4.4: Volcanic Hazards

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Given the obvious dangers of living near active volcanoes, there must be some reason why humans like living around them. The main reason is that the soil tends to be fertile, and thus there is the potential to grow enough food to live. For example, some parts of the area around Mt. Merapi in Indonesia can support populations of 8 to 10 people per hectare. In comparison, the typical farm in the United States can feed just under 1 person per hectare (US Farm Bureau).

Volcanic soil is good for several reasons. One is that volcanic ash and rock fragments are rich in volcanic glass and under weathering conditions glass breaks down quickly to clay minerals so that productive soil can form within 200 to 300 years in favorable climates. Another is that the clays that form from volcanic parent materials are effective at holding onto nutrients such as phosphorous. A third is that volcanic lava or tephra are typically quite rich in some important plant nutrients, such as magnesium and sulphur.

There are two classes of volcanic hazards, direct and indirect. Direct hazards are those that can directly kill or injure people or destroy property or wildlife habitat. Indirect hazards are volcanism-induced environmental changes that lead to distress, famine or habitat destruction. Indirect hazards of volcanism have accounted for many times more deaths during historical times than direct hazards. Some of the more important types of volcanic hazards are summarized in the table below. Do not memorize this; it is here for your exploration and reference.

Table $\PageIndex{1}$: A Summary of the Important Volcanic Hazards
Type	Description	Risk
Tephra emissions	Small particles of volcanic rock are emitted into the atmosphere	Respiration problems for some individuals. Short-term climate cooling and potential famine. Aircraft engines at risk.
Gas emissions	The emission of gases during an eruption, or other event	Short-term climate cooling leading to crop failure and famine. Poisoning is widespread in some cases.
Pyroclastic density current	A very hot (several 100°C) mixture of gases and volcanic tephra flows rapidly (up to 100s of km/h) down the side of a volcano	Extreme hazard. Virtually anything in the way will be destroyed.
Pyroclastic fall	Vertical fall of tephra in the area surrounding an eruption	Areas close to the eruption (km to 10s of km) can be covered in thick tephra. Roofs may collapse; structures may burn.
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	Anything within the channel will be severe risk. 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 reason, leading to an avalanche of debris	Anything in the past of the debris avalanche will be at severe risk.
Lava flow	The flow of lava away from a volcanic vent	People and infrastructure are at risk, but lava flows tend to be slow (typically a few meters per hour on average) and relatively easy to avoid.

Volcanic Gas and Tephra Emissions

Large volumes of tephra (rock fragments, mostly pumice, and volcanic ash) and gases are emitted during major explosive eruptions at composite volcanoes, and a large volume of gas is also released during some high-volume effusive eruptions. One of the major implications of these emissions is cooling of the climate by up to 1˚ 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 time this happened was in 1991 and 1992 following the large eruption of Mt. Pinatubo in the Philippines. 1˚ C may not seem like a lot, but that was the global average amount of cooling, and cooling was more severe in some regions and at some times.

Over an 8-month period in 1783 and 1784 a large 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, and a significant volume of hydrofluoric acid (HF). The sulphate aerosols that formed in the atmosphere led to dramatic cooling in the northern hemisphere. In Iceland poisoning from the HF resulted in the death of 80% of sheep, 50% of cattle, and the ensuing famine, along with HF poisoning, resulted in the over 10,000 human deaths, about 25% of the population. The Laki eruption also resulted in many deaths in Europe, although the total number isn’t known, it is estimated that there were approximately 20,000 deaths in the United Kingdom because of very cold weather,^[1] and it seems likely that other parts of northern Europe would have been similarly affected.

Volcanic ash can also have serious implications for aircraft because it can destroy jet engines. In 2010 the eruption of Iceland’s Eyjafjallajökull volcano led to the closure of the European airspace for several days, and the cancellation of numerous trans-Atlantic flights.

Not all the environmental effects of volcanism are related to eruptions of magma. The craters of dormant volcanoes are commonly filled with water (such as Crater Lake in Oregon). Within Lake Nyos in west central Cameroun, gases emanating from the underlying magma chamber continually percolate upward into the muddy lake sediment and bottom waters. One August night in 1986 a landslide, an earthquake or a minor eruption disturbed the lake sediment and the water and released approximately 100 million cubic meters of carbon dioxide from the lake bottom. The CO₂ quickly bubbled up through the water and out into the air above the lake. The gas spilled over the lip of the crater and descended in a white cloud down into the valleys surrounding the crater. Over 1700 people and 3000 cattle were killed in their sleep.

Pyroclastic Density Currents

In a typical explosive eruption at a composite volcano the tephra and gases are hot enough to be buoyant in the air and they are forced high up into the atmosphere. As the eruption proceeds, and the ejected materials start to cool, parts become heavier than air and they can then flow downward along the flanks of the volcano (Figure $\PageIndex{1}$). As they descend, they cool more and so flow faster, reaching speeds up to several hundred km/h. The temperature of this material can be as high as 1000˚ C. The most famous examples are the one that destroyed Pompeii in the year 79 AD, killing an estimated 18,000 and the one that destroyed the town of St. Pierre, Martinique in 1902, killing an estimated 30,000.

