9.1. Volcanoes

Question 1

Global Distribution of Volcanoes

Answer 1

• most found at plate boundaries
• 3/4 found on the Pacific Ring of Fire
• some exceptions - Hawaii

Question 2

Types of Volcanoes

Answer 2

1) Stratovolcano
2) Shield Volcano
3) Cinder Cone

Question 3

Stratovolcano (composite) features

Answer 3

• steep-sided cones formed from layers of ash and acidic lava flows
• contain complex internal networks of lava flows which contain intrusive (below ground) igneous features such as sills and dykes
• eruptions from these volcanoes may be a pyroclastic flow rather than a flow of lava
• explosive eruptions - andesitic lava
• viscous lava that cannot travel far

Question 4

Shield volcano features

Answer 4

• low with gently sloping sides formed from layers of lava
• non-explosive eruptions
• produce fast flowing basic (fluid) lava that can flow for many miles
• usually found at divergent (constructive) boundaries and sometimes at volcanic hotspots

Question 5

Cinder cone features

Answer 5

• has a cone shape, but is much smaller than a composite volcano
• rarely reach 300 meters in height but they have steep sides
• composed of small fragments of rock, such as pumice, piled on top of one another
• usually have a crater at the summit

Question 6

Effusive eruptions

Answer 6

• Gentle eruptions
• Magma has low viscosity (runny) which means that gases escape easily and when magma erupts at the surface, it forms lava flows
• Common at constructive boundaries and hotspots
• Often shield volcanoes
• Has basaltic lava

Question 7

Explosive eruptions

Answer 7

• Explosive eruptions
• Magma has high viscosity (sticky) which means that gases are trapped and build up in pressure, and the magma erupts explosively out of the volcano
• Common at destructive boundaries
• Often stratovolcanoes
• Has andesitic lava

Question 8

Volcano Primary Hazards

Answer 8

1) Pyroclastic Flows
2) Lava Flows
3) Gases

Question 9

Pyroclastic Flows

Answer 9

a fast-moving cloud of extremely hot gas, ash and
rock fragments, which can reach temperatures of about 1000°C and travel at speeds of up to 700km/h.
- most hazardous of volcanoes
- ‘glowing avalanche’ - Nuées Ardentes

Question 10

Lava Flows

Answer 10

streams of molten rock that pour from an erupting vent - during either non-explosive activity or explosive lava fountains

Question 11

Gases

Answer 11

Magma contains dissolved gases that are released into the atmosphere during eruption. Gases that have highest hazards are sulphur dioxide, carbon dioxide, and hydrogen fluoride.

Question 12

Volcano Secondary Hazards

Answer 12

1) Lahars
2) Volcanic Landslides
3) Tephra

Question 13

Lahars

Answer 13

Hot or cold mixture of water and rock fragments, flowing down the slopes of a volcano and/or river valleys. It carries rock debris ranging in size from clay to boulders.

• almost always occur on stratovolcanoes
• can crush almost anything in its the path

Question 14

Volcanic Landslides

Answer 14

Large masses of rock and soil that fall, slide, or flow very rapidly under the force of gravity

• destroys everything in its path
• may generate a variety of related activity such as causing explosive eruptions, bury river valleys with rock debris, generate lahars, trigger waves and tsunamis

Question 15

Tephra

Answer 15

General term for fragments of volcanic rock and lava regardless of size that are blasted into the air by explosions or carried upwards by hot gases in eruption columns or lava fountains

• from less than 2mm to more than 1m in diameter
• Ash is less than 2mm - covers everything - disruptive
• Lapilli range from 2 to 6mm in diameter
• volcanic bombs are larger than 64mm in diamter

Question 16

Volcanoes definition

Answer 16

Surface landform resulting from the extrusion of magma from underground

Question 17

What makes some volcanoes more hazardous than others?

Answer 17

1) Viscosity of magma
2) Plate Margins
3) Explosiveness of eruptions
4) Materials Ejected
5) Proximity to Population centres
6) Frequency of eruptions and perception of risk
7) Predictions, forecast and reactions

Question 18

Why do people live near volcanoes?

Answer 18

etc. Mount Fuji, Japan
- Fertile soils near volcanoes
- Resources such as building materials and hot water are present
- Tourism

Question 19

Effect of materials ejected on volcanic hazards

Answer 19

• Non-explosive eruptions - lava flow - not much of a hazard to people. but are to farmland and buildings
• Explosive eruptions - ash clouds, pyroclastic flows, lahars, tsunamis

Question 20

Volcanic Explosivity index

Answer 20

• Explosiveness of volcanic eruptions can be measured by the Volcanic Explosivity index (VEI)
• 0 to 8
• 0 for non-explosive, 8 for explosive eruptions larger than any in history
• The higher the VEI, the greater the potential hazard

