volcano hazard v2.notebook

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May 01, 2013

Apr 24­5:30 PM

Volcano Hazards

Apr 24­5:31 PM

What hazards do you expect from volcanoes?

Apr 24­5:47 PM

Hazards

1. Lava Flows

2. Pyroclasts

3. Lahars

4. Gas

5. Other Hazards

Apr 24­5:49 PM

Lava Flows• Magma becomes lava when it hits the surface

• Lava is hot, it can melt through objects that enter its path

• Volcanologists use two Hawaiian terms to describe solidified lava flows on land: pahoehoe (puh­HOH­eh­HOH) and aa (AH­ah)

• Whether it comes from underwater eruptions or flows from land into the water, lava that cools underwater looks very distinctive, and is called pillow lava.

Apr 24­5:55 PM

Pahoehoe vs AaPahoehoe

> slow moving (anywhere from0.1 km/h to 30 km/h)

> starts hot and cools slowly> low viscosity, flows easily> low silica content> the smooth surface helps

prevent heat loss

Aa> fast moving > initial temperature is lower

than Pahoehoe> high viscosity, high silica

content> fast speed results in more

contact with cool air> cools into jagged rock

Apr 24­6:09 PM

Pyroclasts• Volcanoes that produce no lava can still cause problems• Particles of rock ejected during an eruption are called pyroclastic material or pyroclasts

• Pyroclastic material is classified by size, they are (smallest to largest): Ash, Lapilli, and Block or Bomb

Ashdiameter less than 2 mm

Lapillidiameter from 2 to 64 mm

Block or Bombdiameter greater than 64 mm

volcano hazard v2.notebook

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May 01, 2013

Apr 24­6:19 PM

Pyroclastic Flow

In some violent eruptions, pyroclastic material combines with hot gases to form a pyroclastic flow ­ dense, superheated cloud that travels downhill with amazing speed. These flows reach temperatures up to 1000 C and can travel faster than 100 km/h, with some believed to travel up to 700 km/h at times.

Apr 24­7:34 PM

Lahars• A lahar is a type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water.

• Lahars are extremely destructive, can flow at tens of metres per second, and be more than 100 metres deep.

• Lahars have the consistency, viscosity, and approximate density of concrete: fluid while moving, solid at rest.

Apr 24­7:41 PM Apr 24­7:51 PM

Gas

Most of the gas released by a volcano is water vapour, but volcanoes also release carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S), flourine gas (F2), hydrogen flouride (HF), and other gases, many of which are poisonous or toxic.

Carbon dioxide is not poisonous, but it displaces normal oxygen­bearing air, and is odorless and colorless. Because it is heavier than air, it collects in depressions and can suffocate people and animals who wander into pockets where it has displaced normal air.

Apr 24­7:51 PM

GasSO2 can combine with water vapor in the air to form sulfuric acid (H2SO4), a corrosive acid; H2S is also very acidic, and extremely poisonous even in small amounts.

One of the nastiest, although less common gases released by volcanoes is fluorine gas (F2). This gas is yellowish brown, corrosive and extremely poisonous and tends to collect in low areas. Its companion acid, hydrogen fluoride (HF), is highly corrosive and toxic, and causes terrible internal burns.

Apr 24­7:49 PM

Other Hazards• The ash spewed out during an eruption can be blown around the globe by wind patterns, and be kept aloft for long periods of time, partially blocking out the Sun

• Landslides are common on volcanoes because their massive cones typically rise hundreds to thousands of meters above the surrounding terrain, and are often weakened by the very process that created them­­the rise and eruption of molten rock. Each time magma moves toward the surface, overlying rocks are shouldered aside as the molten rock makes room for itself. > Furthermore, the tremendous mass of thousands of layers

lava and loose fragmented rock debris can lead to internal faults and fault zones that move frequently as the cone "settles" under the downward pull of gravity.