CHAPTER 1 THE TECTONIC CYCLE

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3 The Geography of Volcanoes
Volcanic activity how, why and where it occurs
How volcanoes and their effects may be predicted
Volcanic activity and its effects on the Irish
landscape and worldwide
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Chapter 3 The Geography of Volcanoes

• What is a volcano? A feature with the
  following
• Magma chamber from which molten magma moves
• Vent
• Crater
• Volcanic cone

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Write down the location of each volcano
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Chapter 3 The Geography of Volcanoes

• Where does volcanic activity occur?
• Where plates separate (constructive plate
  boundaries)
• Where plates collide (destructive plate
  boundaries)
• At hotspots

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Chapter 3 The Geography of Volcanoes

• Three stages in the lifecycle of a volcano
• Active
• Dormant
• Extinct

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Chapter 3 The Geography of Volcanoes

• Causes of a volcanic eruption
• Rock of subducted plates melts and forms molten
  magma
• Molten magma pushes towards the surface and
  builds up in a magma chamber
• Gases dissolved in magma expand
• Causes the magma to rise
• Magma forces its way up through cracks/fissures
  in the volcano
• Reaches the surface, pressure is released and a
  volcanic eruption occurs

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Chapter 3 The Geography of Volcanoes

• Will the volcanic eruption will be violent or
  not?
• Silica
• Silica produces thick lava which traps gases
• The more silica present in magma, the greater the
  likelihood that a violent eruption will occur

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Chapter 3 The Geography of Volcanoes

• Volcanic materials ejected during volcanic
  activity
• Volcanic ash
• Rock particles known as pyroclasts
• Dust
• Gases
• Lava

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Chapter 3 The Geography of Volcanoes

• Lava
• There are two types of lava
• Acid lava
• Basic lava

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Chapter 3 The Geography of Volcanoes

• Acid lava
• High in silica content
• Tends to be thick
• Doesnt flow too far
• Gases become trapped
• Violent eruptions
• Volcano will have steep sides/cones

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Chapter 3 The Geography of Volcanoes

• Basic lava
• Low in silica content
• Tends to be runny
• Flows greater distances
• Gases escape
• Eruptions gentler
• Volcano will have gently sloping sides/cones

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Chapter 3 The Geography of Volcanoes

• There are three main types of lava flow
• Pahoehoe lava
• Aa lava
• Pillow lava

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Memory time
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Chapter 3 The Geography of Volcanoes

• Intrusive and extrusive structures
• Rocks formed within the Earth are called
  plutonic/intrusive rocks, e.g. granite
• Extrusive structures are those formed on the
  surface of the Earth
• Magma erupts through a crack/fissure/volcano to
  form extrusive rocks

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Chapter 3 The Geography of Volcanoes

• Intrusive structures
• Batholith
• Sills
• Dykes
• Laccolith
• Lopolith

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batholith
sills
laccolith
dykes
Were intrusivenow?
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Intrusive volcanic features
Intrusive features Sills magma cools and
hardens in a horizontal direction between two
layers of rock in the crust. Dykes Magma cools
and hardens vertically in the crust. Batholith
a large dome of magma that forced its way up
through the crust. Laccolith a small, upturned
mound of magma that cooled in the
crust. Lopolith a small, downturned mound of
magma that cooled in the crust.
Intrusive rock Granite
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Chapter 3 The Geography of Volcanoes

• Extrusive structures
• Volcanoes
• Island chains
• Hot spots
• Fissure eruptions

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Chapter 3 The Geography of Volcanoes

• Different shapes of volcanoes
• Shield volcanoes
• Volcanic domes
• Composite volcanoes
• Cinder volcanoes

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Chapter 3 The Geography of Volcanoes

• Hydrothermal areas
• Areas where volcanic activity once occurred
• Moisture has collected and is heated by magma
  then reaches the surface. Types
• Geysers
• Hot springs
• Black smokers

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Chapter 3 The Geography of Volcanoes

• How volcanic activity and its effects can be
  predicted
• Geologists forecast volcanic eruptions
• The type and date of materials and distribution
  of deposits can give insight into volcanic
  activity
• Tiltmeters identify bulging in the side of a
  volcano
• If a bulge is evident it may indicate that an
  eruption is imminent
• Seismographs measure earthquake activity they
  are positioned around the sides of a volcano

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Chapter 3 The Geography of Volcanoes

• How volcanic activity and its effects can be
  predicted - (continued)
• When magma begins to move it creates great heat,
  rocks begin to crack and break and this leads to
  vibrations of the Earths crust which can be a
  clear indication of a volcanic eruption being
  imminent
• Gases emitted from a volcano may suggest that an
  eruption will soon follow
• Steam coming out of vents, the appearance of
  geysers and of hot springs also indicate possible
  volcanic activity

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Chapter 3 The Geography of Volcanoes

• Positive effects of volcanic activity
• Fertile soils
• Tourism
• Geothermal energy
• Creation of new land
• Building materials

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Chapter 3 The Geography of Volcanoes

• Negative effects of volcanic activity
• Lahars
• Nuée ardente
• Loss of life
• Destruction by fire
• Damage to property
• Effect on the environment

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Chapter 3 The Geography of Volcanoes

• Mt St Helens case study
• Earthquake 18 May, 1980 at 8.32 am
• Measured 5.1 on the Richter scale
• Occurred directly below the northern slope of Mt
  St Helens
• Triggered a landslide, the largest in recorded
  history
• Travelled at speeds of 175 to 250 km/hr

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Chapter 3 The Geography of Volcanoes

• Mt St Helens case study
• Gigantic clouds of ash hovered some 16 miles
  above the mountain
• 57 human fatalities
• 7000 deer, elk and bear died. Birds and other
  small mammals were also killed.
• Elevation of the summit before the eruption was
  2,950 m (9,677 feet)
• Reduced by 400 m (1,314 feet) to its
  post-eruption elevation of 2,549 m (8,363 feet)

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