Not only would this volcanic cloud affect visibility, but it could also cause airplane engine failure.
"The large particles of rock, which have been blown into the upper atmosphere, are then ingested into the engines. These engines are delicate machines, the rocks clog the engines which means the engines die," Aviation expert Chris Yates told ABC News.
The U.K. Met Office says the cloud was directly over Heathrow at around 11 a.m. local time.
Heathrow is a major air travel hub with more than 1,200 flights and 180,000 passengers passing through it every day. It is one of the world's main connection points, and this event will have major impacts on global travel.
"Heathrow is one of the most important airports in the whole world for connections between North America, Europe, then into Europe and then very importantly from Asia into Europe," Hans J. Weber, president and owner of TECOP International, a San Diego-based aviation consulting firm told ABC News.
"It's really one of the key airports. It's really disruptive."
So far today more than 100 flights from the U.S. to Great Britain have been canceled. And that is just counting those from U.S. carriers.
For the 219 people on Continental Airlines flight 4 last night, the volcano provided them with a seven-hour trip to nowhere. Their Boeing 767 left Houston four hours late at 10:30 p.m., according to airline flight tracking company FlightAware. At 1:55 a.m., the plane turned around and headed back to Houston, landing at 5:32 a.m.
The ash cloud Thursday afternoon was estimated to be around three times the size of the U.K, according to the Met Office, which tracks weather for the airlines and government aviation agencies. The U.K. airspace has never been closed before, but the size of the volcano eruption is much smaller than others, such as Mount Pinatubo and Krakatau, the Met Office said.
Passengers at Heathrow were told to contact the airlines directly and those due to travel today have been advised not to go to the airport.
At Heathrow, about every five minutes an announcement explained the situation and apologized for any inconvenience. Similar announcements were made on London public transport systems warning people not to travel to the airport.
British Airways announced it will not be offering accommodation or compensation during the delays, but the airline released a statement saying, "customers booked to travel on a cancelled flight can claim a full refund or rebook their flight for a later date."
"The airlines' cost will be racking up like there's no tomorrow," according to Yates, "we could be talking many tens of millions of pounds."
Not only will passengers have to be compensated, refunded or accommodated, but aircraft will be in the wrong place and will have to be repositioned. "It could take several days for Europe's aviation system to return to normal," he said.
While it is still unclear how long flights will be grounded, at least one airline expects it will take at least several days to clear out any backlog. Delta Airlines is letting passengers booked through Sunday to fly in or out of several European airports including London, Amsterdam, Dublin, Paris, and Brussels -- to change their tickets for free.