Volcanic Hazards: Lahars and Ash Fall

• Study notes on volcanic hazards are described below

Lahars

• A lahar is a specific type of mudflow composed of volcanic debris.
• Lahars can form in several ways: when small slope collapses gather water, through rapid melting of snow and ice during an eruption, from heavy rainfall on loose volcanic debris, when a volcano erupts through a crater lake, or when a crater lake drains because of overflow or wall collapse.
• Lahars move rapidly down valleys, and may occur with or without a volcanic eruption.
• These pick up material as they travel and cause damage to structures.
• Lahars and excess sediment leads to serious economic and environmental damage to river valleys and flood plains.

Ash Fall

• Ash fall refers to all explosive eruption products, including particles larger than 2 mm.
• Volcanic ash consists of fragments of pulverized rock, minerals and volcanic glass created during a volcanic eruption.
• Volcanic ash forms during explosive volcanic eruptions when dissolved gases in magma expand and escape violently into the atmosphere.
• Ash fall poses hazards to aviation and causes disruption and damage to buildings, transportation, water and wastewater systems, power supply, communications equipment, and agriculture.
• Ash fall also has heath impacts to humans and animals.

Pyroclastic Flows

• Pyroclastic flows are high-density mix of hot lava blocks, pumice, ash and volcanic gas.
• The move at very high speed down volcanic slopes, typically following valleys.
• Pyroclastic flows have two parts: a lower basal flow of coarse fragments that moves along the ground, and a turbulent cloud of ash that rises above the basal flow.
• Pyroclastic flows can form from the collapse of an eruption column during a highly explosive eruption.
• These flows may also result from boiling over from an eruptive vent during an explosive eruption.
• Pyroclastic flows may also form from collapsing lava domes or flows due to gravitational force.

Volcanic Gases

• Magma contains dissolved gases, providing the driving force for volcanic eruptions.
• As magma rises and pressure decreases, gases are released from the liquid portion and travel upward, eventually being released into the atmosphere.
• Carbon dioxide trapped in low-lying areas can be lethal to people and animals.
• Sulfur dioxide is irritating to eyes, skin and the respiratory system.
• Hydrogen sulfide is very toxic in high concentrations, while hydrogen halides (HF, HCI, HBr) are strong, toxic acids.

Lava Flows

• Lava flows are streams of molten rock pouring or oozing from an erupting vent.
• Lava is disrupted during either non-explosive activity or explosive lava fountains.
• Factors influencing lava flow speed include type of lava and its viscosity, steepness of the ground, whether the lava flows as a broad sheet or through a confined channel or lava tube, and the rate of lava production.
• These flows may instigate other types of hazards.
• Advancing lava flows will knock over, surround, bury or ignite anything in their path.
• When lava erupts beneath a glacier or flows over snow and ice, melt water can result in far-reaching lahars.
• If lava enters a body of water, or water enters a lava tube, the water may boil violently and cause an explosive shower of molten spatter.
• Methane gas, produced as lava buries vegetation, can migrate in subsurface voids and explode when heated.
• Thick viscous lava flows, especially those building a dome, can collapse to form fast-moving pyroclastic flows.

Ballistic Projectiles

• Ballistic projectiles are rocks that an erupting volcano may hurl into the air.
• These blocks and bombs travel like cannonballs and usually land within 2 km of the vent Ballistic Projectiles may occur without warning and in the absence of a larger magnetic eruption. Tephra falls and ballistic projectiles endanger life and property.
• The force of falling fragments only occurs close to an eruption.
• Agricultural lands can be lost if burial is greater than 10 cm depth.
• Suspensions of fine-grained particles in air and water can clog filters and vents of motors, human lungs, industrial machines, and nuclear power plants.
• Noxious gases, acids, and salts may be carried, and close to the vent, there is heat.
• Burial by tephra can collapse roofs of buildings, break power and communication lines and damage or kill vegetation.