Upload
bruno-watkins
View
217
Download
0
Tags:
Embed Size (px)
Citation preview
Volcanic activity – how, why and where it occurs Volcanic activity – how, why and where it occurs
How volcanoes and their effects may be predicted How volcanoes and their effects may be predicted
Volcanic activity and its effects on the Irish landscape Volcanic activity and its effects on the Irish landscape and worldwide and worldwide
33 The Geography of VolcanoesThe Geography of Volcanoes33 The Geography of VolcanoesThe Geography of Volcanoes
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
Where does volcanic activity occur?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
Chapter 3: The Geography of Volcanoes
Causes of a volcanic eruptionCauses 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
Will the volcanic eruption will be violent or not?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
Chapter 3: The Geography of Volcanoes
Volcanic materials ejected during volcanic activityVolcanic 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
LavaLava
There are two types of lava:
1. Acid lava
2. Basic lava
Chapter 3: The Geography of Volcanoes
Acid lavaAcid 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 lavaBasic lava
Low in silica content
Tends to be runny
Flows greater distances
Gases escape
Eruptions gentler
Volcano will have gently sloping sides/cones
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 Intrusive and extrusive
structuresstructures
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
Chapter 3: The Geography of VolcanoesIntrusive structuresIntrusive 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
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
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)