Talk:Shield volcano

Rift eruptions rare in Iceland?

The current article makes the bizarre claim that "Rift zones are a prevalent feature on shield volcanoes that is rare on other volcanic types. The large, decentralized shape of Hawaiian volcanoes as compared to their smaller, symmetrical Icelandic cousins can be attributed to rift eruptions". What part of this makes any sense at all? Has the author ever even compared a map of Hawaii with a map of Iceland? It's Iceland that's "large", "decentralized" and "asymmetrical" full of fissure/rift eruptions. Wikipedia itself spends 3 times as long talking about Iceland's fissures as it does Hawaii's. Iceland is far more fissure eruption prone than Hawaii, lying on a divergent plate boundary. We've got a fissure eruption ongoing right now on Bárðarbunga/Holuhraun that's already erupted more than any Hawaiian eruption in modern history, at a faster rate than any of them too. And it's hardly a record setter - one of our fissure eruptions in the 1700s was over 20 kilometers long, released 120 million tonnes of SO2, and is responsible for as many as 6 million deaths worldwide. And they're also implying that Iceland doesn't have shield volcanoes, when our biggest volcanoes are all shields. (including Bárðarbunga). Basically, what on Earth is this article talking about here? -- 213.176.153.100 (talk) 12:55, 9 October 2014 (UTC)

If anything, Hawaii's volcanoes are remarkable for precisely the opposite - how centralized, symmetric, and consistent they are in where they erupt, which is what's allowed Mauna Loa to develop to such a tremendous height, versus Iceland which is much larger and and has a significantly higher average annual flow rate, but whose largest mountain is only 2100 meters tall (well, okay, glaciers helped with that too...). -- 213.176.153.100 (talk) 13:03, 9 October 2014 (UTC)

Viscosity?

Shield volcanoes are formed by lava flows of low viscosity — lava that flows easily.

This should be changed to:

Shield volcanoes are formed by lava flows of high viscosity — lava that flows easily. —The preceding unsigned comment was added by 24.83.141.120 (talk • contribs).

Not according to my reading of the viscosity article. --Pmsyyz 22:23, 19 July 2007 (UTC)

Pmsyyz is correct. hike395 03:45, 20 July 2007 (UTC)

actually lava with a low viscosity moves slowly while lava with a high viscosity moves quickly —Preceding unsigned comment added by 24.13.71.167 (talk) 00:02, 5 February 2008 (UTC)

Viscosity is the resistance to flow, therefore a low viscosity would have low resistacne and flow easily. Auwerdajn (talk) 03:23, 8 December 2008 (UTC)

Similarity between shield volcano and caldera

• This is evident. I propose to merge both articles or, at least, citate here the one on caldera. --Fev (talk) 03:47, 6 July 2008 (UTC)

Many shield volcanoes have calderas, that's the only similarity.Guanlongwucaii (talk) 09:56, 29 May 2009 (UTC)Guanlongwucaii

FAQ/ Information

Changed the section title "FAQ" which is very unencyclopedic to "Information" which isn't much better. More thought needed... 217.34.222.220 (talk) 13:17, 14 July 2008 (UTC)

Shield volcano or stratovolcano?

What exactly is the difference between a Shield volcano and a stratovolcano? The article doesn't really explain this very well. --The High Fin Sperm Whale (talk) 22:27, 21 May 2009 (UTC)

I've now mentioned and (linked to) stratovolcanoes, since they do provide a useful contrast. But I don't think a detailed comparison is appropriate here. I think the separate articles already cover the important differences. -- Avenue (talk) 04:10, 22 May 2009 (UTC)

Composite Volcanoes

Why does the Encyclopedia of Volcanoes refer to both shield volcanoes and stratovolcanoes as 'composite volcanoes'? --Guanlong wucaii 14:30, 11 February 2010 (UTC)

