A volcano that erupted in Iceland last week is still disrupting travel plans for some in St. Louis.  The ash cloud is blocking air traffic across a wide swath of Europe, grounding planes on a scale unseen since the 2001 terror attacks as authorities stopped all flights over Britain, Ireland and the Nordic countries. Thousands of flights were canceled, stranding tens of thousands of passengers, and officials said it was not clear when it would be safe enough to fly again.

Since the eruption, many Americans have been trapped overseas as European flights were canceled or delayed, and the eruption is preventing a group of Russian doctors and healthcare professionals from making a visit here. The group was scheduled to arrive next week to learn new techniques to stop smoking. It's hoped that the volcano will stop smoking as well. Limited flights in Europe will resume today.

The ash plume, which rose to between 20,000 feet and 36,000 feet, lies above the Atlantic Ocean close to the flight paths for most routes from the U.S. east coast to Europe. An aviation expert said it was the first time in living memory that an ash cloud had affected some of the most congested airspace in the world, while a scientist in Iceland said the ejection of volcanic ash — and therefore disruptions in air travel — could continue for days or even weeks. It was not the first time air traffic has been halted by a volcano, but such widespread disruption has not been seen the Sept. 11, 2001 terror attacks.

The highly abrasive, microscopic particles that make up volcanic ash pose a threat to aircraft because they can affect visibility and get sucked into airplane engines, causing them to shut down. The ash can also block pitot tubes, which supply vital instruments such as air speed indicators, or latch onto engine blades, forming a glassy substance that may cause engines to surge or stall. Ash will also damage all forward-facing surfaces on an aircraft, such as the cockpit windshields, the wings' leading edges, the landing lights and air filters for the passenger cabin.