A volcanic event can have a range of impacts, affecting the area immediately around the volcano or the entire planet.
Volcanic effects become a hazard when they impact upon the human and bilt environments, killing and injuring people, buring and collapsing buildings, destroyign the infrastructure and bringing agricultural activities to a halt.
Effects can be categorised into primary and secondary.
Primary effects consist of:
Tepha – solid material of varying grain size, from volcanic bombs to ash particles, ejected into the atmosphere
Pyroclastic flows – very hot (800 degrees Celsuis), gas-charged, high velocity flows made up of a mixture of gases and tephra
Volcanic gases – including carbon dioxide, carbon monoxide, hyrdoen sulphide, sulphur dioxide and chlorine. Emissions of carbon dioxide from Lake Nyos in Cameroon in 1986 – 1,700 people suffocated
Secondary effects include the following:
Lahars – volcanic mud flows such as those that devastated the Columbian town of Armero after the eruption of Nevado del Ruiz in November 1995
Flooding – melting of glaciers and ice caps such as the Grimsvotn glacial burst on Iceland in November 1996
Tsunami – giant sea waves generated after violent caldera-forming events such as that which occurred in Krakatoa in 1883 – the tsunamis from this eruption are belived to have drowned 36,000 people
Volcanic landslides – large amounts of rocks falling down a mountainside
Climatic change – the ejection of vast amounts of volcanic debris into the atmosphere can reduce global temperatures and is believed to have been an agent in past climatic change
The intial effect of an earthquake is ground shaking. The severity of this will depend upon the magitude of the earthquake, the distance frm the epicentre and the local geological conditions. In Mexico City earthquake of 1985, for example, the sesmic waves that devastated the city were amplified several times by the anicwent lake sediments upon which the city is built.
Secondary effects can be seen below:
Soil liquefaction – when violently shaken, soils with a high water content lose their mechanical strength and start to behave like a fluid
Landslides/avalanches – slope failure as a result of ground shaking
Effects on people and the built environment – collapsing buildings, destruction of road systems and other forms of communications, destruction of service provision such as gas, water and electricity transmiision systems, flooding, disease, food shortages, disruption to the local economy. Some the of the effects on the human evironment are short term; others occur over a long period and will depend to a large extent on the ability of the area to recover
Tsunamis – giant sea waves (tsunami means ‘harbour wave’ in Japanese) generated by shallow-focus underwear earthquakes (the most common cause, volcanic euprionts, undeerwater debris slodes and large landslides into the sea