A volcanic eruption 22 million years ago froze a snapshot of the Andes in their youth, revealing a slow-and-steady formation of this formidable mountain range. The eruption, which originated from the Lauca Caldera in what is now northern Chile, blanketed the landscape in a formation known as ignimbrite. This rock is left over after a pyroclastic flow ‪—‬ a mix of rock, volcanic gas and ash that sweeps over the landscape like a broiling tsunami. Pyroclastic flow is what buried the Roman town of Pompeii after the eruption of Mount Vesuvius in A.D. 79, preserving buildings and people where they fell. Now, a new study published Sept. 11 in the journal Science Advances extrapolates the landscape ...