years ago, a volcanic eruption of extraordinary scale swept across what is now Chile — and in its very destructiveness, it became an act of preservation. The ash and rock that buried the ancient Andean landscape sealed it from the erosion of deep time, creating what geologists now recognize as a geological Pompeii: a frozen moment in the long, slow rise of one of Earth's greatest mountain ranges. Scientists studying this volcanic time capsule are finding within it rare and precise evidence of how the Andes were configured at a critical stage of their formation, of