A volcano is an opening in the Earth's crust through which magma and gases erupt. It consists of three main elements - a vent, conduit, and magma chamber. There are different types of volcanoes defined by their structure, including composite/stratovolcanoes with steep slopes built up of layers of pyroclastic material, shield volcanoes with broad dome shapes built up by fluid lava flows, and cinder cone volcanoes built from loose bits of ejected magma. Volcanic activity occurs at constructive and destructive plate boundaries, with different eruption styles depending on the tectonic setting and rate of pressure release.

1. volcano

2. A volcano is an opening, or rupture, in a planet's surface or crust, which
allows hot magma, volcanic ash and gases to escape from below the
surface.
Volcanism –The processes by which magma and gases are transferred
from the Earth’s interior to the surface.

3. • To understand the cause of volcano
eruptions it is first necessary to
define the elements of a volcanic
system.
A volcano is composed of three basic
elements.
• A vent through which volcanic
material erupts, the conduit which is
the passage way allowing the
magma to reach the surface, and the
reservoir or magma chamber deep
in the crust of the Earth.
•
An eruption simply takes place when
the magma stored in the reservoir
reaches sufficient pressure to
overcome the resistance of the rock
that forms the roof of the magma
chamber.
.

6. • A volcanic vent is an opening exposed on the earth's surface
where volcanic material is emitted. All volcanoes contain a
central vent underlying the summit crater of the volcano.
• The volcano's cone-shaped structure, or edifice, is built by
the more-or-less symmetrical accumulation of lava and/or
pyroclastic material around this central vent system.
• The central vent is connected at depth to a magma
chamber, which is the main storage area for the eruptive
material. Because volcano flanks are inherently unstable,
they often contain fractures that descend downward toward
the central vent, or toward a shallow-level magma chamber.
Such fractures may occasionally tap the magma source and
act as conduits for flank eruptions along the sides of the
volcanic edifice. These eruptions can generate cone-shaped
accumulations of volcanic material, called parasitic cones.
Fractures can also act as conduits for escaping volcanic
gases, which are released at the surface through vent
openings called fumaroles.

9. • Composite Volcanoes
These types of volcanoes are also known as a stratovolcano. This is
the type of volcano most commonly imagined.
It is a cone shaped mountain with steep, smooth, barren slopes often
with a single plume of smoke emitted from a single central vent. It is
composed of alternating layers or strata of material created by
pyroclastic (lava) flows. Liquid magma is emitted from a reservoir
deep in the earth's crust.
In Costa Rica the Poas, Arenal, and Irazu volcanoes are examples of
composite volcanoes.

10. • Shield Volcanoes
A large volcanic structure with long gentle slopes built up almost
entirely from fluid lava flows.
The shape of this type of volcano is more like a dome than a tall
cone. This type of volcano is built slowly and can have multiple
vents producing lava flows.
The largest active volcano in the world is a shield volcano in
Hawaii named Mauna Loa. It stands almost 5 1/2 miles high (3 miles
under water and 2 1/2 miles above sea level). Almost 5 miles of this
volcanic structure is buried into the Pacific plate below ground level
which if included makes this structure about 10 miles in height!

11. • Cinder Cone Volcanoes
A cone structure built by an accumulation of
loose bits of magma called scoria that fall
around a vent or crater after being expelled
during moderately explosive activity.
They are the simplest of volcano structures and
are prevalent in Western North America and
elsewhere in the world.

12. • The mechanism of vulcanicity and volcanic eruptions is
closely associated with several interconnected process
such as (1) gradual increase of temperature with
increasing depth at the rate of 1 degree C per 32 m due to
heat generated from the disintegration of radioactive
elements deep within the earth, (2) origin of magma
because of lowering of melting point (partial melting)
caused by reduction in the pressure of overlying
superincumbent load due to fracture caused by spliting of
plates and their movement in opposite direction, (3) origin
of gases and vapour due to heating of water which reaches
underground through percolation of rainwater and melt-
water. (4) the ascent of magma forced by enormous
volume of gases and vapour and (5) finally the occurrence
of volcanic eruptions of either violent explosive central type
or quiet fissure type depending upon the intensity of gases
and vapour and the nature of crustal surface.
•

