4.6: Volcanic Hazards and Monitoring

Volcanic Hazards

While the most obvious volcanic hazard is lava, the dangers posed by volcanoes go far beyond lava flows. For example, on May 18, 1980, Mount St. Helens (Washington, United States) erupted with an explosion and landslide that removed the upper 400 m (1300 ft) of the mountain. The initial explosion was immediately followed by a lateral blast, which produced a pyroclastic flow that covered nearly 600 km² (230 mi²) of the forest with hot ash and debris [23]. The pyroclastic flow moved at speeds of 80-130 km per hour (50-80 mph), flattening trees and ejecting clouds of ash into the air.

Figure \(\PageIndex{1}\): General diagram of volcanic hazards around a stratovolcano. (By USGS; public domain.)

The USGS video provides an account of this explosive eruption that killed 57 people [24].

Figure \(\PageIndex{2}\): Left: Mount St. Helens, the day before the May 18, 1980 eruption. Right: 4 months after the major eruption of Mount St. Helens.

Figure \(\PageIndex{3}\): Series of still images of the May 18, 1980, eruption of Mount St. Helens, Washington showing largest recorded landslide in history and subsequent eruption and pyroclastic flow (By The Associated Press via The Atlantic)

In 79 CE, Mount Vesuvius, located near Naples, Italy, violently erupted sending a pyroclastic flow over the Roman countryside, including the cities of Herculaneum and Pompeii. The buried towns were discovered in an archeological expedition in the 18th century [25]. Pompeii famously contains the remains (casts) of people suffocated by ash and covered by 10 feet (3 m) of ash, pumice lapilli, and collapsed roofs [26].

Figure \(\PageIndex{4}\): Human remains from the 79 CE eruption of Vesuvius. (By Gary Todd; public domain via Wikimedia Commons.)

Pyroclastic Flows

The most dangerous volcanic hazard is a pyroclastic flow. These flows are a mix of lava blocks, pumice, ash, and hot gases between 200°C-700°C (400°F-1300°F). The turbulent cloud of ash and gas races down the steep flanks at high speeds up to 193 km per hour (120 mph) into the valleys around composite volcanoes [24]. Most explosive, silica-rich, high viscosity magma volcanoes such as composite cones usually have pyroclastic flows. The rocks tuff and welded tuff are often formed from these pyroclastic flows.

Figure \(\PageIndex{5}\): The material coming down from the eruption column onto the flanks is a pyroclastic flow.

There are numerous examples of deadly pyroclastic flows. In 2014, the Mount Ontake pyroclastic flow in Japan killed 47 people. The flow was caused by magma heating groundwater into steam, which then rapidly ejected with ash and volcanic bombs. Some were killed by inhalation of toxic gases and hot ash, while others were struck by volcanic bombs [27]. This video shows an eye-witness account of the Ontake pyroclastic flow.

In the early 1990s, Mount Unzen erupted several times with pyroclastic flows. The pyroclastic flow shown in this short video killed 41 people.

 In 1902, on the Caribbean Island Martinique, Mount Pelee erupted with a violent pyroclastic flow that destroyed the entire town of St. Pierre and killing 28,000 people in moments.

Figure \(\PageIndex{6}\): The remains of St. Pierre after the eruption of Mount Pelee.

Landslides and Landslide-Generated Tsunamis

The steep and unstable flanks of a volcano can lead to slope failure and dangerous landslides. These landslides can be triggered by magma movement, explosive eruptions, large earthquakes, and/or heavy rainfall. During the 1980 Mount St. Helens eruption, the entire north flank of the volcano collapsed and released a huge landslide that moved at speeds of 160-290 km per hour (100-180 mph).

Figure \(\PageIndex{7}\): Sequence of events for Mount St. Helens, May 18, 1980. Note that an earthquake caused a landslide, which caused the “uncorking” of the mountain and started the eruption.

If enough landslide material reaches the ocean, it may cause a tsunami. In 1792, a landslide caused by the Mount Unzen eruption reached the Ariaka Sea, generating a tsunami that killed 15,000 people. When Mount Krakatau in Indonesia erupted in 1883, it generated ocean waves that towered 40 m (131 ft) above sea level. The tsunami killed 36,000 people and destroyed 165 villages [24].

Tephra

Volcanoes, especially composite volcanoes, eject large amounts of tephra (ejected rock materials), most notably ash (tephra fragments less 2 mm). Larger tephra is heavier and falls closer to the vent. Larger blocks and bombs pose hazards to those close to the eruption such as at the 2014 Mount Ontake disaster in Japan discussed earlier.

Figure \(\PageIndex{8}\): A man sweeps ash from an eruption of Kelud, Indonesia.

Hot ash poses an immediate danger to people, animals, plants, machines, roads, and buildings located close to the eruption. Ash is fine-grained (< 2mm) and can travel airborne long distances away from the eruption site. Heavy accumulations of ash can cause buildings to collapse. In people, it may cause respiratory issues like silicosis. Ash is destructive to aircraft and automobile engines, which can disrupt transportation and shipping services [24]. In 2010, the Eyjafjallajökull volcano in Iceland emitted a large ash cloud into the upper atmosphere, causing the largest air-travel disruption in northern Europe since World War II. No one was injured, but the service disruption was estimated to have cost the world economy billions of dollars [29].

