A key mechanism responsible for a systematic bias in the global geomagnetic paleointensity database.

Many eroded calderas expose associated postcollapse plutons, but detailed fieldwork‐supported studies have rarely focused on the internal structure that can contribute to understanding of emplacement dynamics. The Alamosa River monzonite pluton is a postcollapse intrusion at the Platoro caldera complex that erupted six large ignimbrites between 30.2 and 28.8 Ma in the Southern Rocky Mountains volcanic field. Magnetic fabrics in this intrusion indicate the pulsed emplacement of a vertically extensive pluton. The magmatic pulses are documented by three concentric domains of magnetic foliations elongated in ~NE‐SW direction, corresponding to structural trends at the Platoro caldera complex and preexisting regional structures. As no evidence for deformation of wall rocks and the adjacent resurgent block has been identified, we interpret the Alamosa River pluton as a postresurgent intrusion. The space‐opening process involved magmatic stoping and small‐scale magma wedging. New SHRIMP‐RG U/Pb zircon dates (28.98 ± 0.18, 27.42 ± 0.35, and 27.32 ± 0.38 Ma) suggest a magmatic lifespan of ~1.7 My for the Alamosa River pluton. Our results indicate that postcaldera magmatism includes pulsed and protracted activity from large intracaldera resurgent plutons to smaller postresurgent stocks and sheeted complexes. As demonstrated by the Alamosa River pluton, some intrusions are emplaced shortly after collapse and resurgence, but postcaldera volcano‐plutonic systems may remain active for several million years or more. We also suggest that subvolcanic magma bodies may be assembled incrementally and that the record of early composite magma lenses preserved as magma wedges are later obliterated by convective flowage and crystallization.

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Data on the evolution of magmatic plumbing systems are essential for the understanding of magma motion in monogenetic volcanoes and associated volcanic hazard. Combined field (relationships and petrography) and laboratory (rock magnetism, anisotropy of magnetic susceptibility, and paleomagnetism) analysis of a quarried scoria cone offers a unique opportunity to study its magmatic plumbing system. In this study magma emplacement processes were constrained using these methods at two vents, Cerro Pequeño and Cerro Grande, of the composite Cienega scoria cone volcano, in New Mexico, USA. The dikes and lava flows associated with Cerro Pequeño were emplaced in two stages within a short period of time relative to secular variation. In the first stage, the magma was injected into the main conduit of the Cerro Pequeño and then channeled into secondary conduits (e.g., the exposed dike system) moving away from the vent. In the late stage, magma from the northern Cerro Grande was injected into the Cerro Pequeño dike system toward its main vent. The dynamic evolution of the magmatic system observed at the Cienega volcano is similar to that observed in other larger volcanic edifices. As in this study, the investigation of the magma plumbing systems of many scoria cones reveals that the subsurface magmatic system involves multiple feeder conduits that evolve during the life of the volcanic system. This study provides significant insight for the characterization of magma propagation at the shallow level beneath the volcanic edifice and highlights the related hazards associated with small active volcanic systems worldwide.