Chapter 3: The Geography of Volcanoes What is a volcano? 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 What determines whether a volcanic eruption will be violent or not? Presence or absence of 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 1. Batholith 2. Sills 3. Dykes 4. Laccolith 5. Lopolith

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 and 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 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)