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
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
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
Chapter 3: The Geography of Volcanoes
Three stages in the ‘lifecycle’ of a volcano
1. Active
2. Dormant
3. Extinct
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
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
Chapter 3: The Geography of Volcanoes
Volcanic materials ejected during volcanic activity
1. Volcanic ash
2. Rock particles known as pyroclasts
3. Dust
4. Gases
5. Lava
Chapter 3: The Geography of Volcanoes
Lava
There are two types of lava
1. Acid lava
2. Basic lava
Chapter 3: The Geography of Volcanoes
Acid lava
 High in silica content
 Tends to be thick
 Doesn’t flow too far
 Gases become trapped
 Violent eruptions
 Volcano will have steep
sides/cones
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
Chapter 3: The Geography of Volcanoes
There are three main types of lava flow:
1.
Pahoehoe lava
2.
Aa lava
3.
Pillow lava
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
Chapter 3: The Geography of Volcanoes
Intrusive structures
 Batholith
 Sills
 Dykes
 Laccolith
 Lopolith
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.
Chapter 3: The Geography of Volcanoes
Extrusive structures
1.
Volcanoes
2.
Island chains
3.
Hot spots
4.
Fissure eruptions
Chapter 3: The Geography of Volcanoes
Different shapes of volcanoes
1.
Shield volcanoes
2.
Volcanic domes
3.
Composite volcanoes
4.
Cinder volcanoes
Chapter 3: The Geography of Volcanoes
Hydrothermal areas
1.
Areas where volcanic activity once occurred
2.
Moisture has collected and is heated by magma & then reaches the
surface. Types:
I.
Geysers
II.
Hot springs
III. Black smokers
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
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 Earth’s 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
Chapter 3: The Geography of Volcanoes
Positive effects of volcanic activity
 Fertile soils
 Tourism
 Geothermal energy
 Creation of new land
 Building materials
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
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
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)