ugrc 144 science and technology in our lives/geohazards · •tephra falls destroy vegetation,...

College of Education

School of Continuing and Distance Education2014/2015 – 2016/2017

UGRC 144

Science and Technology in Our

Lives/Geohazards

Session 6 – Volcanic Hazards

Lecturer: Dr. Patrick Asamoah SakyiDepartment of Earth Science, UG

Contact Information: [email protected]

Session Overview

• This session introduces students to Hazards associated with volcanoes.

• The eruption of molten material in the form of lava on the surface of the earth has the potential to cause hazards.

• Hazards associated with volcanoes are dependent on how volcanic materials like lava, rock fragments and gases are produced and ejected to the surface of the earth.

• The extent of travel of lava on the surface and potential hazard associated with it, also depends on the temperature, composition and viscosity of the lava. All these will be discussed in this session.

Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 2

Session Outline

The key topics to be covered in the session are as follows:

• Topic One – Components of Volcanic Eruption

• Topic Two – Volcanic Hazard - Primary Effects

• Topic Three – Volcanic Hazard -Secondary & Tertiary Effects

• Topic Four – Beneficial Aspects of Volcanism

• Topic Five - Predicting Volcanic Eruption


Reading List

• Chapter 5 of Environmental Geology 4th Edition – by Carla W Montgomery (1995)

• Read Unit 1, Section 5 of UGRC 140 II Geohazards –Institute of Continuing and Distance Education


COMPONENTS OF VOLCANIC ERUPTION

Topic One


Components of Volcanic Eruption

• The components of volcanic eruption are;

– Lava Flow

– Hot Ash Flows (pyroclastic flows)

– Ash Falls (tephra falls)

– Volcanic Gases

– Mudflows (lahars)

– Volcanic Landslides (debris flows and debris avalanches)


VOLCANIC HAZARD - PRIMARY EFFECTS

Topic Two


Volcanic Hazard - Primary Effect

Lava Flows

• Lava flows have been known to travel as fast as 64 km/hr; most are slower and give people time to move out of the way.

• However, lava flows are most damaging to property, as they can destroy anything in their path.



Lava Flows• Control of lava flows has been attempted with

limited success by bombing flow fronts to attempt to divert the flow, and by spraying with

water to cool the flow.



Violent Eruptions and Pyroclastic Activity

• Pyroclastic materials is the name given to particles produced in volcanic eruptions. Hot pyroclastic flows cause death by suffocation and burning.

• Pyroclastic material can travel so rapidly that few humans can escape.

• Recall the 2010 volcanic eruption in Iceland that produced a lot of ash and disrupted air travel for more than one week?




• Pyroclastic materials is the name given to particles produced in volcanic eruptions. Hot pyroclastic flows cause death by suffocation and burning.

• Pyroclastic material can travel so rapidly that few humans can escape.

• Recall the 2010 volcanic eruption in Iceland that produced a lot of ash and disrupted air travel for more than one week?




• Tephra falls can blanket an area like snow, but they are far more destructive because tephra deposits have a density more than twice that of snow and tephra deposits do not melt like snow.

• Tephra falls destroy vegetation, including crops, and can kill livestock that eat the ash covered vegetation.

• Tephra falls can cause loss of agricultural activity for years after an eruption, a secondary or tertiary effect.

* Tephra is a mixture of volcanic ash and other fragmentsDr. Patrick A. Sakyi, Dept. of Earth Science Slide 12



• Tephra falls can blanket an area like snow, but they are far more destructive because tephra deposits have a density more than twice that of snow and tephra deposits do not melt like snow.

• Tephra falls destroy vegetation, including crops, and can kill livestock that eat the ash covered vegetation.

• Tephra falls can cause loss of agricultural activity for years after an eruption, a secondary or tertiary effect.

* Tephra is a mixture of volcanic ash and other fragmentsDr. Patrick A. Sakyi, Dept. of Earth Science Slide 13

Blanket of Tephra


Poisonous Gas Emissions

• Volcanoes emit gases that are often poisonous to living organisms. Among these poisonous gases are: Tephra falls destroy vegetation, including crops, and can kill livestock that eat the ash covered vegetation.

– Hydrogen Chloride (HCl), - Hydrogen Sulfide (H2S),

– Sulfur Dioxide (SO2), - Hydrogen Fluoride (HF),

– Carbon Monoxide (CO), - *Carbon Dioxide (CO2), and

– *Water Vapour (H2O).



Poisonous Gas Emissions

• For example, in 1984, CO2 gas escaping from the bottom of Lake Monoun, a crater lake in the African country of Cameroon, killed 37 people.

• Again, in 1986, an even larger accumulated CO2 gas emission from Lake Nyos in Cameroon killed more than 1700 people and 3000 cattle.

– The best defense against hazardous volcanic gases is to get well away from the erupting volcano or escaping gases as quickly as possible


Volcanic Hazard – Secondary and Tertiary Effects

Mudflows (Lahars)

• Volcanoes can emit voluminous quantities of loose, unconsolidated tephra which become deposited on the landscape.

• Such loose deposits are subject to rapid removal if they are exposed to a source of water

• The source of water can be derived by;

– melting of snow or ice during the eruption,

– emptying of crater lakes during an eruption, or

– rainfall that takes place any time with no eruption.Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 16


Mudflows (Lahars)

• Mudflows are a mixture of water and sediment.

