Talk:Volcanic arc

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)

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)

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)

The reason why Volcanic Island Arcs form an arc is due to the same physical properties that occurs in a river or a glacier. The zone of minimum resistance is in the centre of the flow and that not only includes the cross-section but the zone furthest from the bank. Look at a map of any volcanic arc and there is usually a transform system associated with the ends of the arc. The centre if the arc is where the pressure is at its greatest and the resistance to the forward motion is at a minimum. Just like a bow - as in bow and arrows. There is another feature that is useful to non-geologists and indeed anyone who wants to understand. The centre of the arc ) points to the subducting plate. The Caribbean Arc, South Sandwich Island arcs all point east - so the subducting plate is moving west. The volcanoes form above the descending plate where it attains a depth of about 100 km (60 miles) below the surface. There is more but it really does get rather technical. For the avoidance of any doubt I am "The Geologist" and not to be confused with the other person who signs their posts as "Geologist."The Geologist (talk) 17:06, 3 February 2014 (UTC)

Theoretical Objects & Empirical Objects

Volcanic arcs existed long before plate tectonics, and they will still be there if the theory is replaced. Perhaps the first sentence should not tie the arcs so intimately with one theory.

Done. The explanation is not the mechanism, reworded a bit. Vsmith (talk) 22:37, 23 October 2009 (UTC)

Ah. Thank you very much. That was very timely. However, I was thinking of something a little more drastic. :-)

Naturally I can't find my copy of Bucher's 'The Deformation of the Earth's Crust', in which Bucher distinguished descriptive statements about empirical objects from explanatory statements about theoretical objects. However, (we) scientific positivists do not especially believe science will ever explain those observable phenomena within its domain. We distinguish empirical objects (like volcanic arcs) from theoretical objects (like rising plutons). Theories connect the two. If an observation was used in developing a theory, the theory explains it. If the observation was not made before the theory, the theory predicted it. Even the most abstract objects of mathematical physics are scientific because they are tied to empirical observations by a string of currently accepted theories. The distinction is important. Blurring them confuses people and leads to the precipitation of 'creationists' and such.

Here's an example that distinguishes empirical objects from theoretical objects. Some facts I borrowed from Harold Jeffrey's 1959 'The Earth':--

A volcanic arc is one of a chain of cuspate, or arcuate, volcanic islands or mountains that takes a path resembling part of a circle. This was first noticed by P. Lake in 1931, off the coasts of Alaska and Northern Asia. Lake explained their partly circular shape as the intersection of a planar ocean bottom with a spherical continental side -- decades before the current theory of plate tectonics. In 1951, J. Tuzo Wilson and A.E. Scheidegger used a shrinking Earth theory and seismic studies by Guterberg & Richter to hypothesize that the oceanic crust actually explanded, causing the collapse of cones, and explaining the arcuate shapes of volcanic mountains. (Kind of. :-)

Volcanic chains were later extended around the Pacific Ocean and around some seas. According to the current theory of plate tectonics, such chains lie inward from the curve of descent of an oceanic plate, within the zone of subduction. The volcanic rocks forming them are thought to rise from the subducted oceanic crust, which carried with it some sediments of the continental rise. When these reach a high enough temperature (the thermodynamic solidus temperature), a magma (of the liquidus composition) will precipitate, pool, and rise when its buoyancy exceeds the strength of the rock above. The result would be a string of volcanic mountains. Plate tectonics explains their existence and positions.

Gutenberg, B. & C.F. Richter, 1949. The Seismicity of the Earth. Princeton: Princeton Univ. Press. ; Lake, P. 1931. Geographical J., v. 78, p. 149-160. ; Lake, P. 1931. Geological Mag., v. 68, p. 34-39. ; Scheidegger, A.E. & J.T. Wilson, 1950. Proc. Geol. Assoc. Canada. v. 3. p. 167-190. ; Wilson, J.T., 1951. Proc. Roy. Soc. Tasmania.

The above, of course, is a half-century out of date. Whether plate tectonics explains why volcanic arcs are arced, I have no idea. If not, the next theory may. :-)

Geologist (talk) 04:41, 24 October 2009 (UTC)

Chlorite & Plutonism

How much of the clay in the continental rise is subducted with the basaltic slab is expected to vary with its dip, and is certainly not known to me. However, the sediments would be expected to lose fluids to the overriding mantle continuously. What relation has the decomposition of chlorite to the formation of calc-alkaline magmas? (Veins we see along chlorite-biotite isograds are not calc-alkaline.) The width of forearc basins, the dip of subducting slabs, the amount of clay subducted, and the composition of the overriding mantle all vary. Even if the positions of two phenomena could be correlated, correlation is not a cause & effect relation.

Unlike increasing temperature or pressure, increasing the amount of water does not increase its thermodynamic escaping tendency. The decomposition of chlorite will not suddenly produce calc-alkaline magmas; it could only increase their amount or change their mode (relative amounts of minerals).

Because of the above reasons for doubt, if there is a popular theory that chorite decomposition produces calc-alkaline magmas, could you make this very clear and give more references, by multiple authors? Thanks very much. (You may wish to keep in mind that theories of plutonism change every decade. This might be a current theory, a working hypothesis. or just the latest idea being explored. Geologist (talk) 21:49, 23 October 2009 (UTC)