Volcanic Hazards
This page will discuss the impact of volcanic eruptions on the anthrosphere and the hazards and risks volcanoes have on the surrounding communities.
Basaltic Lava flows (Hawaiian Eruptions)
As mention on the Basic Volcanology page basaltic lava has low viscosity which means it is relatively runny and travels far distances at a slow rate. It is unlikely that these lava flows are life threatening but they can destroy anything in its path as it slowly engulfs the landscape and it is very hard to stop or divert a lava flow but you can easily out run it as it travels at a slow speed (Marshak, 2015). The hazard with basaltic lava flows is the amount of lava that is erupted to the surface and where that lava goes, they can cover roads, houses and vehicles. “The most disastrous lava flow in recent times came from the 2002 eruption of Mt.Nyiragongo in the East African Rift. Lava flows travelled almost 50km and flooded the streets in the Congolese city of Goma encasing them with a 2-m thick layer of basalt”(Marshak, 2015). In the past authorities have halted and/or diverted lava flows to prevent further damage by saturating the lava in sea water using firehoses, which was seen effective in Heimaey, Iceland in 1973, or by using bulldozers to construct walls to divert the lava flows (Stoiber, 1990). Halting or diverting lava flows could have negative consequences as they could cause the lava to travel to places that would be more hazardous (Stoiber, 1990). |
Figure 3.0 :Heimaey, Iceland, 1973, Firefighters managed to cool the lava flow with firehoses pumping water from the sea and preventing further damage.("Methods And Procedures, Lava-Cooling Operations During The 1973 Eruption Of Eldfell Volcano, Heimaey, Vestmannaeyjar, Iceland, U.S. Geological Survey Open-File Report 97-724, By Richard S. Williams, Jr., Editor")
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Pyroclastic Flows
Pyroclastic flows are debris ejected from an eruption that contain viscous masses of lava, ash and volcanic debris that move as a hot avalanche down the surface of a volcano which destroys everything in its path. They are poorly sorted massive movements of ash <2mm in size, lapilli 2-64mm in size and bombs and blocks >64mm in size, all of different sediment sizes together are known as tephra which forms a rock called ignimbrite when deposited (Marshak, 2015). Pyroclastic flows can travel a speed approximately 60m/s (meters per second) but some reports have seen pyroclastic density currents moving as fast as 100m/s and with temperatures exceeding 800 degrees celsius a pyroclastic flow kills everything in its path (Bobrowsky, 2013). An example of its devastation “during the eruption of Montagne Pelée in 1902, 28,000 people were killed when a cloud of hot gas and ash [pyroclastic flow] destroyed the peaceful village of Saint Pierre, of Martinique…” (Bobrowsky, 2013). The initial hazards of pyroclastic flows can be hard to mitigate as they are not influenced by anthropogenic activities communities vulnerable to such a hazards must implement preparedness and organization. Unlike basaltic lava flows modern technology cannot divert or halt a pyroclastic flows and the only suitable mitigation tactic would be to move out of the path of potential pyroclastic flows (Bobrowsky, 2013). |
Video 3.0 : Mt Unzen, Nagasaki, Japan of a pyroclastic flow soring down the volcano at high speeds and temperatures killing 43 people.("Dome Collapse And Pyroclastic Flow At Unzen Volcano, 2010").
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Lahars/Volcanic Debris Flow
Lahars are volcanic debris flows that are composed of ash, water, rock and other land debris. They can travel an excess of 100km per hour and can compose of large boulders which are carried towards the surface of the lahar due to dispersive forces where larger sediments are pushed upwards due to kinetic energy (Arbogast, 2014). They are sourced from the craters of volcanoes where ash has accumulated either from current or previous eruptions and the ash mixes with water from either rainfall or snow melt (Marshak, 2015). This accumulation of water and ash will eventually breach its walls through either an eruption, earthquake or crater walls giving way to the stress from the accumulation. An example of the wrath of lahars is in November 13, 1985, in the Andes Mountains, Columbia, where an eruption mixed melted ice with volcanic ash. While the town of Armero was asleep the lahar swept through killing 20,000 residents (Marshak, 2015). Lahars can be mitigated through expensive engineering infrastructures that can divert lahars away from communities, although an easier approach would be to map out the paths of lahars and abandon the areas of risk (Stoiber, 1990). Other tactics towards the mitigation of lahars include warning alarms/sirens, monitoring or trip wires across the probably paths of lahars (Stoiber, 1990). |
Video 3.1: Lahar flowing down a river valley in Semeru, Indonesia.("Lahar In Semeru (2003)").
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Volcanic Ash Fall
The most disruptive hazard from volcanoes is the volcanic ash as it is widely distributed in the atmosphere and can be plumed into the troposphere and if the eruption is big enough the stratosphere (Marshak, 2015). Volcanic ash contains gases that create negative effects in the atmosphere, the gases released by volcanic eruptions are “water, carbon dioxide, sulfur dioxide, hydrogen sulfide, carbon monoxide, hydrochloric acid, and hydrofluoric acid” (Wilson & Stewart, 2013). The distribution of the eruption plumes depend on the wind direction and strength, which will determine whether ash is deposited hundreds to kilometres form the volcano (Wilson & Stewart, 2013). The impacts of volcanic ash on humans depend on the amount exposed to the communities. Some exposure can cause respiratory health issues and where the ash does not directly affect humans it can have a toll on infrastructure “such as electricity and water supplies, transport routes, wastewater treatment and drainage systems and communications networks, disruptions to aviation, building damage and impacts on crops and livestock can all lead to significant societal impacts” (Wilson & Stewart, 2013). 455 million people worldwide (9% of the global population) live within 100km of active volcanoes which put them all at risk of volcanic ash hazards and with ash fall being the most disruptive volcanic hazard its affects can be catastrophic if a large eruption were to occur (Horwell & Baxter, 2006). |
Figure 3.1 : Explosive eruption in Ecuador, 2013, 5km high Volcanic Ash plume moving east.(Newsimg.bbc.co.uk. N.p., 2016. Web. 13 Oct. 2016.)
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