Massive clouds of ash belching out of a volcano in Iceland have grounded flights across northern Europe and the U.K. and forced emergency officials to evacuate hundreds away from the glacier near the eruption. The air traffic problem stems from the abrasiveness of the volcanic ash, which is made up of 0.001mm to two mm chunks of volcanic rock, minerals and glass. The gritty particles can sand the paint off a plane’s exterior, completely shut down its engines, and make the flight extremely unpleasant for passengers when sucked into ventilation systems. Travel could be delayed for some time, and could easily be interrupted if the eruptions reoccur—the volcano is expected to continue erupting for days or weeks. This isn’t the first time volcanos have hampered flights, about 100 planes ran into volcanic ash between 1983 and 2000, including two flights that fell several thousands of metres after losing power, narrowly avoiding crashing before their engines restarted.