In need for major work

This article needs a major rewrite big time. I just reworded the first sentence in the introduction from "A Shield volcano is a type of volcano built almost entirely of fluid lava flows." to "A Shield volcano is a type of volcano commonly built almost entirely of fluid lava flows." to make it more obvious. Pyroclastic shields are NOT made almost entirely of fluid lava flows. Instead, they are composed largely of pyroclastic material. Another less common type of shield volcano are those composed of andesite and/or rhyolite. But it seems like this article focuses primarly on the most common shields composed of basalt as far as I'm aware of. I would add a bit for the andesite and rhyolite shields, but I do not know the geology of them too well. Volcanoguy 03:30, 22 August 2010 (UTC)

another factual error: if shield volcanoes are shallow at the bottom, steep in the middle, and flat at the top, they cannot be convex. See that article for definition of convexity. —hike395 (talk) 13:00, 26 August 2010 (UTC)

True. Examples of shields that are not mainly composed of basalt are the Rainbow, Ilgachuz and Itcha ranges in British Columbia. A quote from the book Volcanoes of North America: United States and Canada about the Rainbow Range: "During a period of ~2 m.y., extrusion of highly fluid basic and silicic lava flows built up the gently sloping flanks of the shield volcano. The stratiform flank zone surrounds a central complex of small domes, short, stubby flows, and small intrusive bodies. Alkaline and peralkaline lava flows from four volcanic episodes make up an 845-m composite section on the north flank of the shield volcano. Basal comenditic trachyte flows are unconformably overlain by flows and flow breccias of mugearite. A sequence of 40-60-m-thick columnar-jointed comendite flows blankets the underlyingunits and gives the volcano its shieldlike form. Hawaiite dikes, plugs, and minor capping flows are scattered over the north flank. Comendite flows, which commonly have a glassy selvage at the base, account for 75& of the lavas in the flank zone Anahim Peak, a small center on the northeast flank of the main volcano, consists of a small (<2 km²) pile of 7 thick hawaiite flows cut by a trachyte plug. The unique characteristic of the Rainbow Range is that, although the flank zone is composed dominantly of silicic lava (64-71% SiO²), the form of the volcanic edifice is a shield volcano. The apparent morphological paradox of a silicic shield volcano, made of flows that should be too viscous to form such a shape, is also found in the Kenya rift. It occurs because the peralkaline nature of the silicic lavas decreases the viscosity of the flows a minimum of 10-30 times over that of calc-alkaline silicic flows." Sounds like the Rainbow Range is a unique shield volcano. It erupted lava that is normally too viscous to form shields. Volcanoguy 05:26, 27 August 2010 (UTC)

Organizational recommendation

I was asked to give a recommendation on how to organize this article. I think that the key point about Shield volcanoes is that they form from low-viscosity mafic lavas. That can be used as a starting point from which to investigate the different environments that produce shield volcanoes and as an underlying theme. So perhaps:

• A section of their formation and why the viscosity (and the composition that allows that) is important
• A section of geological features typically associated with shield volcanoes
• A case-study-ish section with subsections on each major geologic environment in which shield volcanoes form, and examples of those environments
• Shield volcanoes and society (hazards, people living on them, etc.)

That's mostly off the top of my head, so don't put too much stock in it, but I sort of like it as a scheme. Cheers, Awickert (talk) 05:46, 13 February 2011 (UTC)

remove etymology section

The section has only two sentences regarding volcanoes which can be easily absorbed in the rest of the article. The rest belongs on shield because it discusses the etymology of that word and has nothing to do with volcanoes. 173.112.160.152 (talk) 04:12, 22 June 2011 (UTC)

Seems reasonable EdwardLane (talk) 17:43, 11 September 2012 (UTC)

Clickable links on world map are all misplced

For example, the popup information for a volcano link in the middle of the Pacific says its in California. Another popup says its in Congo but the link is in South Africa. A third names a volcano on New Zealand but it is down near the Antarctic. Someone should align the image and the links. — Preceding unsigned comment added by 101.103.131.42 (talk) 09:39, 3 January 2012 (UTC)

General tidying work

Ok looking at this page I see a few issues.

