why flights are cancelled

Why ash and aircraft don't mix

A volcanic ash cloud is an abrasive combination of fine rock, minerals and glass particles. Concentrated ash clouds near a volcano can cause catastrophic damage, but even planes that fly through diluted clouds hundreds of miles away must be checked for potential problems.

SOURCE: David Schneider, research geophysicist; U.S. Geological Survey; FAA | Bonnie Berkowitz and Alberto Cuadra/The Washington Post - April 17, 2010