summing-up
1What is a volcano?
Volcanic phenomena are caused by
•
it and is then termed lava.
When the lava reaches the surface, in
•
general, it accumulates and forms a
volcanic structure.
The way in which the magma rises to
•
the surface distinguishes:
–
central volcanoes, in which magma
rises through cylindrical ducts; and
–
linear volcanoes, in which magma
rises along fissures that penetrate
deep into the interior of the Earth.
2The different products of
eruptions
• Volcanoes throw out fluid, solid and
–chemical composition;
–temperature; and
–gas content.
Solid materials are known as
•
pyroclastic material: these are
fragments of solid material ejected
during the explosive activity of a
volcano, together with minute
particles of magma that has been
pulverised by the explosion.
V
• olcanic gases are mainly comprised
of water vapour and gases such as
carbon dioxide.
Shield volcanoes, broad and with
•
not very steep sides, are produced by
central eruptions of very fluid lava
(low in silica).
• Stratovolcanoes, large and coneshaped, are produced by alternating
layers of lava and pyroclastic
materials.
Calderas, large circular depressions
•
with a flat base and steep walls, are
formed following the collapse of the
summit of a volcanic structure.
• Cinder cones are accumulations of
solidified lava fragments.
4Types of eruption
Depending on the viscosity of the
•
variable water content) eruptions are
either:
–
Hawaiian type; or
–
Icelandic type.
If the activity is a mix of effusive•
explosive, but predominantly
effusive (with less fluid magma with
respect to previously) eruptions are
of the Strombolian type.
If the activity is a mix of effusive•
explosive (with viscous magma and
elevated gas content) eruptions will
have:
–
Vulcanian type
–
Plinian type; or
–
Peléan type.
When the activity is only explosive
•
(and there is an interaction between
magma and water) eruptions are of
the hydromagmatic type.
5Phenomena related to
volcanic activity
• In addition to eruptions, there are
ashes present on the slopes of a
volcano become wet, due to rain or
the melting of snow, and then turn
into mud that can flow several
kilometres downstream.
• Sudden emission of gases (as carbon
dioxide) which can be very
dangerous.
• Thermal springs, i.e. hot water
springs, arise when the water flowing
deep underground is heated by a
magma that is cooling in the vicinity.
• Geysers are “fountains” of water,
very warm and rich in minerals,
which are emitted at regular time
intervals.
6The geographical
distribution of volcanoes
• The distribution of active volcanoes
concentrate in long bands.
Volcanic phenomena are associated
•
with three different geographical
situations:
–along oceanic ridges;
–along the edges of a continent or
along an arc of islands, flanked by
deep oceanic trenches; or
–at isolated points (“hot spot”) in the
interior of continents or deep
oceanic plains.
the rise of magma, i.e. molten rock,
to the surface.
• After an eruption, the molten
material loses the gases contained in
gaseous materials.
The fluid products of volcanic
•
eruptions are lavas. Lavas differ from
each other in their:
3Volcanic landforms
The shape of a volcano depends on
•
the type of material erupted.
According to the shape of the
•
volcano geologists distinguish
between shield volcanoes,
stratovolcanoes, calderas, and cinder
cones.
magma and the gas content,
eruptions are distinguished as either
explosive or the less violent effusive.
• Different types of eruption may
alternate in the same volcano, but the
prevalent activity determines the
classification.
• In the presence of dominant effusive
activity (with fluid magma and
other phenomena related to volcanic
activity. Some of these are described
below.
• Lahars are massive mudflows that
may occur during or after an
eruption. This happens when the
on the Earth’s surface and major
eruptions of fluid lava on the ocean
floor is not random, but tends to
Lupia Palmieri, Parotto Osservare e capire la Terra - edizione azzurra © Zanichelli 2012
unità 3•I fenomeni vulcanici
1
summing-up
7Volcanoes in Europe and in
adjacent seas
• In Europe there are about 60 active
volcanoes, concentrated mainly in
three geographical areas.
• The volcanoes of the Atlantic region
are found in Iceland and the
archipelagos of the Azores, Canaries
and Cape Verde.
The volcanoes of Turkey and the
•
Caucasus are stratovolcanoes and
cinder cones.
The volcanoes of the Mediterranean
•
area are distributed in the Italian
Lupia Palmieri, Parotto Osservare e capire la Terra - edizione azzurra © Zanichelli 2012
peninsula, Sicily, the Channel of
Sicily, the Aegean and on the floor of
the Tyrrhenian Sea between the
Aeolian islands and Sardinia.
An area of now extinct volcanoes is
•
situated in Central Europe.
unità 3•I fenomeni vulcanici
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