Closing in on the home stretch of this field trip, things just keep getting bigger and better! We ventured into one of the world’s largest, and best studied “nested caldera” system – the Central San Juan Caldera Complex.

This complex erupted a phenomenal amount of material, 8000 km^3 of magma, in a shockingly short period of 2 million years (trust us, that is short geologically speaking!). For comparison, the cataclysmic 1980 eruption of Mt. St. Helens in Washington, which produced ashfall all the way into Montana, led to lahars for decades, partially dammed the Columbia River, and killed over 50 people, produced 1 km^3 of magma. In this light, the San Juan volcanic field is immense on almost unimaginable proportions. Our official goals for our final two days of the trip are:

[Investigating the] Central  San  Juan  caldera complex,  which  counts  the  twin  distinctions  of:  (1)  being  one  of  the  volumetrically  largest  multicyclic caldera  complexes  of  its  kind  (>8000  km^3 of  tuff  in  eight  units),  and  (2)  being  as  well  mapped  and studied as any Cenozoic caldera system in the world…. We  will  view  many  of  the  highlights  of  this  system  for  the purpose of completing an overview of caldera‐related volcanism. Among these are the Fish Canyon Tuff (at  least  5000  km^3),  the  highly  unusual  Pagosa  Peak  Dacite  (~200  km^3),  the  immense  scale  of  the  La Garita caldera (90 x 45 km),  geologic relations along the margins of  calderas that are nested within the La Garita, the resurgent domes of the La Garita, Creede, and Bachelor calderas, and more.

Basal mudstones, lacustrian deposits (or maybe surges?? [probably not]), draping plinian pumice deposit, and base of the massive Fish Canyon Tuff. This site has, and continues to, cause much controversy