As volcanic ash spreads from an eruption under a glacier in the Eyjafjallajokull region of Iceland, the British have taken the drastic step of locking down the country's airspace. Here's why.

The world's bustling air traffic corridors pass over hundreds of volcanoes just like the one in Iceland — all of which are capable of sudden, explosive eruptions of fire and ash. In the United States alone, aircraft carry many thousands of passengers and millions of dollars of cargo over volcanoes each day. As we see today, volcanic ash can be a serious hazard to aviation even thousands of miles from an eruption. Airborne ash can diminish visibility, damage flight control systems, and most importantly, cause jet engines to fail.

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Volcanic ash normally contains glassy materials, such as silicates, whose melting points are 600 degrees Celsius to 800 degrees Celsius. Since internal temperature of in-flight jet engine exceed 1000 degrees Celsius, glassy particles in volcanic ash inhaled by the engines instantly melt. In the course of exhaust, the glassy materials are rapidly cooled down in the turbine chamber, stick on the turbine vanes, and disturb the flow of high-pressure combustion gases. This disorder of the flow may stop the entire engine in serious cases — such as KLM Flight 867.

Such dangerous and costly encounters between aircraft and volcanic ash can happen because ash clouds are difficult to distinguish from ordinary clouds, both visually and on radar. Also, ash clouds can drift great distances from their source. For example, in less than 3 days, the ash cloud from the June 15, 1991, eruption of Mount Pinatubo in the Philippines traveled more than 5,000 miles to the east coast of Africa. This ash cloud damaged more than 20 aircraft — including the DC-10 shown above — most of which were flying more than 600 miles from the volcano.