Final answer:
The San Juan volcanism stages are part of a complex volcanic history that has evolved over millions of years along the western margin of South America, with activity dating back to the Jurassic period. The exact number of stages or phases is not provided, but it includes both intrusive and extrusive igneous activity.
Step-by-step explanation:
The San Juan volcanism refers to volcanic activity associated with the San Juan Mountains, which is part of the extensive volcanic region along the western margin of South America. This region has experienced multiple stages or phases of volcanic activity due to the subduction of oceanic crust under the continental crust, a process that began in the Jurassic period, around 185 million years ago. The volcanic history in the Central Andes, particularly during the Neogene period between 23 and 2.5 million years ago, includes both intrusive and extrusive igneous rocks. The specific number of stages or phases of volcanic activity in the San Juan region is not explicitly stated in the provided reference material, but it is implied that the volcanic history is complex and evolved over millions of years with both explosive and less hazardous eruptions contributing to the formation of various geological features.