Question 21

Effect of frequency of eruptions and perception of risk on volcanic hazards

Answer 21

• In Hawaii, volcanoes erupt frequently, but the threat minor to people, so building hotels and cafes on the flanks of the mountain are considered worth the risk to build and maintain
• For many people living close to destructive margin volcanoes, eruptions are very infrequent and happen like around every 100 years.
• Memories are short and people are less inclined to worry about an eruption from a volcano that has been inactive for many generations
• The frequency of eruptions therefore has a profound impact on peoples’ perception of the volcanic hazard

Question 22

Effect of predictions, forecast and reactions on volcanic hazards

Answer 22

• Now, active volcanoes are monitored using a vast array of scientific equipment
• Therefore, scientists can provide reasonably accurate forecasts of impending eruptions and people can be evacuated
• However, governments do not always respond to scientific predictions and disasters result

Question 23

Types of Magma (RAB)

Answer 23

1) Rhyolite
2) Andesite
3) Basalt

Question 24

Rhyolitic Magma

Answer 24

• most viscous
• highest silica content (4-6%)
• 70% gas content
• From explosive eruptions
• Cinder cones
• Example: Yellowstone volcano

Question 25

Andesitic Magma

Answer 25

• medium viscous
• Intermediate silica content (3-4%)
• 60% gas content
• From explosive eruptions
• Stratovolcanoes
• Example: Andes Mountains, Mt. St. Helens

Question 26

Basaltic Magma

Answer 26

• least viscous
• low silica content (1-2%)
• 50% gas content
• From effusive eruptions
• Shield Volcanoes
• Example: Kileaua, Hawaii

Question 27

Methods of Predicting Volcanic Eruptions

Answer 27

1) Seismicity
2) Gas Emissions
3) Ground Deformation
4) Thermal (heat) monitoring

Question 28

How can seismicity predict volcanic eruptions

Answer 28

• Measured with seismometers
• Seismic activity always occurs as volcanoes awaken and prepare to erupt
• The types of earthquakes that occur and when they start and end are also key signs
• Example: Mexico City 2000

Question 29

How can gas emissions predict volcanic eruptions

Answer 29

• Measured with Total Ozone Mapping Spectrometers which measure the amount of sulphur dioxide gas released by volcanic eruptions
• As magma nears the surface and its pressure decreases, gases escape
• Sulphur dioxide is one of the main volcanic gases, and increasing amounts of it usually signal the arrival of more magma near the surface
• Example: Mt. Pinatubo 1991

Question 30

How can ground deformation predict volcanic eruptions

Answer 30

• Measured using tiltmeter and InSAR (a technique for mapping ground deformation using radar images of Earth’s surface that are collected by orbiting satellites)
• Swelling of the volcano signals that magma has accumulated near the surface
• An increased rate of swelling accompanied with increasing sulphur dioxide emissions and tremors is a high probability of an eruption
• Mt St. Helens 1980

Question 31

How can thermal (heat) monitoring predict volcanic eruptions

Answer 31

• Volcanoes have thermal features such as steaming vents, hot springs, where its surface temperatures sometimes change before a volcanic eruption
• Temperature measurements can be made from a distance with cameras or instruments that measure thermal infrared radiation (TIR) which is emitted by the heated surface
• Infrared cameras are often used, which produces an image using a color spectrum that correlates to detected temperature
• Infrared satellite sensors can also detect thermal features

Question 32

Reducing Impacts of Volcanic Eruptions

Answer 32

1) Control the Environment
2) Hazard-resistant buildings
3) Community preparedness
4) Hazard mapping and land-use planning

Question 33

How controlling the environment reduces volcanic eruption impacts

Answer 33

• very little can be done to control volcanic eruptions, but lava flows are the only primary hazard which people have attempted to control
• Mt Etna - explosions were used to blow up a section of solidified ridge and channel of molten lava along a new path in order to divert lava flows from houses and farms
• Water sprays were used on Haeimay Iceland, to cool the lava flows in order to protect the harbour of Vestmannaeyjar

Question 34

How hazard-resistant buildings reduce volcanic eruption impacts

Answer 34

• nothing much to do to resist lava, pyroclastic flows and lahars
• ash fallout has the largest spatial impact and designs may help to reduce its impact
• the weight of ash on roofs, especially if it is wet, can be enough to cause roof collapse
• roofs need to be strong and designed to shed ash, with steeply sloping slides

Question 35

How community preparedness reduce volcanic eruption impacts

Answer 35

• Lives can be saved by warnings and advanced preparation

- This includes organised evacuation plans, temporary housing, food and water

Question 36

How hazard mapping and land-use planning reduce volcanic eruption impacts

Answer 36

• before planning land use for areas, hazard maps need to be created to identify hazardous areas
• don’t build in hazardous areas
• In Hawaii, houses in high risk areas are made from timber woods which allow the locals to move their house if necessary
• however, it is still difficult to predict in the long term the timing and scale of volcanic eruptions