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VOLCANIC HAZARDS AND MONITORING
Volcanoes are responsible for a large number of deaths. Volcanic hazards have been famous for centuries, but recent eruptions are better documented. The most obvious hazard is the lava itself found within a lava flow, but the hazards posed by volcanoes go far beyond a lava flow. For example, on May 18, 1980, Mount Saint Helens erupted with an explosion and landslide that removed the upper 1,300 feet (400 m) of the mountain. This explosion was immediately followed by a lateral blast and pyroclastic flow  that covered 230 square miles of forest with ash and debris. The effects of the blast are shown on the before and after images (see figures). The pyroclastic flow (see below) moved at speeds of 50 – 80 miles per hour (80-130 km/hr), flattened trees and ejected a giant ash cloud into the air. Watch the 7-minute USGS video for an account of May 18, 1980, which killed 57 . Pyroclastic flows are common in explosive eruptions of stratovolcanoes.
In 79 AD, 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. 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.
The most dangerous volcanic hazard are pyroclastic flows (video). These flows are a mix of lava blocks, pumice, ash, and hot gases between 400 to 1,300 ℉. The turbulent cloud of ash and gas races down the steep flanks at high speeds up to 120 mph (much faster than people can run) into the valleys around composite volcanoes . Most explosive, silica-rich, high viscosity magma volcanoes such as composite cones usually have pyroclastic flows. The rock tuff and welded tuff is often formed from these pyroclastic flows.
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 . Two short videos below document eyewitness video of pyroclastic flows. In the early 1990s, Mount Unzen erupted several times with pyroclastic flows. The pyroclastic flow shown in this famous 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.
LANDSLIDES AND LANDSLIDE-GENERATED TSUNAMIS
The flanks of a volcano are steep and unstable which can lead to slope failure and generate dangerous landslides. For example, the landslide at Mount St. Helens 1980 released a considerable amount of materials as the entire north flank collapsed. The landslide moved at speeds of 100-180 mph. These landslides can be triggered by movement of magma, explosive eruptions, large earthquakes, and heavy rainfall. In unique situations, the landslide material can reach water and cause a tsunami. In 1792 in Japan, Mount Unzen erupted causing a giant landslide that reached the Ariake Sea and made a tsunami that killed 15,000 people on the opposite shore.
A lahar is an Indonesian word for a mudflow that is a mixture of water, ash, rock fragments, and other debris moving down the flanks of a volcano (or other nearby mountains covered with freshly-erupted ash) and entering adjacent river valleys. They form from the rapid melting of snow or glaciers on volcanoes. They are similar to a slurry of concrete but can flow up to 50 mph while still on the steep flanks. Since lahars are slurry-like, they can travel long distances in river valleys almost like a flash flood.
During the 1980 Mount St. Helens eruption, lahars reached 17-miles (27 km) down the North Fork of the Toutle River. Prehistoric lahar flows have been mapped at significant volcanoes such as Mount Rainier near Tacoma, Washington (Rosi et al. 1999). Prehistoric lahars occupied river floodplains where large cities are located today as shown on the map. Similarly, Mount Baker poses a hazard as shown by this hazards map for Mount Baker north of Seattle, Washington. A recent scenario played out when a lahar from the volcano Nevado del Ruiz in Colombia buried a town in 1985 and killed an estimated 25,000 people.
TEPHRA AND ASH
Volcanoes, especially composite volcanoes, eject large amounts of tephra (ejected rock materials) and ash (fragments less than 0.08 inches [2 mm]). 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. Ash is fine and can be carried long distances away from the vent, and can cause building collapses and respiratory issues like silicosis. Hot ash can be dangerous to those close to the eruption and disrupt services such as airline transportation farther away . For example, in 2010 the Eyjafjallajökull volcano in Iceland created a large ash cloud in the upper atmosphere that caused the most significant air travel disruption in northern Europe since a seven-day airline shut down during World War II. No one was hurt, but the cost to the world economy was estimated to be billions of dollars.
Magma contains dissolved gases. As rising magma reaches the surface, the confining pressure decreases allowing gases to escape; similar to gases coming out of solution after opening a soda bottle. Therefore, volcanoes when not erupting release hazardous gases such as carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S) and hydrogen halides (HF, HCl, or HBr). Carbon dioxide can sink and accumulate in low-lying depressions on the earth’s surface. For example, Mammoth Mountain Ski Resort in Mammoth Lakes, California is located within the Long Valley Caldera. Therefore, the whole ski resort and town are within the caldera. In 2006, three ski patrol members were killed after skiing into snow depressions near fumaroles that had filled with carbon dioxide (info). Therefore, in volcanic areas where carbon dioxide emissions occur, avoid low-lying areas that may trap carbon dioxide. In rare cases, a volcano can suddenly release gases without warning. Called a limnic eruption, this commonly occurs in crater lakes as gases pour from the water. It infamously occurred in 1986 in Lake Nyos, Cameroon, killing almost 2,000 people due to carbon dioxide asphyxiation.
Volcano monitoring requires geologists to use many instruments to detect changes that may indicate an eruption is imminent. Some of the main observations include regular monitoring for 1) earthquakes (including unique vibrational earthquakes called harmonic tremor, caused by magma movement), 2) changes in the orientation and elevation of the land surface, and 3) increase in gas emission. Concise videos (below) summarize how an increased frequency of earthquakes can show that magma is moving and that an eruption may occur soon. Another video (below) shows how gas monitoring is used to monitor volcanoes and predict an eruption. As the magma gets closer to the surface and pressure is released, the gases come out of solution in the magma. A rapid increase of gas emission can indicate an eruption is imminent. The last video (below) shows how a GPS unit and tiltmeter can detect movement of the land indicating that the magma is moving underneath.
Introduction to Physical Geography by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
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