Why are lava fountains silent?

fayaazhussain

Lava fountain is also a form of jet. When other jets produce noise, why do lava fountains not produce any noise?

davenn

have you not experienced a lava fountain up close?
there is a substantial amount of noise of all the hot gasses expanding and escaping as they push the lava into the air.

what "other jets" were you referring to in particular ?

cheers
Dave

fayaazhussain

By other jets i am referring to jets issued from a nozzle for ex. jet engine jets. Considerable amount of noise is produced from the exhaust of jet engines. Why do lava fountains don't create that much noise when compared to the exhaust jets of jet engines?

davenn

thats what i suspected you meant.... well its not a jet engine spinning at 20,000 +- a bit RPM, so its not going to produce a hi pitched scream :) But as i said in my other post, they are quite noisey

Dave

fayaazhussain

I am talking about the aerodynamic noise from the exhaust jets of an engine and not the mechanical noise. Any air flow causes noise take the example of wind. Likewise when a lava fountain comes out of the volcano mouth shouldn't it cause noise which can be heard from quite a distance like in the case of jet engine? Lava fountains are known to rise to heights of even 450m. If the lava is able to reach that much height naturally it's kinetic energy can be compared to that of a jet engine.

This question was actually raised by my teacher. The famous pompeii volcano eruption caused millions of deaths because people couldn't hear the lava fountain and so he asked me why don't they create much noise.

tiny-tim

no, the deaths (less than 50,000 i think) were caused mostly because people couldn't hear or see the very fast (up to 450 mph) completely silent and invisible pyroclastic surges that asphyxiated them, see http://en.wikipedia.org/wiki/Pyroclastic_flow

they knew Vesuvius was erupting, but a lot of people didn't think there was any need to flee (although a lot of people did), since it was 5 miles away, and lava doesn't flow very fast (unlike pyroclastic surges)

davenn

its still a very different situation and my previous response still applies


in my geology activities over the years I have been fortunate to be around lava fountains, much of the noise is mid to low freq ranging, particularly lower freq booms
with lava fountains you also generally dont have the very small orifice of a jet exhaust, which is ~ 1 to 2 metres. The throat of a volcano tends to be much larger ... many metres to in some cases a kilometre or more....
like this one during 1984 in Hawaii...



cheers
Dave

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davenn

Thanks tiny tim

I was going to respond to that as well but was busy with the other response as you posted...
and I agree... Yes it wasnt lava or lava fountains that killed the residents of Pompeii
it was the huge pyroclastic flows

cheers
Dave

fayaazhussain

Thanks to both of you for your replies! :)