Vocca Island
Item
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Title
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Vocca Island
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Description
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We are in the deepest levels of the Basal Complex: it formed between 290 and 282 million years ago in the deepest part of the crust, thanks to continuous injections of magma from the mantle below.
During its slow cooling, the complex consisted partly of crystals that had already solidified and partly of material that was still molten: we could imagine it as a kind of gigantic 'boiling slush'. The resulting rocks outcropping here are gabbros, characterised by light and dark bands depending on the prevalence of two minerals: plagioclase, white, and pyroxene, black (Fig 1). A large dark boulder emerging from the river shows an area of accumulation of the black minerals: the rock forming it is an olivine pyroxenite (Fig. 2).
Here and there on the outcrop one can also see gabbros with traces of deformation that occurred at high temperature (Fig. 3).
The magmas that formed these rocks reached temperatures of over 1200°C: it was the large amount of heat given off by this enormous intrusion that triggered the extraordinary activity of the Sesia supervolcano.
If we take a closer look at the gabbros, we observe an obvious foliated structure: the constituent minerals are elongated and displaced in the same direction.
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Lithology
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gabbro
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Geologic structure type
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Foliation
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Event environment
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crustal setting
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Event process
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intrusion
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Older named age
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Permian
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Numeric age
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290My
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Older numeric age
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280My
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Interest
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Petrographic