The flight bans came amid fears that the ash - a mixture of glass, sand and rock particles - can seriously damage aircraft engines.

BBC: EU emergency talks called on volcanic ash air chaos

Gamma-ray emitters bounce atomic particles off rock formations to see the size, density and flow of oil reservoirs.

FORBES: Magazine Article

Sediment: particles of rock covering a size range from clay to boulders.

UNESCO: L O S S A R Y

Tiny particles of rock ground fine by the glaciers that feed it turn the water milky and give it this astonishing, alien colour.

BBC: A return to Middle-earth

Although barely visible in the air, the ash -- made up of tiny particles of rock, glass, and sand -- poses a serious threat to aircraft, stalling engines and causing electrical failure, experts have said this week.

CNN: Air travel paralyzed as volcanic ash cloud lingers