2. Definition
• A volcano is an opening in the earths crust
that allows molten rock from beneath the
crust to reach the surface.
• If the molten rocks is beneath the surface –
called as magma
• If it flows – called as lava.
3. Causes
• At mid oceanic ridges, where there is
divergent plate – thermal convective currents
from mantle can easily emerge
• The splitting and lateral spreading of plates-
causes pressure release and
• At destructive plate margins – subducted
material melts and become volcanoes
• Weaker zones on crust and at mountain
building regions
4. Mechanism
• Gradual rise in temperature at the rate of 1 deg. C per
32 m due to heat generation from disintegration of
radioactive elements at depth
• Magma originates due to lowering of melting point
caused by fall in the pressure of overlying loads due to
splitting of plates
• Vapour and gases originate due to heating of water
which reaches underground through percolation
• Magma ascents with great force of gases & vapour.
• Depending upon the force and crustal surface , volcano
could be fissure type or explosive type
5. Classification
• On the basis of periodicity of eruption:
• Active volcanoes- constantly eject volcanic
lavas,gases,ashes, & fragmented materials. Ex.
Etna & Stromboli of Mediterranean sea
• Dormant volcanoes – they become quiet after
their eruption without any indication of future
eruption suddenly they erupt violently. Ex.
Mt.Vesuvius, Italy
• Extinct volcanoes – there are no indication of
future eruption. Ex.Mt. Kulal in Kenya
7. Volcanic materials
• Vapour and gases: 60 to 90% of vapour.and gases
include CO2, NOx,Sox, H2,CO, & compound like
HCl, chlorides of iron and pottasium are ejected
• Magma-molten material below the earth surface
• Lava – molten material above the earth surface.
SiO2 is more-acidic ,if SiO2 is less-basic. Basaltic
lava is more fluidic and hottest lava.
• Pyroclastic materials of varying size from mm to
cm size particles are thrown out.
