The Vilama caldera, which is roughly rectangular (35–40 km × 15–18 km) in shape and has central coordinates of 22°24′ S and 66°57′ W, is considered to have formed in a single-stage collapse event. As the topographic rim is only seen on the western side, the extent of collapse and geometry of the caldera are incompletely known and inferred from indirect data. Possible collapse geometries include a slightly asymmetric single-block subsidence and non-chaotic multiple-block collapse. Estimated erupted volumes range from 1800 to 1200 km3, or 1400 to 1000 km3 in dense rock equivalents.
The properties of the Vilama ignimbrite which include a crystal-rich and pumice-poor nature, a high degree of welding and induration and a prodigious volume, suggest that an external drive, rather than volatile overpressures, controlled and maintained the eruption. The best candidate is caldera subsidence triggered by the instability of a magma chamber roof above a batholith-scale magma body. Transtensive/distensive tectonic stresses resulted in the northwest elongation of the magma chamber and/or influenced the roughly rectangular/subelliptical shape of the subsided block.
As most of the Vilama ignimbrite is within the caldera, subsidence must have started early in the eruptive history. Immediately before, or concomitantly with the onset of subsidence, the lower extracaldera unit was deposited from flows that formed during collapse of well-developed plumes with efficient convective phases. Once caldera collapse was completely established, the eruption dynamic changed dramatically, and the extracaldera and intracaldera facies erupted. During this phase, unstable and low plumes (boiling-over collapse fountains) preferentially collapsed towards the interior of the subsiding structure, causing the ignimbrite volume to be concentrated in the caldera. After much less than 1 Ma, resurgence, which might be linked to magma chamber recharge, domed the intracaldera facies and caused post-collapse volcanism to be channeled through subsidence and/or resurgence-related faults.