Essay About Latest Activity And Pyroclastic Flows

Volcanos
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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
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 cones 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