I love the BBC TV movie “Pompeii: The Last Day”. I watch it all the time. Anyway, the narrator says at the end that by the time the eruption was over, Mt. Vesuvius had “spewed out more than 10,000,000,000 tons of pumice, rock, and ash”.
I’m no geologist. I’m interested in geology but can’t even call myself an “amateur geologist”, so I could be completely talking out of my butt. But 10,000,000,000 tons of debris just seems like…a lot. I mean, one ton is 2,000 pounds, and I can’t even multiply that by 10 billion without getting an error on my calculator. Plus, wasn’t the majority of debris pumice and ash, both of which are pretty lightweight?
Well the debris completely covered two good-sized towns. Herculaneam was covered by 23 metres of debris. I imagine most of the weight would be in the pyroclastic surges rather than the ash plume.
Assuming the average density of the ejecta is approximately that of water you’ve got a volume of 10 billion cubic metres, a cube 2,154 metres to a side, which does seem pretty big, but gets more managable if the average density is greater than that of water.
I am not so sure about that. The houses weren’t pushed over as pyroclastic flow would have done, but buried by hot ash from above. It happened so fast that in spite of the high temperature, some wooden artefacts survived.
I think you’re getting density and weight mixed up here.
A pound of feathers and a pound of rocks weigh exactly the same - 1 pound. But the pound of rocks will take up less space.
Ash and pumice aren’t as dense as solid basalt but a ton of pumice or a ton of ash is still a ton, no matter how much space they take up.
To put it into perspective, while that seems like a lot, it’s a burp compared to the size of the Earth, which is roughly 6 * 10^21 tons or 6 sextillion tons. 10 billion tons would be 0.0000000017% of the total mass of the earth.
Another comparison. A cubic yard of water is roughly 1 ton. So, we’d need 10 billion cubic yards of water. What is roughly the same size? That’s about 5000 Sears Tower’s worth of solid water. That’s a lot in human terms, but a very strong rain storm will drop more water than that naturally. And a hurricane definitely drops much more weight in water before it dies out.
Quick googling gets various values for the density of basaltic lava. The values are around 3000 kg/m[sup]3[/sup], more or less. I’ll also simplify by using metric tonnes, 1000 kg.
Doing the math, I get a cube of lava about 1900 meters on each side. That amount of molten rock would mass about 10 billion tonnes, and would make a heck of a lot of ash and pumice if sprayed into the air at high pressure.
I’m sure I messed up the math somewhere, and that someone will correct me. But I think the point is clear–that much mass isn’t really all that much compared to the mountain it came out of, let alone the Earth.
No, it makes sense to me. 10 billion tons of low-density pumice will make a much bigger pile of rock than 10 billion tons of high-density granite. So the debris being low-density pumice makes the debris pile even more implausibly large.
If Pompeii and the surrounding environs were a 20km x 20km (~ 14 miles) area, then they would be buried by only 24 meters (~75 feet). Which is comparable to what “lisiate” and “bob++” said.
Thanks for those links! I’ve always known that, in relation to other eruptions, the eruption of Vesuvius in 79 AD was quite small, but I think it’s really cool to see that visual representation. I’m just wondering why, in the second link you posted, they didn’t include Lake Toba?
The USGS comparison only goes back to the 5000 BC Crater Lake eruption, and most of the eruptions they show are within the last 200 years. Toba was 74,000 years ago, so I assume it didn’t make the cut just based on date.
It is also worth remembering that a decent portion was ejected into the atmosphere (around 10-20%). Ash several inches thick can be found hundreds of miles away. And traces of ash remained in the atmosphere for a couple of years.
Not to mention the large amount of gaseous material. I know that it is low in density but it does propel pretty much everything that comes out and it does contribute to the total ejected mass.
Meh. Once you get to a cube a couple kilometers to a side (which isn’t all that implausible to begin with), in the worse case, what difference does an extra kilometer to a side really make?