Hazards of volcanic activity

Overview

Primary and a secondary volcanic hazard and socio-economic and the environmental hazards of volcanic activity.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Distinguish between a primary and a secondary volcanic hazard.

   Model answer

   A primary hazard is a direct product of the eruption itself, arriving as it happens: lava flows, pyroclastic flows, tephra and ash fall, volcanic gases and volcanic bombs. A secondary hazard is a consequence set off by the eruption but occurring afterwards or elsewhere: lahars, floods from melted ice or a breached crater lake, landslides and flank collapse, tsunami, acid rain, disease in crowded shelters, famine from ruined harvests and climatic cooling. Secondary hazards often affect a far larger area and can persist for years.

2. 2

   Why do lava flows destroy property but rarely kill?

   Model answer

   Because they move slowly. Even fluid basaltic lava usually advances at walking pace or less, and viscous lava barely moves at all, so people have time to leave. Buildings, roads, farmland and infrastructure cannot be moved, however, and are buried, burnt or crushed, and everything within the path is permanently lost because the ground is covered by new rock. The exception is where very fluid lava pours down a steep slope, when speeds can exceed those at which people can escape.

3. 3

   Why is a pyroclastic flow the deadliest volcanic hazard?

   Model answer

   Because it combines speed, heat and asphyxiation and gives no time to react. It is a dense mixture of hot gas, ash and rock fragments that hugs the ground and races downslope at speeds well over 100 km per hour, at temperatures of several hundred degrees. It cannot be outrun, it kills by burning and by filling the lungs with hot ash, it can cross valleys and even short stretches of water, and it destroys everything in its path by heat and by impact.

4. 4

   What harm does ash fall do?

   Model answer

   Ash is abrasive, heavy and dense. Accumulating on roofs, it becomes heavy enough to collapse them, especially once rain has soaked it, and roof collapse is a leading cause of death in ash-fall eruptions. Breathing it causes respiratory illness, and it irritates eyes and skin. It smothers crops and pasture, contaminates water supplies, blocks drains, clogs machinery, shorts electricity supplies, makes roads impassable, and closes airports because it melts inside jet engines and stalls them. It can be carried hundreds or thousands of kilometres downwind.

5. 5

   Which volcanic gases are hazardous, and how?

   Model answer

   Sulphur dioxide reacts in the atmosphere to form acid rain and fine aerosol droplets, damaging crops, acidifying water and causing respiratory illness. Carbon dioxide is odourless and denser than air, so it collects in hollows and valley bottoms and can asphyxiate people and livestock without warning. Fluorine settles onto grass and poisons grazing animals, and it enters the food chain. Hydrogen sulphide and hydrogen chloride add to the respiratory and corrosive hazard.

6. 6

   How can a volcanic eruption affect global climate?

   Model answer

   A large explosive eruption injects sulphur dioxide into the stratosphere, where it converts to a haze of fine sulphate aerosol droplets. These reflect incoming solar radiation back to space, and because the stratosphere has no rain to wash them out they persist for a year or two and spread around the globe. The result is a measurable fall in average global surface temperature, of a few tenths of a degree after a very large eruption, with disturbed rainfall patterns and reduced harvests. Only eruptions powerful enough to reach the stratosphere, and rich enough in sulphur, have this effect.

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.