Distributed-style volcanism is an end member of terrestrial volcanism that produces clusters of small volcanoes when isolated magma bodies ascend from broad magma source regions. Volcano clusters can develop over millions of years, one volcano at a time, and can be used to infer unobserved geologic phenomena, including subsurface stresses and cracks during eruption periods. The Tharsis Volcanic Province covers approximately one-quarter of the martian surface and hosts a large concentration of small volcanoes that formed from distributed volcanism. We present a catalog of 1106 small volcanic vents identified within Tharsis Volcanic Province. This catalog includes morphologic measurements for each cataloged vent. Vent lengths range from 71 m to 51 km, widths range from 40 m to 3.1 km, and 90% of vents have lengths at least 1.5 times their widths. Additionally, 90% of edifices associated with vents have topographic prominences <100 m. Vents are found throughout Tharsis, though they generally form clusters near large volcanoes or among large graben sets. Older regions with volcanic eruption ages of >1 Ga are found at the Tharsis periphery in the Tempe-Mareotis region and Syria Planum. Vents in the Tharsis interior have reported ages <500 Ma. Regional trends in vent orientation and intervent alignment are dependent on nearby central volcanoes and fossae. We use these findings to hypothesize that within the most recent 500 Ma, magma was present under and to the east of the Tharsis Montes and that some of this magma erupted and built hundreds of small volcanoes in this region.

The Blackfoot Reservoir volcanic field (BRVF), Idaho, USA, is a bimodal volcanic field that has hosted explosive silicic eruptions during at least two episodes, as recently as 58 ka. Using newly collected terrestrial and marine gravity data, two large negative anomalies (−16 mGal) are modeled as shallow (<1 km) laccoliths beneath a NE-trending alignment of BRVF rhyolite domes and tuff rings. Given the trade-off between density contrast and model volume, best-fit gravity inversion models yield a total intrusion volume of 50−120 km3; a density contrast of −600 kg m-3 results in model intrusion volume of 63 km3. A distinctive network of 340°−360° trending faults lies directly above and on the margins of the mapped gravity anomalies. Most of these faults have 5−10 m throw; one has throw up to ∼50 m. We suggest that the emplacement of shallow laccoliths produced this fault zone and also created a ENE-trending fault set, indicating widespread ground deformation during intrusion emplacement. The intrusions and silicic domes are located 3−5 km E of a regional, 20 mGal step in gravity. We interpret this step in gravity as a change in the thickness of the Upper Precambrian to lowermost Cambrian quartzites in the Meade thrust sheet, part of the Idaho-Wyoming Thrust Belt. Silicic volcanism in the BRVF is a classic example of volcanotectonic interaction, influenced by regional structure and creating widespread deformation. Exogeneous and endogenous domes are numerous in the region. We suggest volcanic hazard assessments should account for potentially large-volume silicic eruptions in the future.