Volcanic hazards and effects to cpmmunity

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Volcanic hazards

a)         Toxic gases

The gases within the magma when contained under pressure may explode and cause volcanic ejects to be projected through the air. Toxic gases emitted include:  H₂S, Co₂, SO₂, Co and hydrogen chloride. These gases may be carried tens of km downwind. The gases are toxic to plants, animals and human.

b)         Climatic changes

Causes local rise in temperature and consequently changes in the system of wind and rainfall in the area. Climatic changes are more pronounced when eruption occurs for a long period.

c)         Pollution

A lot of ashes (Fig 2.2), gases and solid materials emanated cause environmental pollution. Natural surface water is polluted. Gases combined with atmospheric vapour to create acidic rainfall.

Fig.2.2: Volcanic ash spreads downwind (internet)

d)        Lava, Lahar and pyroclasts

Lava is magma at a very high temperature that flows on the earth’s surface. Cool very slow depending on their thickness. For example a lava flow of 1m thick is estimated to cool from  b1100^oc to 750^oc in 12 days. The viscosity of magma will determine how easily it will flow. Basaltic lava with lower silica content tends to be more fluid than those magmas such as andesite and rhyolite that are richer in silica. Basaltic lava can therefore flow over long distances at rates of several meters per second. The fastest flow is always near the source depending on the topography. Down slope the flow is very fast. The damage caused by lava flows including both lives and properties (Fig.2.3).

Fig.2.3: Lava flow (Internet)

Lahars are volcanic mudflow. Lahars flow at great speeds; they can cover over 100km. These are the most damaging of all volcanic events. 

Pyroclasts/ debris may be thrown across thousands of square km. Ash can be carried by wind. Big rock blocks do cause building to collapse and also destroy crops.

e)         Coastal flooding

The sea wave (tsunamis) produced by a volcanic explosion. It can travel long distance and flood coastal settlements. Tsunamis differ from other earthquake hazards in that they can cause serious damage thousands of kilometres from the causative faults. Once generated, are nearly imperceptible in mid-ocean, where their surface height is less than a metre. The tsunamis travel at incredible speeds, as much as 900 km/hr, and the distance between wave crests can be as much as 500 km.  As the waves approach shallow water, a tsunami's speed decreases and the energy is transformed into wave height, sometimes reaching as high as 25 m, but the interval of time between successive waves remains unchanged, usually between 20 and 40 minutes. When tsunami is near to the coastline, the sea recedes, often to levels much lower than low tide, and then rises as a giant wave. The effects of tsunamis can be greatly amplified by the configuration of the local shoreline and the sea bottom.

f)          Landslides

 Volcanic eruption tends to trigger mass movement on unstable hill slopes. Landslides are caused with shaking of the ground and overloading of the material on slope.

Management strategies for volcanoes

Cooke and Doornkamp (1990) recommended the following measures to be considered:

i)        Increase public awareness through media, seminars /workshops 

  

ii)         Prepare reliable evacuation plan and organize sources of relief and rehabilitation

iii)        Make a hazard zoning/mapping to define major risk sites for proper land-use planning.

iv)       Define statutory prohibition on building within these sites (laws on building sites).

v)         Monitoring Strategy: Monitoring methods are designed to detect and measure changes in the state of a volcano caused by magma movement beneath the Earth’s Crust. Most volcanoes were formed many, many years ago. New volcanoes form every century, but usually in the same area as existing volcanoes. Rising magma typically will (1) trigger swarms of earthquakes and other types of seismic events; (2) cause swelling or subsidence of a volcano's summit or flanks; and (3) lead to the release of volcanic gases from the ground and vents. By monitoring these phenomena, scientists are sometimes able to anticipate an eruption days to weeks ahead of time and to detect remotely the occurrence of certain volcanic events like explosive eruptions and lahars (Fig.2.4).

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