A tear in the tectonic plate
The Cerro Solo Alkaline Volcanic Complex is part of a series of unusual volcanic fields in southern Patagonia. Unlike the famous volcanoes of the Andes, which are fueled by the subduction of an oceanic plate beneath the continent, these volcanoes have a much deeper and stranger origin. Their existence is tied to a "slab window"—a gap that opened in the subducting Nazca tectonic plate starting around 12-14 million years ago. This tear in the plate allows hot, upwelling asthenospheric mantle to rise from depths and melt, a process normally associated with mid-ocean ridges or volcanic hotspots like Hawaii.
The volcanic activity that formed Cerro Solo is geologically recent, part of a "post-plateau" sequence of eruptions that occurred between 7 and 2 million years ago. This activity produced a series of small volcanic structures, including scoria cones and lava flows, covering the older, more voluminous basaltic plateaus of the region. The total erupted volume of this later alkaline phase is estimated to be around 100 cubic kilometers. The lavas themselves are distinct, classified as basanites and nephelinites—alkaline rocks low in silica and high in sodium and potassium.
Geochemical fingerprints of the deep Earth
The chemical composition of the lavas at Cerro Solo provides a direct sample of the Earth's mantle from beneath Patagonia. Geochemists analyze the ratios of specific isotopes, such as strontium (Sr), neodymium (Nd), and lead (Pb), to trace the magma's origin. The lavas here show a strong "Ocean Island Basalt" (OIB) signature. This geochemical fingerprint is characteristic of magma sourced from deep within the asthenosphere, the convecting part of the upper mantle.
This OIB signature confirms that the magmas ascended through the slab window with little interaction or contamination from the overlying South American continental crust or the remnants of the subducted plate. The composition of the lavas, particularly their low concentrations of heavy rare earth elements, indicates that the melting process began at depths near 70 kilometers, just below the garnet-spinel transition zone in the mantle. The resulting magma was produced by a relatively low degree of partial melting, around 7%, of this upwelling asthenospheric source. This makes Cerro Solo and similar Patagonian volcanoes sites for studying deep mantle processes that are usually inaccessible.