Lecture 9: Volcanic Hazards

Question 1

How many active volcanoes in the world?

Answer 1

596

Question 2

Since 1950 there have been how many eruptions from how many volcanoes?

Answer 2

2208 eruptions from 347 volcanoes

Question 3

Average number of eruptions per year since 1950 is

Answer 3

63

Question 4

Volcanic eruptions can cause

Answer 4

loss of life and livelihoods in exposed communities, damage critical infrastructure, displace populations, disrupt business and add stress to already fragile environments.

Question 5

Currently, an estimated…live within 100 km of a volcano that has the potential to erupt.

Answer 5

800 million people

Question 6

Single events have the potential for

Answer 6

devastating mass casualties in a single event

Question 7

Divergent boundaries

Answer 7

new crust is generated as the plates pull away from each other.

Question 8

Convergent boundaries

Answer 8

crust is destroyed as one plate dives under another.

Question 9

Transform boundaries

Answer 9

crust is neither produced nor destroyed as the plates slide horizontally past each other.

Question 10

Hotspots

Answer 10

areas of volcanic activity away from plate boundaries.

Question 11

Spreading ridge volcanism

Answer 11

Volcanism related to plate divergence

Question 12

Composite or stratovolcano

Answer 12

Volcanism at an ocean-continent boundary (similar processes take place at ocean-ocean boundaries)

Question 13

Shield volcano

Answer 13

Volcanism related to a mantle plume

Question 14

Volcanic rocks, lava and ash are classified by the amount of silica they contain

Answer 14

Increased Si = Increased Pressure = More Explosive Volcanism

Question 15

Shield volcanoes are almost exclusively

Answer 15

basalt, a type oflavathat is very fluid when erupted.

Question 16

Shield volcanoes are most likely to produce

Answer 16

smaller geographically constrained eruptions – only explosive if water enter the vent

Question 17

90% of the volcano is

Answer 17

lava rather than pyroclastic material

Question 18

Largest percentage of the Earth’s individual volcanoes (~60%)

Answer 18

Stratovolcano

Question 19

Stratovolcanoes

Most are characterised by eruptions of…

Answer 19

andesite and dacite lava that are more viscous than basalt – allows gas pressure to build explosive eruptions
Usually 50% lava and 50% pyroclastic material

Question 20

Calderas and Caldera Complexes

Answer 20

Caldera complexes are the most explosive volcanoes – when they erupt they collapse in on themselves

Question 21

Common eruptions types are:

Answer 21

Hawaiian
Strombolian
Vulcanian
Plinian
Phreatomagmatic

Question 22

Hawaiian Eruptions

Type of magma

Answer 22

Basic, low viscosity

Question 23

Strombolian eruptions

Type of magma

Answer 23

Moderate viscosity

Question 24

Vulcanian Eruptions

Type of magma

Answer 24

Acid, viscous

Question 25

Plinian Eruptions Type of magma

Answer 25

Acid, viscous

Question 26

Hawaiian Eruptions Nature Of Effusive Activity

Answer 26

Thin, extensive flows from central vent

Question 27

Strombolian eruptions Nature Of Effusive Activity

Answer 27

Flows often absent

Question 28

Vulcanian Eruptions Nature Of Effusive Activity

Answer 28

Flows often absent

Question 29

Plinian Eruptions Nature Of Effusive Activity

Answer 29

Absent

Question 30

Hawaiian Eruptions Nature Of Explosive Activity

Answer 30

Very weak

Question 31

Strombolian eruptions Nature Of Explosive Activity

Answer 31

Weak to violent

Question 32

Vulcanian Eruptions Nature Of Explosive Activity

Answer 32

Moderate to violent

Question 33

Plinian Eruptions Nature Of Explosive Activity

Answer 33

Cataclysmic

Question 34

Hawaiian Eruptions Structures Formed Around The Vent

Answer 34

Broad lava domes and shields

Question 35

Strombolian eruptions Structures Formed Around The Vent

Answer 35

Cinder cones and lava flows

Question 36

Vulcanian Eruptions Structures Formed Around The Vent

Answer 36

Ash cones, explosion craters

Question 37

Plinian Eruptions Structures Formed Around The Vent

Answer 37

Large explosion calderas

Question 38

Phreatomagmatic eruptions

Answer 38

Volcanic eruption resulting from the interaction between magma and water

Question 39

Phreatomagmatic eruptions Surtseyan

Answer 39

volcanic eruption caused by shallow-water interactions between water and lava

Question 40

Phreatomagmatic eruptions Subglacial

Answer 40

volcanic eruption characterized by interactions between lava and ice, often under a glacier

Question 41

Volcanic Explosivity Index (VEI)

Answer 41

A relative measure of the explosiveness of volcanic eruptions. Volume of products, eruption cloud height, and qualitative observations are used to determine the explosivity value. The scale is logarithmic, with each interval on the scale representing a tenfold increase in observed ejecta criteria.

