I thought I’d highlight a comment from Mike the geologist, posted in response to the last post.
If it’s any help – ash is pulverised rock and glass with the consistency of anything from fine sand to powder (although it often sticks together like snow flakes). Ash is produced when magma foams like the head on beer (or a soda). The stickier the magma and the more gas, the more ash is produced.
Most Icelandic volcanoes produce very little ash because they’re generally erupting basalt which isn’t sticky and doesn’t contain much gas. What we’re seeing here is the head of the beer, very soon the majority of the gas will be blown off and the volcano will start disgorging liquid lava. If you want to know how an eruption progresses, take a can of soda, give it a vigorous shake and then open it. You get a jet of froth which gradually subsides leaving nothing more than flat liquid in the can.
By contrast, the volcanoes of Indonesia, like those all the way around the Pacific Rim, the Caribbean and southern Italy, have extremely sticky magma containing huge amounts of gas. They explode and throw huge amounts of ash into the stratosphere. These are the real plane killers as the ash is right at the flight level and higher. The eruptions are known as Plinian (after Pliny the Elder who observed the 79CE eruption of Vesuvius) – if you want to see a *small* one, here’s Mount Redoubt in Alaska (which has also been stirring in the last couple of weeks.
This is excellent information for the likes of YT who has been wondering just HOW LONG flights will be grounded. As it happens, I’m due to fly out to Brussels with a group of Icelandic journos and bloggers on Monday, at the invitation of the European Union. Was starting to think the trip might be postponed [and of course there are as yet no guarantees ...]. More on that later, perhaps.
[ps - that photo Mike links to is very cool!]