Through most of the Mesozoic the Sierra Nevada of California was an active subduction-related continental arc like the Cascades or Andes of today. This boulder of granite from the Rocky Hill Pluton in the Sierra Nevada has major and trace element compositions common in continental rocks but radiogenic isotope signatures of mantle affinity. Samples like this may contain the key to understanding the role of island arc magmatism is the growth of continents over Earth history. In this picture, Moore Scholar A.M. Celâl Sengör, visiting Caltech from Instanbul Technical University, is moved to a public display of affection towards this rare and potentially important specimen.

Photo courtesy of Paul Asimow