Credit: © Will Beback

Eruptions of potassium-rich lavas in the Sierra Nevada were caused by oblique rifting of the Earth's crust, which in turn triggered a small extent of partial melting of the underlying layer, finds a new study. This is contrary to previous work attributing the volcanic activity to the removal of the dense root of the mountain range.

Keith Putirka from the California State University-Fresno and Cathy Busby from the University of California-Santa Barbara in the USA1 analysed the compositions of potassium-rich rocks of the Sonora Pass region. Their geochemical model simulations show that small extents of melting and subsequent crystallization are capable of enriching the melts in potassium and in chemically similar elements such as rubidium and barium. This, combined with their finding that the melts tend to be water-rich and hence sufficiently buoyant for magmas to rise up through the thick crust of the Sierra Nevada, explains the composition of the rock samples.

The onset of oblique rifting of the Sierra Nevada lithosphere 10 million years ago coincides with the ages of the lavas and could have provided a pathway for the eruption of these magmas. The researchers suggest that the potassium-rich volcanism in the Sierra Nevada attests to the birth of a plate boundary.