Figure $\PageIndex{1}$: The eruption of Mt. Mayon, Philippines. in 1984. Although most of the eruption column is ascending into the atmosphere, there are pyroclastic density currents flowing down the sides of the volcano in several places.

Pyroclastic Falls

Most of the tephra from an explosive eruption ascends high into the atmosphere, and some of it is distributed around the Earth by high altitude winds. The larger components (larger than 0.1 mm) tend to fall relatively close to the volcano, and the amount produced by large eruptions can cause serious damage and casualties. The large 1991 eruption of Mt. Pinatubo in the Philippines resulted in the accumulation of tens of centimeters of ash in fields and on rooftops in the surrounding populated region.

Lahars

A lahar is any mudflow or debris flow that is related to a volcanic eruption or volcanic mountain. Most are caused by melting snow and ice during an eruption, as was the case with the lahar that destroyed the Columbian city of Armero in 1985 (Figure $\PageIndex{2}$). Lahars can also happen when there is no volcanic eruption, and one of the reasons is that, as we’ve seen, composite volcanoes tend to be weak and easily eroded.

Figure $\PageIndex{2}$: Part of the City of Armero Following the 1985 Lahar.

For several reasons, there are significant lahar risks from Washington State’s Mt. Rainier:

It is the tallest and largest of the Cascade Range volcanoes,
It has numerous large glaciers and accumulates a great deal of snow in winter,
It has a significant volume of rock that has been weakened by alteration,
It has been active within the past 125 years, and
There are several large communities close to the mountain and within the channels of known past lahars (Figure $\PageIndex{3}$).

According to a US Geological Survey (USGS) publication^[2], Mt. Rainier is a lahar risk because there is a likelihood of melting snow and ice during an eruption, and also because there is a risk of slope failure at any time, but especially if there is a large earthquake in the region, or if there is movement of magma within the volcano itself. The USGS, Pierce County Department of Emergency Management, and Washington State Emergency Management Division have established an array of motion sensors in the drainage channels around Mt. Rainier. The system is designed to detect the vibrations associated with a lahar and then to alert at risk residents so that they have time to get to higher ground.

Figure $\PageIndex{3}$: The Debris Flow and Lahar Risk Around Mt. Rainier in West-Central Washington.

Lava Flows

Lava flows at volcanoes like Kilauea do not advance very quickly, and in most cases, people can get out of the way. Of course, it is more difficult to move infrastructure, and so buildings and roads are typically the main casualties of lava flows.

That was the case with the massive Kilauea eruption in 2018 where lava flowed through a mostly rural area (Figure $\PageIndex{4}$), resulting in 24 injuries, no deaths, $800 million in damages, and the loss of over 800 homes as a well as a geothermal energy plant. But the situation at Mount Nyiragongo in the Republic of the Congo in 2002 was quite different. There a similar-sized lava stream flowed through Goma, a city of 200,000 people, destroying thousands of buildings, and leaving about 120,000 people homeless. A total of 245 people died, most from carbon-dioxide asphyxiation.

Figure $\PageIndex{4}$: Kilauea’s Lower Puna Lava Flow on May 19th, 2018

Media Attributions

Figure $\PageIndex{1}$: Pyroclastic Flows at Mayon Volcano, by C. Newhall, US Geological Survey, public domain, via Wikipedia, https://en.Wikipedia.org/wiki/File:P...on_Volcano.jpg)
Figure $\PageIndex{2}$: Armero Mudflow and Ruins by N. Banks, US Geological Survey, Public domain, via Wikimedia Commons, https://commons.wikimedia.org/wiki/F..._and_ruins.jpg
Figure $\PageIndex{3}$: Debris Flow and Lahar risks map, Public domain, from Driedger, C. & Scott, W. (2008). Mount Rainier: Living safely with a volcano in your backyard. US Geological Survey, Fact Sheet 2008-3062, p.3. https://doi.org/10.3133/fs20083062
Figure $\PageIndex{4}$: Kīlauea Volcano’s Lower East Rift Zone from US Geological Survey, 2018, public domain, via Wikimedia Commons, https://commons.wikimedia.org/wiki/F...aFile-2062.jpg

Witham, C. S., Oppenheimer, C. (2004). Mortality in England during the 1783–4 Laki Craters eruption. Bulletin of Volcanology 67, 15–26. https://doi.org/10.1007/s00445-004-0357-7 ↵
Driedger, C. & Scott, W. (2008). Living safely with a volcano in your back yard. U.S. Geological Survey, Fact Sheet 2008-3062, p.4. https://pubs.usgs.gov/fs/2008/3062/fs2008-3062.pdf ↵

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