The first stumbling block is the definition of 'shield volcano'

Shield volcanoes are one of the three major types of volcanoes, distinguished from the two other major volcanic types, stratovolcanoes and cinder cones, by distinct differences in structure and composition. Stratovolcanoes are built up by the accumulation of thick, viscous lavas, whereas cinder cones are constructed of tephra ejected in explosive eruptions. In comparison, shield volcanoes are built of relatively weakly viscous basaltic lavas that erupts in longer cycles than that of a stratovolcano.[3] Shield volcanoes are distinctive products of hotspot volcanism, but can form at rift and subduction zones as well.

Ok so assuming that is correct I see there is a section further down about pyroclastic shields - incidentally that page has no citations for it's definition. It says it is an uncommon type of shield volcano - it seems (based on the quoted definitions) that they are more like an uncommon type of cinder cone - in that it looks like a shield volcano rather than being steep sided.

If we take that idea as true then moving the subsection on pyroclastic shields into the cinder cone article would make sense, and then there is no contradiction to the given definition, low viscosity lavas etc etc . Also that then allows the three "major" types of volcano (shield/strato/cinder) to be cleanly defined as non conflicting entities.

I don't have a source to back up that proposal - I'm just looking at the conflict that is already within this particular page and thinking about how it might be tidied up. Anyway if felt just slightly more confident that I was right I'd have been bold and moved the pyroclastic shield section into the cinder cone article. What do you think? EdwardLane (talk) 10:44, 13 September 2012 (UTC)

As mention above, not all shield volcanoes are made of basalt. Cinder cones are not made of ignimbrite and pyroclastic shields are way larger than cinder cones. The Global Volcanism Program mentions "a less common type of shield volcano is a pyroclastic shield, whose broad low-angle slopes are formed by accumulation of fragmental material from powerful explosive eruptions (source)." Here are a few pyroclastic shields: Billy Mitchell, Emi Koussi, Purico Complex and Sacabaya. Volcanoguy 09:42, 25 November 2012 (UTC)

If it's called a pyroclastic shield it is obviously a shield volcano. If they were a form of cinder cone they would be called a pyroclastic cone. Volcanoguy 06:27, 26 November 2012 (UTC)

GVP is pretty good most of the time - and I do understand that if they are called 'pyroclastic shields' then they appear to be a subcategory of shield volcano. Clearly they match in terms of shape, but if you look at the quote above from the article, plus the quote from GVP (ignoring for the moment the words "a less common type of shield volcano is " - which I know is unreasonable, but as a hypothetical).

Then, you get "a pyroclastic shield, whose broad low-angle slopes are formed by accumulation of fragmental material from powerful explosive eruptions " and "cinder cones are constructed of tephra ejected in explosive eruptions. In comparison, shield volcanoes are built of relatively weakly viscous basaltic lavas " which would clearly show a resemblance between cinder cone and pyroclastic shield.

So, morphologically the pyroclastic shield clearly fits with the shield volcano but I think GVP is being slightly confusing calling a pyroclastic shield a "less common type of shield volcano" - I'd have thought they would be clearer if they said something similar to "a pyroclastic shield was a less common type of cinder cone morphologically resembling a shield volcano".

But I accept that we don't have that definition from the GVP - so we have the problem that the two small chunks of text I quote above don't have a contradiction - but taking the two entire definitions does - so either my intial premise is makes sense or the section I quoted at the top of this thread needs a fairly big rewrite to include pyroclastic shields in the 'shield volcano' definition.

Suggest changing it to say something like the following:

"Shield volcanoes are one of the three major types of volcanoes, distinguished from the two other major volcanic types, stratovolcanoes and cinder cones by it's shield like structure and we can subdivide these into the more common basaltic shields (built of relatively weakly viscous basaltic lavas that erupts in longer cycles than that of a stratovolcano) and the uncommon pyroclastic shields (whose broad low-angle slopes are formed by accumulation of fragmental material from powerful explosive eruptions)"

If that becomes the definition of shield volcano I'm not sure we can continue to say "Shield volcanoes are distinctive products of hotspot volcanism, but can form at rift and subduction zones as well" (not that I think that line actually says very much as it is - hotspot or rift or subduction is pretty much everywhere).

