Ora Caldera

Eruption

The eruptions of the Ora Formation between 277-274 Ma were characterized by crystal-rich (~25 to 55%) and ubiquitously welded. This means the eruption was highly explosive and high volume ignimbrite.^[2]

Ora is the youngest eruption of the larger Athesian Volcanic Group (AVG) during the Carboniferous. Volcanism here is tied to the closing of the Paleo-Tethys Ocean, large scale extensional and strike-slip tectonics, and the collapse of the Variscan Orogenic Belt.^[2] This belt covers over 2,000 square kilometres (770 mi²) in northern Italy. Pre eruption strata includes two unconformities'. The lowest is made up of a Variscan South-Alpine metamorphic basement, while the upper layer is continental red beds (Val Gardena Sandstone).^[6]

Evidence shows the eruption started in the south portion of the caldera and progressed to the north, forming the two different caldera depressions. There is also evidence of multiple magma chambers in the Ora system.^[7] This also might show that there were multiple vents erupting at once, incremental caldera filling of subtly compositionally different pyroclastic flow pulses, and an eruption of lower intensity.^[7]

Volcanic rock characteristics

The Andriano Formation is one of the best and most well preserved formations in the Ora complex. It is a massive, rhyolitic lava of red-orange to brick red or red-purple color, with diffuse flow bands of variable thickness (5 to 20 cm). The shape of flow bands range from flat-parallel or slightly wavy to strongly wavy and some flow bands even intersect each other with variable angles.^[6]

Volcanic rocks show a porphyric structure with idiomorphic phenocrysts (1-4 mm) of feldspar and quartz that appears to have completely recrystallized under a microscope. It is also crystal-rich, poorly sorted, and has common fiamme and pumice.^[6][2]