Talk:Volcanic arc

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[edit] Clarity

this either does contains contradictions or seems to where it first says what volcanic arcs are, then says something like 'contrary to popular belief'...... Regardless, 'contrary to popular belief' whatever he says is weaselish, and i dont really know how to fix it because i am not educated on this subject.

—Preceding unsigned comment added by Thuglas (talk • contribs) 06:57, 16 December 2006

Reworded it a bit - hopefully better now - Vsmith 14:18, 16 December 2006 (UTC)

Yes, this reads nicely; and island arcs appear an elegant oceanic 'facies' of volcanic arcs. Though it's too attractive to avoid, I'll agree: it may offer a very similar origin for all volcanic arcs; a similarity that may not always be there.

The positivist philosophy of separating observation and measurement from theory appears to have died with Eskola: A volcanic arc is chain of volcanic islands or mountains formed by plate tectonics... ."

Nevertheless, there was a geologist (whose name escapes me) who published a book (highly praised in Germany), in which he attempted to separate descriptions of singular geological features of the globe, such as island arcs, from their then current, popular explanations. The language used here is very different, and very common in contemporary geology. This new theory-laden language has the advantage of drawing one's attention to the similarities & differences of island arcs and their continental extensions. And, terms should be simple.

Logically, however, should plate tectonics someday be disproved, language like the above would suggest volcanic arcs are theoretical structures that no longer exist. In contrast, the German book of the 1930s (someone help me here with a reference*) is relatively ageless, of value today, though it was written just after Stille had made the geosynclinal theory very attractive in Europe and to the book's author.

These observations belong in a more general setting than this topic, and don't adversely criticize the very clear and otherwise excellent description given here. Geologist 12:51, 6 November 2007 (UTC)

*Bucher, W.H, 1933. The deformation of the Earth's crust. Princeton, NJ: Princeton University Press. Bucher separates 46 objective 'laws' from an equal number of subjective 'opinions'. Following Suess's 'Face of the Earth', chains of volcanoes are termed 'welts' and arcuate troughs termed 'furrows'. Law 6 supports using volcanic arc as a general term. It states that welts & furrows occur at ocean bottoms and upon continents, and these display the same form and dimension. Geologist 07:47, 7 November 2007 (UTC)

[edit] Merging Island arc into Volcanic arc

After editing both of these articles today, it seems clear to me that this one should be merged into the other. All island arcs are volcanic arcs, so this article is just a subset of the other. There is already major overlap in content in the two articles, so this was an easy decision to make. Better to have one nicer, more comprehensive article than just leave these two as they are now. --Seattle Skier (talk) 01:16, 13 April 2007 (UTC)

Well, the merge is now complete, and I think it turned out even better than I expected, since the text of both articles complemented each other nicely. I've also smoothed the prose and expanded the text in places, and added 3 maps to illustrate some examples of arcs. These maps are all kind of ugly, though, so they should be replaced by some nicer ones eventually. --Seattle Skier (talk) 02:21, 13 April 2007 (UTC)

[edit] Why are volcanic arcs arced?

Should someone offer here geologists' explanations of why volcanic arcs or island arcs are arced? Geologist 13:02, 6 November 2007 (UTC)

Basically it is because the intersection of a plane (the subducting slab) with the surface of a sphere is a curve. Cheers Geologyguy 14:50, 6 November 2007 (UTC)

Ah, thank you. Have you a reference? (This might be in secondary texts.) I don't mean to show my ignorance; but, why is the subducted plate spheroidal in shape and the subducting plate planar in shape? (In actual fact, I once suggested your explanation during a geophysics class at Dartmouth in 1973. We quickly dragged in a globe & piece of paper, but no reasonable angles made the arcs.) Is there a reasonable hypothesis yet?

It has been my experience that tracing a 'common sense' explanation in college texts will often lead one back through texts of older generations to a paper that just suggested an explanation and specifically remarked that it was unlikely. This is where discoveries are yet to be made. As an example, relevant to this discussion, is columnar basalt. We were taught that random cooling centers along its surface would create a network of cracks. Each must be the same size, and closely resemble a sphere. The only network that satisfies these properties is an hexagonal network. This clearly lacks something.

Now glance at the illustration in this Wikipedia under the topic 'Giant's Causeway', labeled 'Columnar Basalt'. Pretty. The photo on the first page of 'Voronoi diagram' shows a mathematical model of cracks caused by random cooling centers. Do they look similar? (The nucleation of triangular stress cracks & their propogation makes a better hypothesis.) My point, I suppose, is that a theory should 'explain' the origin of 'in your face' properties common to, in this case, all volcanic arcs. Does plate tectonics provide this explanation? If not, perhaps plate tectonics should be distanced a bit from volcanic arcs (at least in definition). Geologist 06:06, 7 November 2007 (UTC)

Good observations, good questions, and probably somewhat beyond me, but: this PDF, pages 7-8, has some information about the Euler theory of movement on a sphere that may apply. Euler poles of rotation for divergent boundaries are fairly straightforward, but for convergence I think some complex vector algebra (well beyond me) is involved, but likely results in interactions along great circles on the spherical surface, manifested as arcs. Sorry I can't be of more help. Cheers Geologyguy 16:20, 7 November 2007 (UTC)

Thanks, I read the pages. No, I don't think there exists an explanation beyond you, or even me. If so, we should at least be told so. Euler's proof that a translation on a sphere's surface is a rotation was nice in contributing to one explanation of transform faults on a single, rigid plate. Triple junctions require a bit more thought, I agree. However, I don't immediately see a relation between elementary complex algebra (which geologists do study geometrically, using stereographic projections), and arcuate plates.

Even if one should propose an hypothesis, it would demand much testing. The columnar basalts were only an example of an accepted explanation that no one (for a long time) tested. They do bear a coincidental resemblance to the creation of plates by the precipitation of three divergent cracks, offering an hypothesis why divergent margins could being arcuate. Unfortunately, they are not. It's the convergent ones that are. Not only are they convergent, the arcs differ in the signs of their curvatures, and even have cusps. The 'Expanding Earth Theory' has a ready explanation, but your description of volcanic arcs draws upon plate tectonics: a theory which appears to fail when confronted with a phenomenon in your title.

I have no objection to explaining Andean volcanoes by the rise of calc-alkaline magma, though whether this occurs is still one of the great unsolved problems in petrology; but if there are no hypotheses at all that explain why volcanic arcs are arced, I suggest this be mentioned in the article. It indicates a failure of, or failure to complete, plate tectonic theory. Geologist 06:28, 10 November 2007 (UTC)