Volcano Mount Vesuvius

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  • Topic: Mount Vesuvius, Volcano, Pyroclastic flow
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  • Published : March 8, 2000
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Mount Vesuvius is a volcano located in southern Italy, near the bay of Naples and the city of Naples. It is the only active volcano on the European mainland. Vesuvius rises to a height of 1277 m (4190 ft). Vesuvio (Vesuvius) is probably the most famous volcano on earth, and is one of the most dangerous. Mount Vesuvius is a strato-volcano consisting of a volcanic cone (Gran Cono) that was built within a summit caldera (Mount Somma). The Somma-Vesuvius complex has formed over the last 25,000 years by means of a sequence of eruptions of variable explosiveness, ranging from the quiet lava outpourings that characterized much of the latest activity (for example from 1881 to 1899 and from 1926 to 1930) to the explosive Plinian eruptions, including the one that destroyed Pompeii and killed thousands of people in 79 A.D. At least seven Plinian eruptions have been identified in the eruptive history of Somma-Vesuvius (1). Each was preceded by a long period of stillness, which in the case of the 79 A.D. eruption lasted about 700 years. These eruptions were fed by viscous water-rich phonotitic to tephritic phonolitic magmas that appear to have differentiated in shallow crustal conditions. They are believed to have slowly filled a reservoir where differentiation was driven by compositional convection. A minimum depth of about 3 km was inferred for the top of the magmatic reservoir from mineral equilibria of metamorphic carbonate ejecta (2). Fluid inclusions ([CO.sub.2] and [H.sub.2]O-[CO.sub.2]) in clinopyroxenes from cumulate and nodules indicate a trapping pressure of 1.0 to 2.5 kbar at about 1200 [degrees]C, suggesting that these minerals crystallized at depths of 4 to 10 km (3). The differentiated magma fraction was about 30% of the total magma in the reservoir, and a volume of about 2 to 3 [km.sup.3] was inferred for the reservoir (4). The magma ascent to the surface occurred through a conduit of possibly 70 to 100 m in diameter (5). A thermal model predicts that such a reservoir should contain a core of partially molten magma (6) that can be detected by high-resolution seismic tomography.

The earliest outcropping volcanic deposits date back to about 25,000 years ago. The lavas observed at a -1125 m bore-hole are about 0,3-0,5 million years old. It is known for the first eruption of which an eyewitness account is preserved, in 79 AD. Geologically, Vesuvio is unique for its unusual versatility. Its activity ranging from Hawaiian-style release of liquid lava, fountaining and lava lakes, over Strombolian and Vulcanian activity to violently explosive, plinian events that produce pyroclastic flows and surges. Vesuvius is a complex volcano. A complex volcano is "an extensive assemblage of spatially, temporally, and genetically related major and minor [volcanic] centers with there associated lava flows and pyroclastic flows." Vesuvius has a long history. The oldest dated rock from the volcano is about 300,000 years old. It was collected from a well drilled near the volcano and was probably part of the Somma volcano. After Somma collapsed about 17,000 years ago, Vesuvius began to form. Four types of eruption have been documented: a) Plinian (AD 79, Pompeii type) events with widespread air fall and major pyroclastic surges and flows; b) sub-Plinian to Plinian, more moderately sized eruptions (AD 472, 1631) with heavy tephra falls around the volcano and pyroclastic flows and surges; c) small to medium-sized, Strombolian to Vulcanian eruptions (numerous events during the 1631-1944 cycle, such as 1906 and 1944) with local heavy tephra falls and major lava flows and small pyroclastic avalanches restricted to the active cone itself. The fourth type it is the smallest of all eruption types observed at Vesuvio. It is the persistent Strombolian to Hawaiian style eruption that characterizes almost all of an eruptive sub-cycle, such as was the case during the period 1913-1944. Activity of...
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