Talk:Pyroclastic flow

When/why are flows unlikely to happen?

From the Hekla article, a volcano in Iceland:

Up until now, it has always been assumed that Hekla was incapable of producing that most dangerous of volcanic phenomena, the pyroclastic flow.

Is there a reason scientists would believe this, is it something about the shape of the mountain, kind of material that the mountain can eject, or simply because there was little evidence of previous pyroclastic flows? In the pyroclastic flow article it says that the phenomena happens in "some" eruptions, but doesn't really expound on how/why they can happen, anyone have some more info? --Fxer 18:03, 30 January 2006 (UTC)

Re: When/why are flows unlikely to happen?

Flows occur due to a high ratio of solid to gas. They are extremley dense and continuous. Hekla's magma may not have the right composition needed to produce flows, the eruptions may be too fluid and instead produce sheets. By the discription given it looks as though Hekla is a fissure volcano which produces extensive lava sheets, there also was a great ash deposit. Ash deposits indicate a pyroclastic air fall, not a pyroclastic flow.

Unlikely but true?

Can someone supply a reference for the statement Volumes range from a few hundred cubic metres to more than a thousand cubic kilometres. Do we have a first idea how big a 1000 cubic kilometres is? Interestingly, the Dutch article mentions a range van enkele honderden kubieke meters tot enkele kubieke kilometers (from a few hundred cubic metres to a few cubic kilometres.) Who's wrong? --Radioflux 21:31, 23 September 2006 (UTC)

Well, Britannica talks about eruptions of up to 2500 km³ at Yellowstone, so I guess that's okay. I will update the Dutch article. --Radioflux 10:03, 24 September 2006 (UTC)

1000 cubic kilometres is equivalent to a cube of 10 x 10 x 10. Since it would be unlikely that a pyroclastic flow would be 10 km high, some of other dimensions need to change. However pyrolclastic flows can be several metres thick, and be kilometres wide and long.The Geologist (talk) 14:23, 16 April 2012 (UTC)

@The Geologist: No change required, at least not for this reason. "1000 cubic km" does not refer to a cube 10 x 10 x 10 km, but to the volume of such a cube. Surely you've heard of medicines and other fluids being measured in cubic centimeters; do you think a liquid will hold a cubic form?

However, the relevant part of § Size and effects says

The volumes range from a few hundred cubic meters to more than a thousand cubic kilometres. The larger ones can travel for hundreds of kilometres, although none on that scale have occurred for several hundred thousand years.

If we suppose a huge, ancient pyroclastic flow 10 meters thick (that's 0.01km) and 1,000 km in length, it would still have to be 100,000 kilometers wide to reach a volume of 1000 cubic kilometers. The circumference of the Earth is only 40,000km! Decuple (i.e., ×10) the thickness to 100 meters and you still need a flow 10,000 kilometers wide to get a volume of 1000km³.

It's pretty clear that that volume estimate is absurd. I suspect that somebody got confused between meters and kilometers in the process of calculating the cube; after all, 10³ = 1,000. Volumes range from a few hundred cubic metres to more than a thousand cubic metres would probably be more accurate, but that would badly underestimate the ancient flows.

--Thnidu (talk) 04:25, 17 December 2015 (UTC)

User:Thnidu, I suspect that you may be the somebody "who got confused between meters and kilometers". You think that 0.01 km thick by 1,000 km long by 100,000 km wide equals 1,000 cubic km. I recommend that you check your arithmetic. 0.01 km thick by 1,000 km long equals 10 square km. To have a volume of 1,000 cubic km, a flow would require a width of 100 km, not 100,000 km. Pyroclastic flow volume estimates of thousands of cubic km remain valid. GeoWriter (talk) 17:09, 17 December 2015 (UTC)

@GeoWriter: Oops, how very embarrassing! Thanks for the correction. )oo( --Thnidu (talk) 17:21, 17 December 2015 (UTC)

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Seminal paper published in 1992

Everyone seems to have missed it: Branney and Kokelaar, Pyroclastic density currents and the sedimentation of ignimbrites. Completely new thinking, pretty much abolishes terms like flow and surge. Widely accepted. Main tennet: Deposits only represent the conditions at the flow boundary layer, deposits formed by proggressive aggradation, so terms like fully dilute and granular fluid only apply to the base of the current. Russjass (talk) 09:37, 13 May 2008 (UTC)

Gravity current?

