Volcanic Hazards (Part 1)

Introduction

Volcanoes produce a wide variety of hazards that can prove fatal and harmful to people as well as destroy property. Large explosive eruptions can endanger people’s lives and obliterate property hundreds of miles away and even affect global climate.

Various Volcanic Hazards
Source: http://pubs.usgs.gov/fs/fs002-97/

Classification of Volcanic Hazards

• Primary volcanic hazard: occurred as a result of volcanic activity itself.
• Secondary volcanic hazard: caused by a primary effect.
  

List of Volcanic Hazards

• Volcanic gases (primary effect)
• Lava flows and domes (primary effect)
• Pyroclastic flows (primary effect)
• Volcanic landslides (secondary effect)
• Lahars (secondary effect)
• Earthquakes (secondary effect)
• Tsunamis (secondary effect)
  

Some of the deadliest volcanic eruptions in the world

Volcanic Gases

Volcanoes emit gases during eruptions. Even when a volcano is not erupting, cracks in the ground allow gases to reach the surface through small openings called fumaroles. he gaseous portion of magma varies from ~1 to 5% of the total weight. More than 90% of all gas emitted by volcanoes is water vapor (steam), most of which is heated ground water (underground water from rain fall and streams). Other common volcanic gases are carbon dioxide, sulphur dioxide, hydrogen sulfide, hydrogen, and fluorine. Sulphur dioxide gas can react with water droplets in the atmosphere to create acid rain, which causes corrosion and harms vegetation. Carbon dioxide is heavier than air and can be trapped in low areas in concentrations that are deadly to people and animals. Fluorine, which in high concentrations is toxic, can be absorbed onto volcanic ash particles that later fall to the ground. The fluorine on the particles can poison livestock grazing on ash-coated grass and also contaminate domestic water supplies.


Lava Flows and Domes

Lava flow moves through an intersection on the south flank of Kilauea
Source: http://volcanoes.usgs.gov/Hazards/What/Lava/lavaflow.html

Lava flows are streams of molten rock that pour or ooze from an erupting vent. Lava is erupted during either nonexplosive activity or explosive lava fountains. Lava flows destroy everything in their path, but most move slowly enough that people can move out of the way. The speed at which lava moves across the ground depends on several factors, including (1) type of lava erupted and its viscosity; (2) steepness of the ground over which it travels; (3) whether the lava flows as a broad sheet, through a confined channel, or down a lava tube; and (4) rate of lava production at the vent.

Low-silica basalt lava can form fast-moving (10 to 30 miles per hour) streams or can spread out in broad thin sheets up to several miles wide. Since 1983, Kilauea Volcano on the Island of Hawaii has erupted basalt lava flows that have destroyed more than 200 houses and severed the nearby coastal highway.

In contrast, flows of higher-silica andesite and dacite lava tend to be thick and sluggish, traveling only short distances from a vent. Dacite and rhyolite lavas often squeeze out of a vent to form irregular mounds called lava domes. Lava domes often grow by the extrusion of many individual flows >30 m thick over a period of several months or years. Such flows will overlap one another and typically move less than a few meters per hour.


Pyroclastic Flows

Pyroclastic flows descend the south-eastern flank of Mayon Volcano, Philippines.
Source: http://volcanoes.usgs.gov/Imgs/Jpg/Mayon/32923351-020_large.jpg

High-speed avalanches of hot ash, rock fragments, and gas can move down the sides of a volcano during explosive eruptions or when the steep side of a growing lava dome collapses and breaks apart. These pyroclastic flows can be extremely hot and move at speeds of 100 to 150 miles per hour. Such flows tend to follow valleys and are capable of knocking down and burning everything in their paths. Lower-density pyroclastic flows, called pyroclastic surges, can easily overflow ridges hundreds of feet high.

The climactic eruption of Mount St. Helens on May 18, 1980, generated a series of explosions that formed a huge pyroclastic surge. This so-called "lateral blast" destroyed an area of 230 square miles. Trees 6 feet in diameter were mowed down like blades of grass as far as 15 miles from the volcano.

How collapse of a growing lava dome generates the nuée ardente.
Source: http://www.geology.sdsu.edu/how_volcanoes_work/Images/Diagrams/PFDomeCollaps_crop_med.GIF

Pyroclastic flows erupted by Mount Pinatubo on June 15, 1991,
buried the Marella River valley with pumice, ash, and other volcanic rocks
Source: http://volcanoes.usgs.gov/Imgs/Jpg/PFeffects/3041135-092_large.JPG

Tephra and Ash Fall

Tephra is a general term for fragments of volcanic rock and lava regardless of size that are blasted into the air by explosions or carried upward by hot gases in eruption columns or lava fountains. Such fragments range in size from less than 2 mm (ash) to more than 1 m in diameter. Large-sized tephra typically falls back to the ground on or close to the volcano and progressively smaller fragments are carried away from the vent by wind. Volcanic ash, the smallest tephra fragments, can travel hundreds to thousands of kilometers downwind from a volcano.

Tephra consists of a wide range of rock particles (size, shape, density, and chemical composition), including combinations of pumice, glass shards, crystals from different types of minerals, and shattered rocks of all types (igneous, sedimentary, and metamorphic). A great variety of terms are used to describe the range of rock fragments thrown into the air by volcanoes. The terms classify the fragments according to size, shape, or the way in which they form and travel.

Tephra deposit about 9 cm thick blankets former U.S. Clark Air Base, Philippines, about 25 km east of Mount Pinatubo.
Source: http://volcanoes.usgs.gov/Imgs/Jpg/Pinatubo/16112441-008_large.jpg