Incidentally I struck out my own text as I think we have to accept the consensus (GVP) definition even if it doesn't seem consistant to me, I left it there as I feel the argument makes sense and that there is a small chance there are other sources for conflicting definitions of pyroclastic shields. EdwardLane (talk) 10:20, 27 November 2012 (UTC)

Well, pyroclastic shields are not a common form of shield volcano. It's also worthy to note that there are shield volcanoes made of felsic lava rather than mafic lava as well. For example, the Ilgachuz Range, the Rainbow Range and the Itcha Range, which are mostly made of trachytes and rhyolites. Volcanoguy 11:54, 27 November 2012 (UTC)

The folks at GVP are seemingly redefining "shield volcano", are there any other sources which support the inclusion of pyroclastic shield shaped features as "shield volcanoes"? The standard definition is a volcano built by highly fluid mafic lavas and pyroclatic piles are not fluid or mafic. Yes, "shield shaped" piles of pyroclastics do exist and perhaps "pyroclastic shield" is a valid term, but they don't fit the basic definition of a shield volcano. Methinks we need supporting sources here for such a basic redefinition - as in peer reviewed publications. Vsmith (talk) 13:59, 27 November 2012 (UTC)

The single ext link in the unref'd pyroclastic shield article is a one liner stating: A pyroclastic shield volcano is a shield volcano covered with pyroclastic deposits from later eruptions.[1] and that is not a WP:RS. Vsmith (talk) 14:04, 27 November 2012 (UTC)

I did see several uses of "pyroclastic shield" and "ignimbrite shield" while doing a Google search, but I found this geology glossary, which describes an ignimbrite shield as "a shield volcano built of rhyolitic ash flows, with a collapse caldera at its summit." Volcanoguy 15:32, 27 November 2012 (UTC)

And cinder cones don't have collapse calderas just a plain volcanic crater. Volcanoguy 15:36, 27 November 2012 (UTC)

Cinder cones are relatively small features.

Using google scholar I do get a few relevant hits, the first an abstract from J. Volcanology uses it with shield in quotes. Another from Bulletin Volcanology uses basaltic pyroclastic shield. A search of GSA website (member) returned nothing. So, the phrase is used, but is "pyroclastic shield" a shield volcano? I'd think not, rather a term used to describe broad piles of pyroclastics. Haven't seen anything yet to indicate that "pyroclastic shield" is a "shield volcano", but I await enlightenment :) Vsmith (talk) 16:22, 27 November 2012 (UTC)

I think the pyroclastic shield article should be moved to ignimbrite shield since that is probably a better term based on sources. Volcanoguy 17:14, 27 November 2012 (UTC)

I don't understand why you think "pyroclastic shields" are not shield volcanoes. How is it not a shield volcano if it's structure has the same profile as a shield volcano? From what I understand, the term "shield volcano" comes from the shape of these particular volcanoes. So "felsic shields" arn't shield volcanoes either just because they are made of felsic rocks? Doubt it. Volcanoguy 17:34, 27 November 2012 (UTC)

Basically because I haven't seen that connection made in a WP:RS. I have noted use of pyroclastic shield in published papers, but not the connection to shield volcano, and as the definitions of shield volcanoes I've seen don't include or discuss felsic pyroclastic piles - we don't WP:SYN and connect the two. Do you have a source that makes the connection? Vsmith (talk) 20:47, 27 November 2012 (UTC)

This GVP page describes the Purico Complex as a pyroclastic shield and this page describes Emi Koussi as a pyroclastic shield. According to WP:RS the Global Volcanism Program website is a reliable source.