Pyroclastic flows are not gravity currents. They are the same as turbidity currents. they are driven by the density contrast, they can move on a slope of less than 1 degree —Preceding unsigned comment added by Russjass (talk • contribs) 09:42, 13 May 2008 (UTC)

Pyroclastic flows flow down-slope under the influence of gravity, whereas pyroclastic surges can climb up slopes often steep and over-run the slope summit before descending the opposite side. You might like to read the description of snow avalanches which behave in a similar manner to pyroclastic flows, and also read the account of the Franks Slide - also known as the "Mountain that ran away."The Geologist (talk) 14:23, 16 April 2012 (UTC)

Pyroclastic clouds are not only related to geological phenomena

Herbert Huppert is Professor of Theoretical Geophysics and Director of the Institute of Theoretical Geophysics at the University of Cambridge. He states the dust cloud created during the WTC collapse of 9/11 to be a Pyroclastic flow.

http://plus.maths.org/issue20/features/huppert/ —Preceding unsigned comment added by Cmatrix (talk • contribs) 21:57, 3 October 2008 (UTC)

Read Talk no. 4 - "And yes, the 9-11 collapse was a gravity flow, but it doesn't seem to fit the volcanologic usage of the term pyroclastic flow."Guanlong wucaii 12:35, 22 June 2009 (UTC)Guanlongwucaii

The cloud behaved "... Like a Pyroclastic flow..." BUT since it was not created by volcanic activity it was not a pyroclastic flow.The Geologist (talk) 14:23, 16 April 2012 (UTC)

I think also violent firestorms, like the Great Peshtigo Fire, may make phenomenons similar to pyroclastic flows. Several times the Peshtigo Fire is compared to a volcano eruption, and with respect to the energy emission and the volume of the ash particles, temperature etc., this probably is a relevant comparison. After the fire, they found melted spades, rails, nails etc., which indicate temperatures at 1100-1600 °C or more.Pål Jensen (talk) 09:01, 26 May 2012 (UTC)

New article for nuée ardentes

I have heard that nuée ardentes differ from pyroclastic flows in that they are formed by the collapse of a lava dome, whereas pyroclastic flows are formed when the ash cloud collapses on itself. Do nuée ardentes deserve their own article? --The High Fin Sperm Whale (talk) 04:27, 16 October 2009 (UTC)

A nuée ardente is simply a "Glowing cloud." Contrary to what some have claimed it is not because they glow in the dark but because they glow. Lacroix who coined the term, described watching several nuées on Martinique and described them as a "...Dark clouds which glowed red as the cloud billowed..." The description given and not believed by people who witnessed the Montagne Pelee eruption in 1902 also mentioned a dark swirling billowing mass of cloud which at times glowed red.The Geologist (talk) 14:23, 16 April 2012 (UTC)

nuee ardentes

"a name for some pyroclastic flows." Can someone make that more specific please? I am doing a project on pyroclastic flows and surges. —Preceding unsigned comment added by 124.182.30.125 (talk) 03:42, 14 March 2010 (UTC)

Flows tend to be metres thick can be homogenous, whilst surges tend to show bedding and laminar layering, and crucially are a few centimtres thick.

Densities

What is missing from the article is an explanation of the mechanism at work in a pyroclastic flow which allows rock particles and heated gas to behave in association as they do. In normal circumstances, hot gas in the atmosphere will flow straight up by convection, while rock particles, no matter how hot, will roll along the ground. What is the mechanism which makes two components with very different relative densities act together? Peter Bell (talk) 23:05, 21 April 2010 (UTC)

You could try reading Branney and Kokelaar, Pyroclastic density currents and the sedimentation of ignimbrites, Published by the Geological Society of London. It has become the definitive work regarding PDC's.The Geologist (talk) 14:23, 16 April 2012 (UTC)

Improvement from the Spanish wiki?

I removed the following: {{Expand Spanish|Flujo piroclástico|date=October 2011}} I did a cursory examination of the Spanish article. It is shorter than the english article and does not appear to have additional info. However, I do not read Spanish. Can someone who can read Spanish please check this? -Arch dude (talk) 00:25, 23 February 2013 (UTC)

"Fully Dilute"

I am not an expert in this field, and this may be biasing my opinion, but the term "fully dilute" is used here without explanation and without links to any explanations. I am completely uncertain as to its meaning - the common meaning of "dilute" indicates it isn't thick with pyroclast, but what does it mean to be "fully" dilute as opposed to "partially" dilute (if such a thing even exists)? Non-experts (like me) might have difficulty with this. 73.206.173.11 (talk) 14:03, 11 October 2016 (UTC)

Combine with pyroclastic surge into one article on pyroclastic density currents?

I would like to propose major edits and combine this article with the article on [surges].

Currently, research on the topic uses the term pyroclastic density current rather than flow or surge. This is due to the understanding that a flow and a surge represent two ends on spectrum of flow regime (ie dilute and turbulent or dense and granular) but a single PDC can span multiple flow regimes. A surge can become a flow over time and a flow can become a surge etc. Hutvh (talk) 23:18, 14 August 2020 (UTC)