Three major cycles of volcanism during the Miocene and Pliocene formed a layered succession of calc-alkaline eruptive materials in the western San Juan Mountains nearly 1.5 miles thick and having a volume greater than 1,000 cubic miles. Each cycle was characterised by major eruptions followed by subsidence in the vent areas, and the resulting structure was a great volcanic plateau surrounding a complex of nested cauldrons. In the first cycle, cruption of several hundred cubic miles of tuff breccia and subordinate lavas was followed by subsidence that created the San Juan volcanic depression, about 15 miles wide and 30 miles long. During the second cycle, pyroclastic rocks and lava flows accumulated within this depression and on its borders, and the depression subsided further. During the third cycle, ash flows spead widely from centres within the depression, and their eruption resulted in formation and subsidence of the nearly circular comagmatic Silverton and Lake City cauldrons, each about 10 miles across, within the earlier depression. Cauldron subsidence in the second and third cycles was followed by resurgence and doming of the central blocks. Keystone grabens formed along the distended crests of the domed floors; graben faults formed in the third cycle were in part controlled by those formed in the second cycle. The distribution of post-cauldron radial and concentric fractures, dikes, and intrusive plutons, particularly around the Silverton cauldron, suggests that the underlying magma chamber must have been appreciably larger than the associated cauldrons.