Figure \(\PageIndex{9}\): Micrograph of silica particle in volcanic ash. A cloud of these is capable of destroying an aircraft or automobile engine.

Volcanic Gases

As magma rises to the surface, the confining pressure decreases and allows dissolved gases to escape into the atmosphere. Even volcanoes that are not actively erupting may emit hazardous gases, such as carbon dioxide (CO₂), sulfur dioxide (SO₂), hydrogen sulfide (H₂S), and hydrogen halides (HF, HCl, or HBr).

Carbon dioxide tends to sink and accumulate in depressions and basins. In volcanic areas known to emit carbon dioxide, low-lying areas may trap hazardous concentrations of this colorless and odorless gas. The Mammoth Mountain Ski Resort in California, is located within the Long Valley Caldera, is one such area of carbon dioxide-producing volcanism. In 2006, three ski patrol members died of suffocation caused by carbon dioxide after falling into a snow depression near a fumarole (article).

In rare cases, volcanism may create a sudden emission of gases without warning. Limnic eruptions (limne is Greek for lake), occur in crater lakes associated with active volcanism. The water in these lakes is supercharged with high concentrations of dissolved gases. If the water is physically jolted by a landslide or earthquake, it may trigger an immediate and massive release of gases out of solution. An analogous example would be what happens to a vigorously shaken bottle of carbonated soda when the cap is opened. An infamous limnic eruption occurred in 1986 at Lake Nyos, Cameroon. Almost 2000 people were killed by a massive release of carbon dioxide [24].

Lahars

Lahar is an Indonesian word and is used to describe a volcanic mudflow that forms from rapidly melting snow or glaciers. Lahars are slurries resembling wet concrete and consist of water, ash, rock fragments, and other debris. These mudflows flow down the flanks of volcanoes or mountains covered with freshly-erupted ash, and on steep slopes can reach speeds of up to 80 km per hour (50 mph).

Figure \(\PageIndex{10}\): Mud line shows the extent of lahars around Mount St. Helens.

Several major cities, including Tacoma, are located on prehistoric lahar flows that extend for many kilometers across the flood plains surrounding Mount Rainier in Washington (see map). A map of Mount Baker in Oregon shows a similar potential hazard for lahar flows [24]. A tragic scenario played out in 1985, when a lahar from the Nevado del Ruiz volcano in Colombia buried the town of Armero and killed an estimated 23,000 people.

Figure \(\PageIndex{11}\): Old lahars around Tacoma, Washington.

Monitoring

Geologists use various instruments to detect changes or indications that an eruption is imminent [30][31]. The three videos show different types of volcanic monitoring used to predict eruptions 1) earthquake activity, 2) increases in gas emission, and 3) changes in land surface orientation and elevation.

This video shows how monitoring earthquake frequency, especially special vibrational earthquakes called harmonic tremors, can detect magma movement and possible eruption.

This video shows how gas monitoring may be used to predict an eruption. A rapid increase of gas emission may indicate magma that is actively rising to the surface and releasing dissolved gases out of solution, and that an eruption is imminent.

The last video shows how a GPS unit and tiltmeter can detect land surface changes, indicating the magma is moving underneath it.

References

23. USGS. The Volcanoes of Lewis and Clark - Mount St. Helens. (2017). Available at: https://volcanoes.usgs.gov/observatories/cvo/Historical/LewisClark/Info/ summary_mount_st_helens.shtml. (Accessed: 14th March 2017)

24. USGS. USGS: Volcano Hazards Program. (2012). Available at: https://volcanoes.usgs.gov/vhp/hazards.html. (Accessed: 30th July 2016)

25. Scandone, R., Giacomelli, L. & Gasparini, P. Mount Vesuvius: 2000 years of volcanological observations. Journal of Volcanology and Geothermal Research 58, 5–25 (1993).

26. Luongo, G. et al. Impact of the AD 79 explosive eruption on Pompeii, II. Causes of death of the inhabitants inferred by stratigraphic analysis and areal distribution of the human casualties. J. Volcanol. Geotherm. Res. 126, 169–200 (2003).

27. EarthScope. Mount Ontake Volcanic Eruption. (2014). Available at: http://www.earthscope.org/science/geo-events/mount-ontake-volcanic-eruption. (Accessed: 30th July 2016)

28. USGS. Dual volcanic tragedies in the Caribbean led to founding of HVO. (2001). Available at: http://hvo.wr.usgs.gov/volcanowatch/archive/2001/01_05_03.html. (Accessed: 30th July 2016)

29. Perkins, S. 2010’s Volcano-Induced Air Travel Shutdown Was Justified. (2011). Available at: http://www.sciencemag.org/news/2011/04/2010s-volcano-induced-air-travel-shutdown-was-justified. (Accessed: 30th July 2016)

30. Tilling, R. I. The critical role of volcano monitoring in risk reduction. Adv. Geosci. 14, 3–11 (2008).

31. Stovall, W. K., Wilkins, A. M., Mandeville, C. W. & Driedger, C. L. Fact Sheet. (2016).