volcanic hazards

Ash

dust sized particles of rock produced by the explosive eruption of some volcanoes. - May be carried in the air for long distances from the volcano which formed it

composite volcano

Large,steep-sided symmetrical cone-shaped volcano formed from alternating layers of lava flows,volcanic ash,cinders,blocks and bombs

Lahars

Mudflows composed of pyroclastic material and water that flows down form a volcano usually along a river valley

Magma

Molten rocks that is found beneath the surface of the earth

Tsunamis

Giant sea waves generated by shallow focus underwater earthquakes,violent volcanic eruptions,under water debris slides and landlslides into the sea

volcanic bombs

Rocks that are more than 5mm in diameter that are thrown into the air by a volcanic eruption

volcanic explosive index

a scale used to measure the explosiveness of volcanoes

volcanic activity is found at

along constuctive plate margins

on or near subduction zone

over hotspots

constructive plate margins

where plates are moving apart and magma forcing its way to the surface. In some locations along oceaniic ridges where the lava build up to the ocean surface,volcanic islands form e.g iceland.

Also form withing continental rift valleys

volcanoes formed here have gentle sides due to the low viscosity of the basaltic lava

Eruptions are frequent but gentle (effusive)

subduction zones

ring of fire that surrounds the pacific ocean associated with plate subduction.The deeper the oceanic plate descends,the hotter the surroundings become.together with the heat generated from friction,begins to melt the oceaninc plate into magma in a part of the subduction zone known as the benioff zone

As it is less dense than the surrounding material,this molten magma begins to exploit weakness in the crust and rises as columns of magma.Magma collects huge sub surface reservoirs called plutons .Eventually some of the magma reaches the surface and form volcanoes

characteristics of the lava formed at subduction zones

The andesitic lava which has a viscous nature (flows less easily) creates complex,composite and explosive volcanoes

over hotspots

The hawaiian islands in the north pacific ocean is an example of shield volcanoes that are located away from plate boundaries over a hotspot.

Low angles slopes made from low viscosity,basaltic lava that flows great distances from the volcanic vent itself

viscosity and its relation to volcanoes

viscosity is the measure of how thick or sticky a liquid is it describes how easily a substance moves.

if the lava is not viscous (a thin fluid) the gases may escape easily.

if the lava is highly viscous (thick and pasty) the gases will not move freely and build up tremendous pressure within the volcano. - the resulting explosive eruptions can blow volcanic dust into high atmosphere reducing the amount of incoming solar radiation and causing short term global climate change

volcanic explosivity index

the main method of measurement of magnitude - logaritmic scale running from 0 to 8

frequency of eruptions

its previous history of activity can be interpreted by volcanologists using the deposits associated with the volcano itself

primary effects of volcanic activity

Tephra

Pyroclastic flows

Lava flows

Volcanic gases

Tephra

solid material of varying grain size ranging from volcaninc bombs to ash,all ejected into the atmosphere

pyroclastic flow (nuees ardentes)

very hot ( over 800 c) gas charges ,high velocity flows made up of a mixture of gas and tephra

a super-heated mixture of gas and tephra that flows at speeds of up to 700 km per hour

• This is one of the deadliest volcanic hazards as they can travel long distances and destroy everything in their path

• E.g. when Fuego volcano erupted in Guatemala in 2018, pyroclastic flows destroyed several nearby towns

lava flows

rarely cause injury to people due to their relatively low velocity they are however often unstoppable and can therefore damage crops and buildings and block roads

lava flows travel at different speeds depending on slope, temperature and viscosity

• Most move slowly enough that they are not a risk to human life but can cause significant damage to property and infrastructure

• The fastest flows can reach speeds of up to 60 mph

• In the 2021 eruption of Mount Nyiragongo in the Democratic Republic of Congo, lava flows travelled almost 10km destroying properties and killing over 30 people

volcanic gases

include carbon dioxide,carbon monoxide,hydrogen sulphide,sulphur dioxide and chlorine

eruptions can release large quantities of gases such as sulphur dioxide and carbon dioxide

• They are hazardous to human health causing respiratory problems and deaths

secondary effects

lahars

flooding

volcanic landslides

tsunamis

acid rain

lahars ( volcanic mudflows)

unconsolidated ash from a recent eruption combines with water may be swept down river alleys in the form of hot dense fast moving mudflow.

flooding

when an eruption melts glaciers and ice caps , serious flooding can result

volcanic landslides

the high velocity and great momentum of landslides allows them to cross between valleys and run up slopes several hundred metres high

tsunamis

sea waves generated by violent volcanic eruptions

acid rain

volcanoes emit gases which include sulphur.When this combines with atmospheric moisure acid rain results.

climate change

the ejection of huge amounts of volcanic debris into the atmosphere can reduce global temperature and is believed to have been an agent in past climatic change

responses to volcanic hazards: preparedness

early prediction means that people can be evacuated from the danger zone and lives can be saved

many earthquakes are now monitored continually either by local observatories or by satellites

a study of the previous eruption history of any volcano is important along with the type of activity it has produced it is also possible to identify areas at greatest risk and frequency of eruptions from these layers of deposits

mitigation

risk assessments are one form of mitigation. Governments of countries at risk of volcanic hazards such as Philippines carry out risk assessments and produce a series of alert levels to warn public of the threat.

some physical actions can be undertaken to reduce the impact

most common way to reduce the impact on people is to evacuate vulnerable areas when risk becomes intolerable

However, if an evacuation is carried out needlessly future evacuations are more difficult to manage

prevention

there is no way to prevent a volcanic eruption

adaptation

perhaps the best response is to move away from the area surrounding the volcano

Eyjafjallajokull, Iceland April 2010

Characteristics of the hazard

VEI 4

Glacier covered stratovolcano

Constructive boundary

Interaction between lava and ice produces ash

Steam creates highly explosive eruptions and fine grained ash

eruption lasted one month as magma chamber from 1821 reactivated

0.25 billion m3 ash in total

High pressure system -northly winds long lasting

Characteristics of the place

28th richest country in the world officially nobody lives below poverty line

rural area,sparsely populated

Prediction- ongoing monitoring,risk mapping informs settlement planning

Protection-Community preparation and education

mitigation- board up houses/bring in animals

exclusion zone (evacuation)

close roads (infrastucture designed to be swept away by lahar

impact

0 deaths

£130m per day cost to airlines

homes and roads damaged and services distrupted

crops damaged by ash

roads washed away

Mt Nyiragongo ,Congo January 2002

characteristics of the hazard

constructive boundary (continental)

large basaltic stravolcano

13km fissure

VEI 1

fast moving lava (90kmph)

characteristics of the place

Dense urban population in goma (15km)

228th poorest country in the world

68% live below poverty line

DR Congo is slowly recovering from a civil war which led 5m deaths between 1994 and 2003 but many eastern areas are still plagued by violence as various rebel groups continue to operate there

UN peacekeeping mission

prediction- ongoing monitoring,warning of lava flows issued

protection-

none:rapid growth of informal housing in Goma due to rural to urban migration and refugee movements

Aid: 350,000 fled to Rwanda camps (15m red cross)

slow return as lava cooled (hard to dig graves) 320000 left homeless

impact

1/3 Goma destroyed

147 dead

350000 fled

drinking water poisoned

dysentery looting and public disorder