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 3 The 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 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

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

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 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

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)