Tertiary volcanic hazards what are they?

Question

Tertiary volcanic hazards what are they?

Answer 1

www.tulane.edu/~sanelson/geol204/volhaz&...

Answer 2

Primary Effects of Volcanism

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      Lava Flows

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            Lava flows are common in Hawaiian and Strombolian type of eruptions, the least explosive.

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            Although lava flows have been known to travel as fast as 64 km/hr, most are slower and give people time to move out of the way.

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            Thus, in general, lava flows are most damaging to property, as they can destroy anything in their path.

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            Control of lava flows has been attempted with limited success by bombing flow fronts to attempt to divert the flow, and by spraying with water to cool the flow.  The latter is credited with saving the fishing harbor during a 1973 eruption of Heimaey in Iceland.



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      Violent Eruptions and Pyroclastic Activity

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            Pyroclastic activity is one of the most dangerous aspects of volcanism.

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            Hot pyroclastic flows cause death by suffocation and burning.  They can travel so rapidly that few humans can escape.

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            Lateral blasts knock down anything in their path, can drive flying debris through trees.

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            Tephra falls can cause the collapse of roofs and can affect areas far from the eruption. Although tephra falls blanket an area like snow, they are far more destructive because tephra deposits have a density more than twice that of snow and tephra deposits do not melt like snow.

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            Tephra falls destroy vegetation, including crops, and can kill livestock that eat the ash covered vegetation.

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            Tephra falls can cause loss of agricultural activity for years after an eruption, a secondary or tertiary effect.

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      Poisonous Gas Emissions

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            Volcanoes emit gases that are often poisonous to living organisms. Among these poisonous gases are: Hydrogen Chloride (HCl), Hydrogen Sulfide (H2S), Hydrogen Fluoride (HF), and Carbon Dioxide (CO2).

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            In 1984, CO2 gas escaping from the bottom of Lake Monoun, a crater lake in the African country of Cameroon, killed 37 people.

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            In 1986 an even larger CO2 gas emission from Lake Nyos in Cameroon killed more than 1700 people and 3000 cattle.

LakeNyos.GIF (8328 bytes)



Secondary and Tertiary Effects of Volcanism

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      Mudflows (Lahars)

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            Volcanoes can emit voluminous quantities of loose, unconsolidated tephra which become deposited on the landscape.  Such loose deposits are subject to rapid removal if they are exposed to a source of water.

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            The source of water can be derived by melting of snow or ice during the eruption, emptying of crater lakes during an eruption, or rainfall that takes place any time with no eruption.

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            Thus, mudflows can both accompany an eruption and occur many years after an eruption.

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            Mudflows are a mixture of water and sediment, they move rapidly down slope along existing stream valleys, although they may easily top banks and flood out into surrounding areas.

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            They have properties that vary between thick water and wet concrete, and can remove anything in their paths like bridges, highways, houses, etc.

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            During the Mt. St. Helens eruption of May 18, 1980, mudflows were generated as a result of snow melt on the volcano itself, and deposition of tephra in streams surrounding the mountain.

StHelensMap.GIF (28927 bytes)

          o On November 13, 1985 a mudflow generated by a small eruption on Nevado del Ruiz volcano in Columbia flowed down slope and devastated the town of Armero, 50 km east of the volcano and built on prior mudflow deposits.  The town had several hours of warning from villages higher up slope, but these warnings were ignored, and 23,000 people died in the mudflow that engulfed the town.

NRuiz.GIF (22507 bytes)



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      Debris Avalanches and Debris Flows

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            Volcanic mountains tend to become oversteepened as a result of the addition of new material over time as well due to inflation of the mountain as magma intrudes.

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            Oversteepened slopes may become gravitationally unstable, leading to a sudden slope failure that results in landslides, debris slides or debris avalanches.  We will cover these types of hazards in more detail later in the course and in the next lecture.

            

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            During the May 18, 1980 eruption of Mt. St. Helens, Washington, a debris avalanche was triggered by a magnitude 5.0 earthquake.  The avalanche removed the upper 500 m of the mountain, and flowed into the Spirit Lake, raising its level about 40 m.  It then moved to the west filling the upper reaches of the North Fork of the Toutle River valley (see map above).

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            Debris avalanches, landslides, and debris flows do not necessarily occur accompanied by a volcanic eruption.  There are documented cases of such occurrences where no new magma has been erupted.

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      Flooding

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            Drainage systems can become blocked by deposition of pyroclastic flows and lava flows.  Such blockage may create a temporary dam that could eventually fill with water and fail resulting in floods downstream from the natural dam.

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            Volcanoes in cold climates can melt snow and glacial ice, rapidly releasing water into the drainage system and possibly causing floods. Jokaulhlaups occur when heating of a glacier results in rapid outburst of water from the melting glacier.

            

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      Tsunami

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            Debris avalanche events, landslides, caldera collapse events, and pyroclastic flows entering a body of water may generate tsunami.

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            During the 1883 eruption of Krakatau volcano, in the straits of Sunda between Java and Sumatra, several tsunami were generated by pyroclastic flows entering the sea and by collapse accompanying caldera formation.  The tsunami killed about 36,400 people, some as far away from the volcano as 200 km.

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      Volcanic Earthquakes and Tremors

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            Earthquakes usually precede and accompany volcanic eruptions, as magma intrudes and moves within the volcano.

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            Although most volcanic earthquakes are small, some are large enough to cause damage in the area immediately surrounding the volcano, and some are large enough to trigger landslides and debris avalanches, such as in the case of Mount St. Helens.

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            Volcanic Tremor (also called harmonic tremor) is a type of continuous rhythmic shaking of the ground that is generated by magma moving underground.

            

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      Atmospheric Effects

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            Since large quantities of tephra and volcanic gases can be injected into the atmosphere, volcanism can have a short-term effect on climate.

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            Volcanic ash can cause reflection of solar radiation, and thus can cause the temperatures to be cooler for several years after a large eruption.

            The 1815 eruption of Tambora volcano in Indonesia, was the largest in recorded history.  The year following the Tambora eruption (1816) was called the "year without summer".  Snow fell in New England in July.

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            Volcanic gases like SO2 also reflect solar radiation.  Eruptions in 1981 at El Chichón Volcano, Mexico, and 1991 at Pinatubo, Philippines, ejected large quantities of SO2 into the atmosphere.   The effects of the El Chichón eruption were masked by a strong El Niño in the year following the eruption, but Pinatubo caused a lowering of average temperature by about 1oC for two years following the eruption.

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            Volcanic gases like CO2  are greenhouse gases which help keep heat in the atmosphere.  During the mid-Cretaceous (about 90 to 120 million years ago) the CO2 content of the atmosphere was about  15 times higher than present.  This is thought to have been caused by voluminous eruptions of basaltic magma on the sea floor.  Average temperatures were likewise about 10 to 12oC warmer than present.

            

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      Famine and Disease

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            As noted above, tephra falls can cause extensive crop damage and kill livestock.  This can lead to famine.

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            Displacement of human populations, breakdown of sewerage and water systems, cut off of other normal services can lead to disease for years after an eruption, especially if the infrastructure is not in place to provide for rapid relief and recovery.