• They move rapidly down slope along existing stream valleys, although they may easily top banks and flood out into surrounding areas.

• They have properties that vary between thick water and wet concrete, and can remove anything in their paths like;

– bridges,

– highways,

– houses, etc. Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 17


Debris Avalanches and Debris Flows

• Volcanic mountains tend to become over-steepened as a result of the addition of new material over time.

• Over-steepened slopes may become gravitationally unstable, leading to a sudden slope failure that results in landslides, debris slides or debris avalanches.


• Occurrence of debris avalanches, landslides, and debris flows are not necessarily accompanied by a volcanic eruption.


Flooding

• Drainage systems can become blocked by deposition of pyroclastic flows and lava flows.

• Such blockage may create a temporary dam that could eventually be filled with water and fail resulting in floods downstream from the natural dam

• Volcanoes in cold climates can melt snow and glacial ice, rapidly releasing water into the drainage system and possibly causing floods.



Tsunami

• Debris avalanche events, landslides, caldera collapse events, and pyroclastic flows entering a body of water may generate tsunami.

• For example, in 1883 a tsunami, triggered by pyroclastic flow entering the sea in Sumatra, Indonesia killed about 36,600 people, some as far as 200 km away from the volcano.



Volcanic Earthquakes and Tremors

• Earthquakes usually precede and accompany volcanic eruptions, as magma intrudes and moves within the volcano.

• Most volcanic earthquakes are small, but some are large enough to cause damage in the area immediately surrounding the volcano,

• Some are large enough to trigger landslides and debris avalanches



Volcanic Earthquakes and Tremors

• Volcanic Tremor (also called harmonic tremor) is a type of continuous periodic shaking of the ground that is generated by magma moving underground.



Atmospheric Effects

• Since large quantities of tephra and volcanic gases can be injected into the atmosphere, volcanism can have a short-term effect on climate.

• Volcanic ash can cause reflection of solar radiation, and thus can cause the temperatures to be cooler for several years after a large eruption.



Atmospheric Effects

• E.g., Volcanic gases like SO2 also reflect solar radiation, thus lowering the average temperature.

• Volcanic gases like CO2 are greenhouse gases which help keep heat in the atmosphere.



Famine and Disease

• Tephra falls can cause extensive crop damage and kill livestock. This can lead to famine.

• Displacement of human populations, breakdown of sewerage and water systems, cut off of other normal services can lead to a number of diseases.


BENEFICIAL ASPECTS OF VOLCANISM

Topic Four


Beneficial Aspects of Volcanism

Even though this course concentrates on the damaging effects of volcanism,

• Volcanism helps renew the soil, and soils around active volcanoes are some the richest on earth.

• Hydrothermal processes associated with volcanism produce rich ore deposits.

• The heat rising around magma bodies can sometimes be tapped to produce geothermal energy.


PREDICTING VOLCANIC ERUPTIONSTopic Five


Predicting Volcanic Eruptions

Active Volcano

• An active volcano is a volcano that has shown eruptive activity within recent (recorded) history, possibly in the past ~10,000. Thus an active volcano needs not be in eruption to be considered active.

• Globally there are about 550 active volcanoes, and each year 50 to 70 volcanoes actually erupt.

• Examples of active volcanoes– Stromboli (Italy)-erupting for more than 1000 years, Sinabung (Indonesia)

– Mt. Etna (Italy), Kilauea Volcano (Hawaii, USA)



Active Volcano

• An active volcano is a volcano that has shown eruptive activity within recent (recorded) history, possibly in the past ~10,000. Thus an active volcano needs not be in eruption to be considered active.

• Globally there are about 550 active volcanoes, and each year 50 to 70 volcanoes actually erupt.

• Examples of active volcanoes– Stromboli (Italy)-erupting for more than 1000 years, Sinabung (Indonesia)

– Mt. Etna (Italy), Kilauea Volcano (Hawaii, USA)



Dormant Volcano

• A dormant volcano (sleeping volcano) is somewhere between active and extinct.

• A dormant volcano is one that has not shown eruptive activity within recorded history, but shows geologic evidence of activity within the geologic recent past.



Dormant Volcano (contd.)

• Thus, it is a volcano that hasn’t erupted in the past 10,000 years, but which is expected to erupt again.

• Dormant volcano normally appears fresh-looking and not too eroded or worn down.

• Examples are;

– Mauna Kea (Hawaii, USA),

– Mauna Loa (Hawaii, USA) &

– Mt Killimanjaro (Tanzania)Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 32


Extinct Volcano

• An extinct (dead) volcano is a volcano that has not shown any historic activity, usually only the heavily eroded lava plug remaining, no fumarolic activity and shows no signs of recent eruptive history.

• Examples are;

– Mt Ashitaka (or Ashitakayama-Japan)

– Yellowstone Caldera (USA)-has not erupted for 640,000 yrs.


Summary

• In this session, we have looked at the components of volcanic eruption such as lava flow, mudflow, pyroclastic material, and gases.

• We have also looked at the primary and secondary effects of volcanic hazards such as flooding, destruction of vegetation as well as causing the death of livestock.

• The session also touched on the beneficial aspects of volcanism such as renewing soil and the formation of ore deposits


END


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