Ora Caldera


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, 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 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 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.
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
. 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 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 Eruption DateRef
Ora calderaItaly8Rhyolite1290~277-274 Ma
Cottonwood Wash TuffUtah 8Dacite200031.1 Ma
Wah Wah Springs CalderaUtah 8Dacite590030 Ma
Lake TobaIndonesia8Rhyolite2800~74,000
Yellowstone Wyoming 8Rhyolite2450~2.1 Ma
La GaritaColorado 8Dacite3000+~27.8 Ma
Mount TamboraIndonesia7Trachyandesite1301815 AD
Lund CalderaUtah8Dacite440029 Ma
Yellowstone Wyoming 8Rhyolite1000630,000 BC
Long Valley Caldera California 7Rhyolite650~764,800 ± 600
KrakatoaIndonesia6Dacite to Rhyodacite18-211883 AD
Mount VesuviusItaly5Tephra, Phonolite6.479 AD
Mount Saint HelensWashington 5Dacite2.51980 AD
Lake TaupōNew Zealand8Rhyolite110025,500 BC
Mount PinatuboPhilippines6Dacite101991 AD
SantoriniGreece6-7Dacite78-861610 BC
NovaruptaAlaska 6Rhyolite171912 AD
Mount MazamaOregon 7Dacite1767700 BC
Valles Caldera New Mexico 7Rhyolite3001.2 Ma
Campi FlegreiItaly6Trachyte, Trachydacite4012,800 BC