13. • Constructive plate boundaries:
• Most of the active fissure volcanoes are found along the mid-
oceanic ridges which represent splitting zones of divergent
plate boundaries. Two plates move in opposite directions
from the mid-oceanic ridges due to thermal convective
currents which are originated in the mantle below the plates.
This splitting and lateral spreading of plates creates fractures
and faults (transform fault) which cause pressure release
and lowering of melting point and thus materials of upper
mantle lying below the mid-oceanic ridges are melted and
move upward as magmas under the impact of enormous
volume of accumulated gases and vapour. The rise of
magmas along with mid-oceanic ridges causes fissure
eruptions of volcanoes and there is constant upwelling of
lavas. It is obvious that divergent or constructive plate
boundaries are always associated with

15. • quiet type of fissure flows of lavas because the pressure
release of superincumbent load due to divergence of plates
and formation of fissures and faults is a slow and gradual
process. Example: Azores islands situated on either side of
mid-Atlantic Ridge.
• Destructive plate boundaries: These are associated with
explosive type of volcanic eruptions. When two convergent
plates collide along Benioff zone (subduction zone),
comparatively heavier plate margin is subducted beneath
comparatively lighter plate boundary. The subducted plate
margin, after reaching a depth of 100 km or more in the upper
mantle, is melted and thus magma is formed. This magma is
forced to ascend by the enormous volume of accumulated
explosive gases and thus magma appears as violent volcanic
eruption on the earth’s surface. Such type of volcanic eruption
is very common along the destructive plate boundaries –
circum-Pacific belt and the mid-continental belt.

16. • The depth of the descending plate increases away from the
trench. These causes several systematic variations in
volcanic activity across an island arc.
• Volcanic activity typically begins abruptly along a line 200 or
300km landward from the trench axis. This line is called the
volcanic front. It occurs because 100km is the critical
depth at which partial melting produces enough magma to
migrate to the surface. Between the trench and the island
arc is a gap where no volcanic activity occurs. This area is
called the arc-trench gap.
• Volcanic island arc: A chain of volcanic islands.

17. Volcanic front
Arc-trench gap

18. • The volcanoes of the island arcs are caused due to
subduction of oceanic crust.
• Honshu island is bordered by Japan trench in the east
and Japan sea in the west. The western part is more
frequented by volcanic activities than the eastern part.
The Japan trench was formed due to subduction of
Pacific oceanic plate under the oceanic crust to the east
Japan. Since Japan is very close to the Japan trench
and hence western part of Japan is more frequented by
volcanic activities. This process is still continuing as the
Pacific plate is being continiously subducted under the
oceanic crust along the Japan Trench.

20. Active volcanoes are associated with mid-oceanic
ridges. Under the influence of rising thermal
convection currents oceanic plates (crusts) are
separated and two plates move in opposite
directions from the ridge crests. Because of
divergence of two plates the confining pressure of
superincumbent load is released and consequently
melting point is lowered which causes partial
melting which moves upward through ascending
thermal convection currents and appears as
fissure flow of basaltic lava. The volcanic
mechanism leads to formation of ridges parallel to
mid-oceanic ridges. The newly formed basaltic
crust is

21. • divided into two equal halves and placed on either
side of the ridge move away from the ridge due to sea-
floor spreading. Iceland presents an ideal example of
this mechanism because it is situated on both sides of
the mid-Atlantic ridge. There is continuous growth in
the surface area of Iceland due to basaltic lava. It is
estimated that the island has grown in size by 400km
since the beginning of Tertiary (65 mya) epoch, which
indicates average growth rate of 0.6 cm/yr. The age of
the lava (basalt) increases away from the ridges as
recent lava is found close to the ridge, 2 million year-
old lava away from the ridge and 65 million year old
lava at the margin of the island.
• The most active volcanic islands are located nearest
to the to the ridge whereas dormant and extinct
volcanoes are located at the farthest distance from the

22. • ridge.
• The volcanic islands are formed near the ridge due to
upwelling of magma from the below. As the sea-floor
spreads these volcanic peaks move away from the
ridge and magma source. When they move far away
from the ridge the supply of magma comes to an end
and thus most of these volcanic islands are
submerged under sea waves and become sea
mounts. Not all the peaks submerge beneath sea
waves as a few of them project from 1500 to 3000m
above sea level.

23. • Iceland is splitting along the
Mid-Atlantic Ridge - a
divergent boundary between
the North American and
Eurasian Plates. As North
America moves westward
and Eurasia eastward, new
crust is created on both sides
of the diverging boundary.
While the creation of new
crust adds mass to Iceland
on both sides of the
boundary, it also creates a rift
along the boundary. Iceland
will inevitably break apart into
two separate land masses at
some point in the future, as
the Atlantic waters eventually
rush in to fill the widening and
deepening space between.