La Garita (Fish Canyon Tuff) is generally considered the most powerful vocanic eruption, and the only VECP 9 eruption (though there are evidences of more powerful ones being uncovered.) Vesuvious is puny if you still measure the amount of ejectamenta in tonnes. Just go the the top-10 volcanic eruptions in history.
But to answer the preceding question, geologists still say that the Chixulub meteor impact is the most violent event to have occurred on the earth’s surface.
Rough rule of thumb - rock is usually about density 3 (3 times density of water) and metal like iron about 6.
Pompeii, IIRC, was “only” buried to a depth of about 30 feet, based on what I saw of areas still being excavated. They are still excavating. that depth may not include material already eroded off. (Also, not long after the eruption, a number of opportunists started burrowing down and into houses looking for buried treasure. Some were killed in tunnel collapses.)
Pompeii is about 10km from the center of Mt. Vesuvius. Also, Pompeii used to be a seaport. Now it’s quite a ways inland. It would not surprise me if the ash and debris eventually washed out to sea and conributed to this expanded shoreline.
A 2km cube of lava is the same as 10m deep (30 feet) covering 800 sq km, or a circle about 30km in radius, if I have my math right. 30km (20 miles) is not a huge debris field in the grand scheme of geological events. It was probably larger but much thinner toward the edges, much thicker in the center.
Doh! Diamete, not radius. A circle about 16km in diameter, 10m deep.
Heck, that’s actually tiny.
IIRC, I think Pliny said the ash was pretty thin in Naples, about 13 km from Vesuvius. Wind direction must have been a factor.
So, a cubic mile, loosely speaking. When you spread that out over, say, a hundred square miles, which is about a six mile radius around the mountain, you’re talking about fifty feet deep. Of course, it was only that deep in a small area, and a lot of the rest was spread out over a larger area, and some of that area was ocean, which absorbs a hell of a lot of ash and rock without showing any effect to someone without scuba gear. That’s still a hell of a lot of mass, but it’s something that you can visualize, at least. It would, very roughly, fill the valley I live in. In a way, that’s a lot, but in another way, it’s only one valley.
Somewhere in the deepest recesses of my brain, I was able to remember (pi)(r squared) and calculate the radius of your circle in my head. Who knew I still knew how to do that?