Natural Hazards - Volcanoes

What Are Volcanoes?

A volcano is a landform created by magma from the earth's interior which penetrates through weaknesses in the Earths surface. Most volcanoes are formed at plate boundaries. They are classed in one of three categories of activity: active, dormant or extinct.

What Causes Volcanoes To Erupt?

Magma contains a mixture of silicates and dissolved gases. As magma rises closer to the Earths surface the pressure decreases and the dissolved gases come out of solution to form bubbles. As the bubbles expand the magma is pushed into the volcanic vent until it breaks through weaknesses in the Earth's crust. The explosive potential of the volcano is dependant on the effervescence of the gases and the viscosity of the magma. High gas effervescence and low viscosity cause the most explosive eruptions.

Where Do Volcanoes Occur?

As previously mentioned volcanoes occur at weaknesses in the Earths surface, these are in one of three tectonic settings: subduction volcanoes, rift volcanoes and hot spot volcanoes. Subduction volcanoes occur where one tectonic plate is thrust and consumed by another. This type is the most explosive and comprises approximately 80% of the worlds active volcanoes. Rift volcanoes occur where tectonic plates diverge, often on the ocean floor. These are generally less explosive. Hot spot volcanoes occur in the middle of plate boundaries where magma exits from weaknesses in the earth's surface. The Hawaiian Islands are an example of hot spot volcanoes. The most active area of volcanic activity occurs along the pacific plate boundary which is often referred to as the 'Pacific ring of fire'.

What Hazards Are Related To Volcanic Eruptions?

There are about 500 active volcanoes thoughout the world. In an average year approximately 50 of these erupt. Volcanic hazards create fewer disasters and deaths compared with earthquakes and severe storms. Many of the deaths associated with volcanoes are indirect hazards such as famine due to crop damage or from secondary hazards such as lahars. Volcanoes are composite hazards. There are both primary and secondary hazards which can be caused by volcanic eruptions. The primary hazards include pyroclastic flows, air-fall tephra, lava flows and volcanic gases. The secondary hazards include ground deformation, lahars (mudflows), landslides and possibly tsunamis in ocean floor volcanic eruptions.

History of Volcanic Eruptions

Mount Vesuvius, Italy (789 A.D)
This Plinian eruption completely destroyed the cities of Pompeii and Herculaneum killing approximately 3,360 people due to ashfall and pyroclastic flows. The bodies were preserved perfectly by the hard ash and mud.
Nevada del Ruiz, Columbia (November 13th 1985)
This eruption is one of the worst volcanic disasters in recent times. The volcanic eruption melted ice and the snow cap of the Nevada del Ruiz volcano causing a torrent of water, mud, debris to travel down all six major river valleys away from the volcano. The lahars increased in size as they travelled down the river channels, eroding loose soil, rocks and vegetation. The lahars travelled 100 kilometres, killed approximately 25,00 people, injuried 5,000 and destroyed over 5,000 homes. The worst town hit was Armero: three quarters of its 28,700 population died during the mud flows.
Mount Pelee, Martinique (1902)
This Peleean eruption destroyed the town of St Pierre which left only two survivors out of a population of approximately 30,000. Peleean eruptions are very explosive and are characterised by pyroclastic flows.
Krakatoa, Indonesia (1883)
This submarine volcanic eruption was one of the violent eruptions in the last 3,000 years. The eruption on the 27th August blow the whole northern and lower areas of volcano, creating an explosion which was heard nearly 3,100 miles away. The discharge of volcanic ash was transported in air currents and was diffused for years, the effects of this ash affected North and South America, Europe, Asia, southern Africa and Australia. This was not the worst consequence of this eruption; the disintegration of the volcano caused a tsunami which inundated coastal areas of the islands of Java and Sumatra, killing 36,000 people.
Mount Pinatubo (1991)
This eruption was not only a disaster in terms of human loss but it had a major effect on the global temperature and ozone depletion in the years following the eruption. Approximately 200 people died because of the ash released during the eruption. During the time of the eruption Tropical Storm Yunya was passing 47 miles northeast to Mount Pinatubo, causing high rainfall rates in the region.

The ash which was released from the volcano mixed with water vapour in the atomosphere to cause a rainfall of tephra. This water saturated ash fall fell on to rooftops which subsequently collapsed due to the excessive weight.

It is thought that had the tropical storm not been present the death toll would have been significantly lower. The gas and ash cloud produced by the eruption reached high into the atmosphere and travelled around the world within a year. During the years of 1992 and 1993 the average temperature in the Northern hemisphere had dropped by approximately 0.6 0C, the entire planets temperature dropped by 0.4 -0.50C.

The eruption also ejected between 15 and 30 million tons of sulphur dioxide gas.
In the atmosphere the sulphur dioxide gas mixes with oxygen and water to form sulphuric acid which aids ozone depletion which caused the ozone hole over Antarctica to enlarge. It is thought this global change in temperature and climate caused other natural disasters such as the 1993 Mississippi floods.

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