The recently discovered Cerro Galan Caldera, an elliptical structure with a diameter of ~35 km, lies along the crest of the Andes Mountain chain in northwestern Argentina. A magnificently exposed related caldera, Cerro Panizos, lies farther north, along the Argentina-Bolivia border. The Cerro Galan caldera formed 2.2 million years ago with a colossal eruption that generated more than 1000 km3 of pumice-bearing pyroclastic flows (ignimbrites). The outflow facies of these flows radiate outward from caldera for distances of over 100 km. Within the caldera, the ignimbrites accumulated to a thickness of over 1.2 km. Resurgence probably began a few thousand years after its birth. At one time, the caldera enclosed a large lake, the remainder of which is present as a salty remnant on the western side of the caldera, called Laguna Diamante. The caldera floor was uplifted asymmetrically along its eastern side, probably as a consequence of a small intrusion of new magma above the one that cause the eruption.

Cerro Galan Caldera -- The caldera walls and resurgent floor of this north-south elliptical structure is evident in this false-colored Thematic Mapper Landsat image. Laguna Diamante lies in the southwest corner of the caldera. Courtesy of Peter Francis.

After resurgence, a thick pile of dacitic coulées erupted non-explosively along the northern caldera ring fracture about 2.1 million years ago. These probably represent the last vestiges of the magma that fed the eruption of pyroclastic flows. The non-explosive nature of these dacitic coulées is due to their low volatile content, which was lost during rapid vesiculation (degassing) of the magma by the caldera-forming eruption.