The volcanics of the San Venanzo area (Terni) were erupted from three local vents that built up small asymmetric pyroclastic structures associated with lava flows: San Venanzo-maar to the north; Pian di Celle tuff-ring, less than one kilometre south of San Venanzo; and Celli lapilli-cone, about 500 metres east of Pian di Celle. The volcanic morphologies, and the maar-diatreme crater structures in particular, are fairly well preserved. Explosive activity produced about 6X106 m3 of pyroclastic material, mostly lapilli-sized, whereas more than 1x106 m3 of lava was outpoured. The overall sequence shows that the initial crater-forming explosions were followed by strombolian activity. The San Venanzo eruptions caused progressive widening and deepening of the craters, which remained set in the sedimentary substratum. The initial products of San Venanzo volcano consist of breccias containing large accessory-blocks coming from the sedimentary substratum. Late products consist of mainly lapilli-sized juvenile deposits. Pyroclastic surge and minor pyroclastic flow are the main depositional mechanisms. The presence of nucleated, concentric-shelled glassy "lapilli" formed in subvolcanic conditions is typical and is associated with the mobilisation and eruption of diatremic breccia (tuffisite). The juvenile fraction of the pyroclastic rocks contains large amounts of calcite up to 50% in vol., whose composition and texture are suggestive of a magmatic origin. Numerous levels of fine-grained primary calcite, with volcanic-sedimentary structures, are considered as products of the fall of carbonatite ash.

Geological map of the San Venanzo volcano (Central Italy): explanatory notes.

STOPPA, Francesco;
1995-01-01

Abstract

The volcanics of the San Venanzo area (Terni) were erupted from three local vents that built up small asymmetric pyroclastic structures associated with lava flows: San Venanzo-maar to the north; Pian di Celle tuff-ring, less than one kilometre south of San Venanzo; and Celli lapilli-cone, about 500 metres east of Pian di Celle. The volcanic morphologies, and the maar-diatreme crater structures in particular, are fairly well preserved. Explosive activity produced about 6X106 m3 of pyroclastic material, mostly lapilli-sized, whereas more than 1x106 m3 of lava was outpoured. The overall sequence shows that the initial crater-forming explosions were followed by strombolian activity. The San Venanzo eruptions caused progressive widening and deepening of the craters, which remained set in the sedimentary substratum. The initial products of San Venanzo volcano consist of breccias containing large accessory-blocks coming from the sedimentary substratum. Late products consist of mainly lapilli-sized juvenile deposits. Pyroclastic surge and minor pyroclastic flow are the main depositional mechanisms. The presence of nucleated, concentric-shelled glassy "lapilli" formed in subvolcanic conditions is typical and is associated with the mobilisation and eruption of diatremic breccia (tuffisite). The juvenile fraction of the pyroclastic rocks contains large amounts of calcite up to 50% in vol., whose composition and texture are suggestive of a magmatic origin. Numerous levels of fine-grained primary calcite, with volcanic-sedimentary structures, are considered as products of the fall of carbonatite ash.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11564/110743
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