I imagine it depends on just where the volcano is. The main reason the Icelandic volcano was such a big deal is that it’s right there next to a whole lot of the transatlantic air routes, and is placed in a particularly bad spot for paralyzing European air travel.
If say… it was to continuously spew ash for 6 months, I imagine that there would be a corresponding increase in train and road travel within Europe, and I also figure that cruise lines might start sailing the transatlantic routes again- there would be a lot of money to be made sailing a big cruise ship from say… Southampton to New York, if people couldn’t fly between Europe and the US, and I suspect that would outweigh whatever money is made schlepping tourists between Caribbean islands, or down the Mexican coast.
There also might be round-about flights that fly out of southern Europe/N. Africa to the US.
I saw a special on TV just a few weeks ago about a commercial air liner that flew through an ash cloud. They didn’t know it was a volcano, but they knew something was wrong. They were seeing sparks along the side of the plane. The cabin was filling with some kind of “smoke” (that didn’t quite act like smoke) and what seemed to be the strangest is that they lost their engines, but they later started back up on their own.
Also, when they landed, more or less, blind since they’re windows were ruined from the ash.
As above, but even worse if it was a major eruption in Japan. Not only would it isolate the country, but it would cause untold loss of life and property because of proximity to population centers. Plus the fact that a major eruption anywhere in East Asia will trash the busiest air routes on the planet. 9 of the top 10 routes in the world are in East Asia or Australia. The economic impact to the world would be off the charts massive.
Assuming the disruption is restricted to air travel, I think there would be little to modest macro-level disruption after the initial shock. Apart from the airline sector, of course. Business can be conducted by phone or internet; personal meetings might require a significant train journey or cruise - I can see Iceland benefiting here. Tourism would be seriously affected but that would only shift where tourist money was spent, so while there would be local disruptions, on a macro scale there would be no overall effect.
Some of the biggies make regular transatlantic cruises anyway; they sail the Caribbean in the winter and the Mediterranean in the summer (see, e.g., Royal Caribbean’s Jewel of the Seas, Freedom of the Seas, etc.), and the semi-annual repositioning cruise is a popular choice. The Disney Magic is currently en route from Florida to Dover, England; she’ll spend the summer in Europe before returning from Spain to New York in September.