Chapter 13 Volcanism
There are two classes of volcanic hazards, direct and indirect. Indirect hazards are volcanism-induced environmental changes that lead to distress, famine, or habitat degradation. It is estimated that indirect effects of volcanism have accounted for approximately 8 million deaths during historical times, while direct effects have accounted for fewer than 200,000, or 2.5% of the total. Some of the more important types of volcanic hazards are summarized in Table 13.3.
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| Type | Description | Risk |
|---|---|---|
| Tephra emissions | Small particles of volcanic rock emitted into the atmosphere |
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| Gas emissions | The emission of gases before, during, and after an eruption |
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| Pyroclastic density current | A very hot (several 100°C) mixture of gases and volcanic fragments (tephra) that flows rapidly (up to 100s of kilometres per hour (km/h)) down the side of a volcano | Extreme hazard — destroys anything in the way |
| Pyroclastic fall | Vertical fall of tephra in the area surrounding an eruption |
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| Lahar | A flow of mud and debris down a channel leading away from a volcano, triggered either by an eruption or a severe rain event | Severe risk of destruction for anything within the channel—lahar mud flows can move at 10s of km/h |
| Sector collapse/ debris avalanche | The failure of part of a volcano, either due to an eruption or for some other reason, leading to the failure of a large portion of the volcano | Severe risk of destruction for anything in the path of the debris avalanche |
| Lava flow | The flow of lava away from a volcanic vent | People and infrastructure at risk, but lava flows tend to be slow (less than km/h) and are relatively easy to avoid |
Volcanic Gas and Tephra Emissions
Large volumes of tephra (rock fragments, mostly pumice) and gases are emitted during major plinian eruptions (large explosive eruptions with hot gas and tephra columns extending into the stratosphere) at composite volcanoes, and a large volume of gas is released during some very high-volume effusive eruptions. One of the major effects is cooling of the climate by 1° to 2°C for several months to a few years because the dust particles and tiny droplets and particles of sulphur compounds block the sun. The last significant event of this type was in 1991 and 1992 following the large eruption of Mount Pinatubo in the Philippines. A temperature decrease of 1° to 2°C may not seem like very much, but that is the global average amount of cooling, and cooling was much more severe in some regions and at some times.
Over an eight-month period in 1783 and 1784, a massive effusive eruption took place at the Laki volcano in Iceland. Although there was relatively little volcanic ash involved, a massive amount of sulphur dioxide was released into the atmosphere, along with a significant volume of hydrofluoric acid (HF). The sulphate aerosols that formed in the atmosphere led to dramatic cooling in the northern hemisphere. There wer