The following will help you understand volcanic processes; their destructive nature (and also their constructive nature) and will help you to understand how we can predict, prepare and manage their hazards. We will also include a few case studies to help you to understand better


Volcano Hazards No.1: Volcanic Gases (1)

50 to 80% of gases are in actual fact, just water vapour

Among the volcanic gases; Carbon Dioxide, Sulphur Dioxide, Hydrogen Sulphide, Carbon Monoxide, Hydrogen and Chlorine are the most common

Most of volcanic gases get released into the atmosphere and are little or no threat

However, volcanic gases can actually create some severe fatalities; e.g. Hawaii suffers from Volcanic Smog (Vog). Vog is no threat to tourists who only visit for short periods of time, but for those that live there permanently it can be fatal.

  • It is created due to the cold (and dense) air descends and the hot (volcanic gas) rises above it, and as the volcanic gases are unable to escape, it creates this hazy Vog
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Volcano Hazards No.1: Volcanic Gases (2)

Case Study: Laki, Iceland 1783-84

Eruption released 14.7km3 of basaltic lavas which contained 8million tons of fluorine gas

122million tons of Sulphur Dioxide and 250million tons of Suplhuric Acid Aerosols remained aloft for two years

The gases infected 75% of cattle and horses and 50% of the cattle died

The aerosols spread and affected crops in Ireland and Scandanavia

Due to the high volume of gases, it created 3 abnormally long and cold winters in the Northern Hemisphere

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Volcano Hazards No.1: Volcanic Gases (3)

Case Study: Lake Nyos, Cameroon August 21st 1986

A small but deep lake rested ontop of an active hot spot volcano in Cameroon, Wester Africa

Gases produced by the volcano were held underneath the lake

The turbulence of the lake eventually allowed the gases to be blown out of the lake

This resulted in 1billionM3 of Carbon Dioxide being released down the side of the volcano in one hour

The result of this was 1,700 people and 3,000 cattle dies under a 50m river of Carbon Dioxide

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Volcano Hazards No.2: Lava Flows (1)

Lava Flows rarely hold any dangers for humans as they are not very fast and follow low lying regions, which makes it easier to predict their paths

A low viscous (sticky) flow is not very rapid, however, if a Lava Tube is present then the speed of the lava may reach up to 50km/hr

A Lava Tube forms when the margins and upper surface of the lava solidifies

Once the eruption ceases, the lava tube drains

Types of Lava Flows

Pahoehoe has a smooth, ropy surface (less viscous)

AA Lava consists of jagged, angular blocks and fragments

Columnar Joints form when lava stops moving; coolingand causing joints to open

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Volcano Hazards No.2: Lava Flows (2)

Polygons form on the surface of the lava flow and are commonly six-sided

Pillow Lava is when lava cools rapidly (generally beneath water) and looks like interconnecting pillows

Case Study: Paracutin, Mexico 1943 and 1952

Flow of lava covered 25km2 and the city laid 5km from the volcano vent.

Only visible thing left was the church tower

Case Study: Kilauea, Hawaii 1983

This volcano has been erupting non-stop since 1983 and has covered a distance of 25km2 and has destroyed 180 houses

The flows have added an extra 120km2 of new land, resulting in tourism

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Volcano Hazards No.2: Lava Flows (3)

Case Study: Nyiracongo, Congo January 10th 1977

Eruption began with a basalt lava followed by a fissure-type eruption

Initial speed of these eruptions reached up to 100km/hr and covered 20km2 of land

Killed several hundred people and wildlife

Immense damage as the lava filled crated had been growing for the last fifty years

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Volcano Hazards No.3: Pyroclastic Flows (1)

Pyroclastic Flows are a mixture of hot rock fragments, lava particles and hot ash thrown up into the air by hot gas

Also known as Nuees Ardentes and are associated with andesitic/rhyolitic volcanoes

They can travel up to 300km/hr and can travel up to 40km from the source. Their temperatures range from 100 to 700*c

Material that is erupted from volcanoes

Tephra: tephra is any material that is erupted from volcanoes and is split up into three categories. Ash (which is less than 2mm in diameter), Lapilli (2 to 64mm) and Volcanic Bombs (more than 64mm). Tephra can also be classified as pyroclasts. Volcanic bombs fall to the ground first and are therefore found nearest to the volcano vent; therefore the further away from the volcano, the decrease in size the tephra is (sequentially) graded. However, ash can become circumglobal

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Volcano Hazards No.3: Pyroclastic Flows (2)

When large amounts of tephra accumulates in the atmosphere, it reflects light and heat from the sun and creates volcanic winters as temperatures drop. If there is then heavy rainfall, the precipitation can become acidic

Ash Fall: ash fall occurs when magma rises closer to the surface, increasing the pressure between its gases and the overlying lithostatic pressure. In felsic magma, expansion is restrained, therefore gas pressure increases and will eventually lead to an explosion.