What I was really refering to was felsic lava piles not felsic pyroclastic piles. Just that you said "the standard definition is a volcano built by highly fluid mafic lavas". Any sources that make that claim? From doing years of research the Rainbow, Ilgachuz and Itcha ranges are known as felsic shield volcanoes. I have sources to back that up as well. Same for Toney Mountain in Antarctica. Volcanoguy 03:28, 28 November 2012 (UTC)

I've been lurking, and thought I'd add a third viewpoint to try to sort this out smoothly. Situation is: 1. No-one is disputing that in the vast majority of cases, shield volcano = basaltic lava shield. 2. In a few instances, a few reputable authors & sources have qualified the expression "shield volcano" to describe other more exotic volcano types which are also scutiform (...get me!), though this is not widespread usage. Sounds like a "add one sentence somewhere, plus hyperlink, job done" sort of situation to me. DanHobley (talk) 05:18, 28 November 2012 (UTC)

Who says it's not widespread usage? In the vast majority of cases, shield volcano = basaltic lava shield most likely because they are the most common type of shield volcano and are the best known. Volcanoguy 05:58, 28 November 2012 (UTC)

And if pyroclastic and felsic lava shields are not shield volcanoes what are they? They arn't cinder cones, stratovolcanoes or calderas. Volcanoguy 06:10, 28 November 2012 (UTC)

Agreed, pyroclastic/ingnimbrite and felsic lava shields are clearly uncommon but definitely scutiform (nice word btw) volcanoes. If I say shield volcano to most people I'll get a blank look, but a good chunk of people will think of a basaltic example such as hawaii, almost no-one will think of the unusual ignimbrite or felsic lava examples. Should the definition in the article say something like:

"Shield volcanoes are one of the three major types of volcanoes, distinguished from the two other major volcanic types, stratovolcanoes and cinder cones by it's shield like structure built of relatively weakly viscous basaltic lavas that erupts in longer cycles than that of a stratovolcano, this term is also used to include other scutiform volcanoes including uncommon pyroclastic shields (whose broad low-angle slopes are formed by accumulation of fragmental material from powerful explosive eruptions) and rare felsic lava shields (whose broad low-angle slopes are formed by unusual magma compositions resulting in low viscosity felsic magmas)."

with hyperlinks and references obviously. Incidentally I'm not keen on the wording that's there in the first place, but I figure that can be fixed up once the meaning of the term Shield Volcano is cleared up. But while I'm thinking about that - 3 major types (are there only 3? fissures also seem like a major type to me, I'm willing to concede Maars are uncommon, and that Caldera are probably either shield or strato first - so might not count as a distinct 'major type' ) , and the words "relatively weakly viscous" can surely be replaced by "low viscosity" EdwardLane (talk) 12:18, 28 November 2012 (UTC)

Yeah, the wording of that intro was bothering me, too. I think we could do with following a WP:RS reference here too, especially for contrasting with the "other types" of volcano. Given we seem to have a decent range of geo-experience represented here, yet also all seem to slightly disagree with each other, I suspect the literature itself won't be consistent (splitters vs lumpers, and all that). If we're going to contrast with other types, we need to follow a notable existing list of those types, per the lists guidelines. If not, we shouldn't be doing it. That said, your form is superior to what we have, IMO. DanHobley (talk) 13:52, 28 November 2012 (UTC)

Only a year or two later I've finally managed to catch up on this thread, and I think (if I read the Diffs correctly) that User:Resident Mario looks to have implemented these changes - just wanted to say thanks EdwardLane (talk) 20:49, 15 November 2015 (UTC)

Two years later and not much has changed. In the Structure subsection I noticed Oregon's Big Obsidian Flow is given as an example of a felsic lava shield. Surely there must be a better example that has its own article. Volcanoguy 10:20, 20 September 2017 (UTC)

Clickable red pyramids on shield volcano map

The one for La Grille mentions that it is in "the Cameron". This should be "Comoros". — Preceding unsigned comment added by 198.49.222.20 (talk) 15:54, 13 September 2012 (UTC)

De Orphan/Merge

Post shield stage is an orphan stub, is it worth an article on its own, or just merge it into a subsection of this article? EdwardLane (talk) 22:17, 17 June 2013 (UTC)

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Science

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