Volcanos

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 this kind is mainly restricted to the

central crater where one or more intracrateral cones form, and to the sides of the cone. Lava

flows from the summit crater or from the sub terminal vents extend beyond the cone's base. A

somewhat particular kind of persistent activity is the slow release of large amounts of lava from

sub terminal fractures to form thick piles of lava with little lateral extension, such as the lava cupola

of Colle Umberto, formed in 1895-1899. (7) Vesuvius lies over a subduction zone. The two

plates are the African plate and the Eurasian plate. The African plate is moving northward at about

one inch (2-3 cm) per year and is slowly closing the Mediterranean basin. As it moves to the

north, the African plate is pushed beneath the Eurasian plate. The rocks at Vesuvius are called

tephrite. A tephrite is basaltic in character and contains the following minerals: calcic plagioclase,

augite, and nepheline or leucite. (8) Eruptive activity of Vesuvio noticeably occurs in cycles that

last several centuries and alternate with repose periods lasting several centuries. Each