Pyroclastic Ash Flow: is fast moving currents of hot gas and ash. They can reach speeds of up to 80km/hr and have a high temperature of 1,000*c. When there are more gases than rock then it can also be called Dilute Pyroclastic Density Currents or Pyroclastic Surges.

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Volcano Hazards No.3: Pyroclastic Flows (3)

Case Study: Krakatoa 1883

Pyroclastic flow reached Sumatra Coast 40km away after it crossed the waters on a cushion of super heated steam. A Tsunami was also generated by the heavy matter and was precipitated out of the flow after the flows initial contact with the water

Case Study: Mt. Pinatubo 1991

Pyroclastic Flows that were 30 to 50km thick

Case Study: Mt. Pelee 1902

There have only been a reported 64 known survivors from the 29,000 population. The Pyroclastic Flow on May 8th sped down the River Blanche Valley at about 190km/hr and reached temperatures of 700*c. Further pyroclastic flows on 20th and 30th of May claimeds a further 2,000 people

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Volcano Hazards No.3: Secondary Volcanic Hazards (


Volcanic mudflows where ash mixes with water to form a dense slurry. They flow downslope at 50km/hr. Water within the mixture comes from melted snow and ice from the top of the volcano.

Case Study: Nevado del Ruiz, Colombia 1985

Lahars spread 20km away from volcano and killed 22,000 people. Reached 8m high and travelled up to 45km/hr. Ice caps on volcano tip melted and, mixed with tephra, released around 10 to 60million m3 of water. 90% of buildings destroyed and replaced with 5 to 8m thick layer of mud which claimed the lives of 15,000 animals

Case Study: Redoubt Volcano, Alaska 1989-90

Three successive lahars that destroyed Containment Berms at Drift River Oil Terminal

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Volcano Hazards No.3: Secondary Volcanic Hazards (


Generated by explosive eruptions or submarine caldera collapse. The waves generated may reach tens of metres in height. They are difficult to detect in deep ocean as they have long wavelengths and low waveheights; however, in shallow waters they increase in height as the wave behind it decreases its distance and the angle of the coast increases.

Case Study: Krakatoa 1883

Tsunamis that reached 30 to 40m in height and travelled at 60mph. The coastal town lying on the Sunda Strait were destroyed within two hours. Some tsunamis carried corals weighing up to 600tonnes. It killed 36,000 people and the actual eruption only claimed a few lives

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Volcano Hazards No.3: Secondary Volcanic Hazards (

Volcanic Landslides

Gravity driven slides of rock and volcanic material. An example occured on Mt. St. Helens when the north side of the volcano collapsed. They can also occur due to heavy rainfall or earthquakes. Ground deformation of volcanic slopes by the volcanoes rising magma may also cause instability and trigger landslides before an eruption (precursor)

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Predicting Volcanoes (1)


Increase in gases: magma rising to the surface will increase the output of steam and will also create rings of gas and steam

Ground Deformation: ground moves slowly and cracks may form or may form a bulge in the volcano

Temperature Increase: this can be due to the magma rising and therefore heating its surroundings

What Makes Predictions Hard?

Dormant Volcanoes: unpredictable as we never know when they will erupt.

Evacuation Procedures: if an accurate prediciton is not given, then people may die or feel that they have been evacuated for too long and will go back

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Predicting Volcanoes (2)

Some gases do not get released beforehand and are kept inside the magma

Lava Tubes: heat will be kept inside the lava tubes

How to Predict

Gas readers, robots (to collect material and find out characteristics of lava etc), temperature recorders and thermal detectors

Prepare and Manage

Not much can be done to prepare and manage a volcanic eruption. The best that can be done is to predict the eruption to as close as possible then warn the surrounding beings of the impending eruption and urge them to evacuate. We can issue out areas of high risk and low risk from different volcanic hazards. In terms of the initial eruption and specific hazards, we can slow down hazards such as lava flows (see revision card) but cannot prevent the eruption. So the best we can do is evacuate people and hope for the best

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very helpfull indeed, think this should be a 4 out of 5 star.

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