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VOLCANOES AND EARTHQUAKES
CHARACTERISTICS
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SUDDEN PROCESSES
ACT SLOWLY
VERY SUDDEN EFFECTS
VOLCANOES AND EARTHQUAKES
VOLCANOES
• Sudden and abrupt explosion of the crust
• Magma, dust, smoke and solid materials
ejected
– Great pressure through a pipe known as vent
– Cone-like feature- volcanic cone
– Funnel shaped depression top of the cone- crater
– Sometimes secondary cones may also be formed
Causes of volcanic eruptions
• High temperature in the interior of the earth
– Melting of rocks at great depths.
• Weaker sections of the earth’s crust
• Gases- water vapour most effective
– Percolated rain water is heated inside
– Sea water along the coast also heated
– Increases heat and pressure within water vapour
• Earthquakes- release of pressure at fault zones
• Plate Tectonics- subduction
Materials ejected by volcanoes
• Gases- mainly steam- 60 to 90% of all gases
– Causes heavy rainfall following condensation
• Liquids- lava- temperature-600°C to 1200°C
– Speed varies- 15 km to 80 km per hour
• Solids- lava solidifies within the lava tubes
– At next eruption, solidified lava ejected out with
force
Types of volcaanoes
• Active- material ejected at frequent intervals
– Etna- 2500 years
– Stromboli- explodes every 15 minutes
• Dormant- eruption does not occur regularly
– Cause great damage to life and property
– Vesuvius-erupted only 10 times in last 1500 years
– Barren Island- India- erupted in 1991
• Extinct- Vent is blocked or moves away from
hot spot
– Rarely becomes active- Krakatoa 1883
Volcanic Landforms
• Ash or Cinder cone- Circular, cooling of granitic lava
around the crater
– average height- 300 m
– Concave sloped sides
• Basic lava cone (Shield)- Flatter, cooling of basic lavawide base, low slope, cools as sheets
• Composite cone- Largest and highest of all types
– Lava deposited parallely on top of each other
• Caldera- large roughly circular depression
• Crater Lake- Funnel shaped crater filled with rain
Influence of volcanoes on man
ADVANTAGES• Release of intense pressure- safety valve
• Helps in studying interior of the earth
• Fertile black soils
• Many valuable minerals
• Geysers, mineral springs and crater lakes
• Geo thermal energy
• Crater lakes- used for irrigation
Influence of volcanoes on man
DISADVANTAGES
• Highly destructive to life and property
• Cause earthquakes
• Lava flow destroys agricultural lands
• Undersea eruptions destroy marine life
• Poisonous gases are released- pollution to air
• Sudden changes in world maps- Krakatoa
Island
Distribution of volcanoes
• Mostly found at sea coasts, percolation of water
• At new fold mountains- thinner crust
• Boundaries of plates
– Bordering Pacific Ocean- 403 active volcanoes
– Mid-world belt- Alpine-Himalaya belt.
– African rift valley– Mid Atlantic ridge- Iceland
– Madagascar island
– Hawaiian Islands
EARTHQUAKES
• Tremors caused by passage of seismic waves
• Any sudden disturbance causes vibrations that travel
in all directions.
• 8000 to 10000 earthquakes occur all over the world,
1 per hour
• More common in the oceans.
• Originate at depths of 50 to 100 km below earth’s
surface
• Place of origin- hypocentre/focus.
• Point vertically above focus- epicentre, earthquake
strongest here
Causes of earthquakes
• Volcanic eruptions- most earthquakes occur in
volcanic regions
• Contraction of the earth- contraction creates
disorder in rocks
• Folding and faulting- compression and tension
in rocks. Bihar earthquake in 1934, Assam in
1950, Uttar Kashi in 1991
• Plate Tectonics- subduction of oceanic plates
• Artificial- running trains, heavy machines,
bomb explosions.
Effects of earthquakes
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Heavy loss to life and property
Disturb rock strata- change course of rivers
Tsunamis
Disrupts roads, creates huge faults
Causes landslides
Helps in studying the interior of the earth
Weathering due to landslides helps in soil formation
New landforms are created
Creation of bays- natural harbours, help in trade
Valuable minerals brought up to the surface
Distribution of earthquakes
• Similar to presence of volcanoes
– Circum-Pacific belt- 68% of world’s earthquakes
occur here
– Mid-world mountain belt- 21% of world’s
earthquakes, parallel to equator, fold mountains,
large depressions and active volcanoes
– Minor belts- African Lakes, Red Sea and Dead Sea
zones
Earthquakes in India
• Mostly confined to Himalayan region, Ganga
valley
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Kutch- 1819
Assam- 1877
Kangra- 1905
Bihar- 1934
Assam- 1950
Himachal- 1975
Uttarkashi- 1991
Kutch- 2001
Protection from earthquakes
• Cannot prevent, but safety can be assured
– Large number of scientific centres to be set up
– Warning system must be strong
– Building structures should be changed
– Prevent changes in environmental conditions
Effects of earthquakes- Tsunamis
• Sea quakes- violent movement in sea water
• Japanese word ‘tsu’ meaning harbour,’nami’
meaning wave
• Series of waves, generate near or below ocean
floor, can travel across oceans
Formation of Tsunamis
• Caused by any endogenous process
• Most destructive due to undersea earthquakes
• Strong earthquake 7.5 on the richter scale, tilts
or deforms oceanic crust, sea water above is
displaced, waves are formed as water tries to
retain its position, this vertical movement
generates tsunami waves
• Subduction zones, displacement of heavier crust
under lighter crust, creation of fault, tsunami
occurs
Propogation of Tsunamis
• Only caused due to vertical propogation of
waves in all directions
• Movement resembles ripples in a pond of
water
• Travels at high speeds( 500-800 km/hr) in
deep seas, long wave-lengths
• Cant be seen or detected from the air.
• As they approach shores, speed is reuced and
they attain enormous height>10 m to 30 m
Effects of Tsunamis
• Flood hundreds of metres of inland, flatten
houses, wipe out villages, uproot electric
poles, throw cars around, toss boats ashore &
drag helpless people back into the sea.
• Large rocks and debris may be transported
inland, it can travel up rivers and streams and
cause damage to crops.
• Tsunamis generated on one side of an ocean
may have its effects felt of the opposite side
Measuring earthquakes
• Measured in terms of intensity and magnitude
• Intensity means destructive power- severity of
ground motion, measured in relation to
effects on human life, directly observed by
people
• Mercalli scale- 1 to 12 intensities
• Intensity depends on– Total amount of energy released
– Distance of place from epicentre
– Types of rocks prrsent
Measuring earthquakes
• Magnitude- energy releases at the epicentre,
estimates severity of an earthquake.
• Exponential scale- 2 to 9
• Each increase in a whole number is 10 times
greater than the previous one