Question 42

Hazards and Impacts

Answer 42

``` Volcanic ash and tephra Pyroclastic flows, surges and blasts Lahars and Floods Debris Avalanches and Landslides Lava Flows Volcanic gases ```

Question 43

Tephra and Volcanic Ash

Answer 43

Anything that comes out of a volcano during an eruption Blocks (>64mm) Lapilli (64 – 2mm) Ash (2mm

Question 44

Pyroclastic flows, surges and blasts

Answer 44

Pyroclastic flows contain a high-density mix of hot lava, blocks, pumice, ash and volcanic gas. They move at very high speed down volcanic slopes, typically following valleys - up to 200 m/s

Question 45

Pyroclastic flows, surges and blasts Hazards

Answer 45

Most lethal volcanic hazard – 1/3 of all fatalities Destroy nearly everything in their path Severe damage to infrastructure, vegetation and agricultural land Extreme temperatures can ignite fires and melt snow and ice On the margins of pyroclastic flows, death and serious injury to people and animals may result from burns and inhalation of hot ash and gases. Can lead to secondary hazards such as flooding and lahars

Question 46

Lahars and Floods

Answer 46

Lahars are fast moving mixtures of volcanic debris and water Commonly occur when intense rain moves loose volcanic rock during an eruption Also caused by volcanic activity melting ice caps and glaciers Can threaten an area for years after an eruption

Question 47

Lahars and Floods Hazards

Answer 47

Large lahars can crush, abrade, bury, or carry away almost anything in their paths. Buildings and valuable land may be partially or completely buried. By destroying bridges and roads, lahars can also trap people in areas vulnerable to other hazardous volcanic activity Confined to valleys and areas close to volcano so can identify vulnerable areas.

Question 48

Debris Avalanches and Landslides

Answer 48

Landslides are large masses of wet or dry rock and soil that fall, slide, or flow very rapidly under the force of gravity. Landslides are common on volcanic cones because they are tall, steep, weakened by the rise and eruption of molten rock and constructed from poorly consolidated deposits. Can lead to lateral volcanic blasts as highly pressurized volcanic interior is exposed Can also cause tsunamis if material enters the sea

Question 49

Several conditions can trigger landslides:

Answer 49

Explosive eruptions Large earthquake directly beneath a volcano or nearby Heavy or long-lived rainfall that saturates the ground.

Question 50

Debris Avalanches and Landslides Hazards

Answer 50

Typically destroy everything in their path Can cause additional hazards Dam lakes leading to lahars Change pressure of magma chamber triggering further eruptions

Question 51

Lava Flows

Answer 51

Streams of molten rock that pour or ooze from an eruption vent

Question 52

Lava Flows Hazards

Answer 52

Destroys everything in its path Deaths are uncommon because it moves slowly Can bury homes and agricultural land under tens of meters of hardened black rock People are rarely able to use land buried by lava flows or sell it for more than a small fraction of its previous worth.

Question 53

The speed at which lava moves across the ground depends on several factors

Answer 53

Type of lava erupted and its viscosity Steepness of the ground over which it travels Whether the lava flows as a broad sheet, through a confined channel, or down a lava tube Rate of lava production at the vent.

Question 54

Volcanic Gases

Answer 54

Magma contains dissolved gases which escape during reduction in pressure as the magma moves towards the surface. Main gas is water vapour (60-99%) which is harmless Other gases include Carbon Dioxide – up to 10% Sulphur dioxide and other sulphur gases – up to 15% Halogens, including chlorine – up to 5%

Question 55

Volcanic Gases Hazards

Answer 55

Fatalities and health impacts Damage eyes and respiratory system CO2 asphyxiation Damage to vegetation, livestock, infrastructure and property Impact varies widely and depends on the amount and type of gas emitted. Environmental hazards Cooling of the Earth and Acid rain

Question 56

Volcano Monitoring

Answer 56

Volcanic eruptions are typically preceded by days to months of precursory activity, unlike other natural hazards like earthquakes. Volcano observatories play a key role in monitoring and early warning due to the localised character and individuality of volcanoes around the world Set up on all active volcanoes that threaten the human population Designed to monitor and potentially predict the eruptive behaviour

Question 57

Volcano Monitoring Methods

Answer 57

``` Seismic activity Increase in local earth quake activity Audible rumblings Ground deformation Upwelling near volcanic vent Changes in ground slope Hydrothermal phenomena Increased discharge or temperature from hot springs or fumaroles Rise in temperature in crater lakes Melting of ice and snow on the volcano Withering of vegetation of volcano slopes Chemical changes Changes in chemical composition of gas from vents ```

Question 58

Hazard Mitigation

Answer 58

The process for mitigating risk differs from country to country. Communities need to understand their risk and take action to mitigate risk.

Question 59

Hazard Mitigation Pyroclastic flows

Answer 59

Prior evacuation of threatened areas (identified by hazard mapping) can reduce loss of life. Effects of pyroclastic flows could be mitigated via long-term urban planning (subways, strong buildings)

Question 60

Hazard Mitigation Lava flows

Answer 60

Hazard maps can be developed for areas of at risk of flow-related damage

Question 61

Hazard Mitigation Lahars and Floods

Answer 61

Hazard mapping to identify high risk areas (valleys). Warning systems that permit evacuation could reduce the lethal effect of lahars. Engineering measures, such construction of sediment dams, can reduce flow mass and resulting destruction. Long-term urban planning can also reduce risk by avoidance of dense population concentrations in valleys and deltas.

Question 62

Hazard Mitigation Tephra

Answer 62

In the developing world, tephra accumulation on roofs of poorly constructed buildings often results in collapse; campaigns to promote ash removal can reduce fatalities and damage from tephra-induced roof collapse. Humanitarian assistance can reduce potential long-term impacts of resulting crop loss, livestock deaths, and economic disruption.

Question 63

Hazard Mitigation Debris avalanches

Answer 63

Prior evacuation the only measure for preventing loss of life.

Question 64

Hazard Mitigation Gas

Answer 64

Identification and ongoing monitoring of hazardous lakes, resettlement, pre-evacuation, and establishing guidelines relative to areas of refuge.

Question 65

USGS employs a nationwide volcano alert-level system. 

Answer 65

Ranked terms to inform people on the ground about a volcano's status Ranked colors to inform the aviation sector about airborne ash hazards.