Saying farewells to the Land of Enchanment, we crossed over from New Mexico into Colorado (although the Rio Grande rift knows no political borders). Venturing out of the Rio Grande basalts and paleo- Lake Alamosa, we set our sights to the massive calderas of the southern San Juan Volcanic Field (SJVF), which occurred in huge bursts between 25-30 Ma. We first visit tuffs sourced from the fascinating Platoro Caldera, in the southeast SJVF:

The Platoro caldera complex was the source of eight, largely dacitic Members of the Treasure Mountain Tuff. These units range in volume up to ~500‐1000 km3 … This  system  is  an  instructive  example  of  the  difficulty  of linking outflow  tuff  sheets, which present  relatively  straightforward  stratigraphic successions,  with  the much  less  well‐preserved  corresponding  units  in  their  caldera  sources.

Did we mention we get to see beautiful rivers and aspen-infused hills of Colorado high-country too?

Densely welded tuff of the Treasure Mtn group. Glassy black areas are fiamme: squashed pumice	Sam Johnstone, USGS Mendenhall Fellow, gets up close an personal with some Treasure Mountain vitrophyre
Mike Dungan protecting some lovely fiamme-bearing ignimbrite	Visiting an overlying, much more poorly welded member of the Treasure Mountain Tuffs
Paleontologist stoked about multiple types of pumice in the ignimbrite!	Paleontologist very happy to see hydrothermally improved rocks in the Platoro Caldera
Incredibly Densely welded intracaldera tuff of the Platoro caldera	Pre-caldera Conejos lava flow in foreground, with overlying thick package of course lava-derived sediments