The Ora Caldera is an extinct Permian supervolcano in the Southern Alps of northern Italy. This and caldera was formed by a supereruption 277–274 million years ago that produced more than of rhyodacitic-rhyolitic ignimbrite. The ignimbrite from this eruption forms the Ora Formation, which represents the youngest eruptive unit of the Athesian Volcanic Group.
Geology Even though Ora erupted between 277 and 274 Ma (also known as The Permian Ora Formation), the preservation of erupted rhyolite is exceptionally good. This is because the southern Alps region has been remarkably stable. This means erosion exposed the Ora formation is largely undeformed and unmetamorphosed, with sub-horizontal dips and essentially no post-emplacement disruption.
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.
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. This belt covers over 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).
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. 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.
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.
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.
Eruption comparison
Below is a comparison of the Ora Caldera eruption to other explosive volcanic eruptions: NameLocationVEIRock TypeEruptive Volume (km3)Eruption DateRefOra caldera Italy8 Rhyolite 1290 ~277-274 Ma Cottonwood Wash TuffUtah (United States)8Dacite200031.1 MaWah Wah Springs CalderaUtah (United States)8Dacite590030 MaLake TobaIndonesia8Rhyolite2800~74,000Yellowstone (Huckleberry ridge)Wyoming (United States)8Rhyolite2450~2.1 MaLa GaritaColorado (United States)8Dacite3000+~27.8 MaMount TamboraIndonesia7Trachyandesite1301815 ADLund CalderaUtah8Dacite440029 MaYellowstone (Lava Creek)Wyoming (United States)8Rhyolite1000630,000 BCLong Valley Caldera (Bishop Tuff)California (United States)7Rhyolite650~764,800 ± 600 KrakatoaIndonesia6Dacite to Rhyodacite18-211883 ADMount VesuviusItaly5Tephra, Phonolite6.479 ADMount Saint HelensWashington (United States)5Dacite2.51980 ADLake TaupōNew Zealand8Rhyolite110025,500 BCMount PinatuboPhilippines6Dacite101991 ADSantoriniGreece6-7Dacite78-861610 BCNovaruptaAlaska (United States)6Rhyolite171912 ADMount MazamaOregon (United States)7Dacite1767700 BCValles Caldera (Bandelier Tuff)New Mexico (United States)7Rhyolite3001.2 MaCampi FlegreiItaly6Trachyte, Trachydacite4012,800 BC
See also List of volcanoes in Italy
References