volcanic activity – how, why and where it occurs

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


Three stages in the ‘lifecycle’ of a volcano

1. Active

2. Dormant

3. Extinct



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



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


LavaLava

There are two types of lava:

1. Acid lava

2. Basic lava


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


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


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 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.


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


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


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


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


Positive effects of volcanic activity

Fertile soils

Tourism

Geothermal energy

Creation of new land

Building materials


Negative effects of volcanic activity

Lahars

Nuée ardente

Loss of life

Destruction by fire

Damage to property

Effect on the environment



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


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)

volcanic activity – how, why and where it occurs

Documents

geography of volcanoeschapter

magma cools

magma expandcauses

magma chambergases

downturned mound of

upturned mound of magma

silica silica

large dome of magma