Talk:Oceanic crust

Thickness of oceanic crust
Under the continents "oceanic crust" should be thinner than it is in the ocean basins, do to isostasy. --Bejnar 01:40, 1 February 2007 (UTC)

Density changes
Editor Karl07 added the sentence: "When the crust becomes dense enough it subducts into the mantle at what is known as a convergent boundary." Unfortunately the rocks cannot change their density as they are pulled/pushed away from the oceanic ridges. One theory has it that the convection currents in the mantle formed early in the earth's history and their current locations are more a matter of history than mechanics. --Bejnar 22:14, 15 February 2007 (UTC)
 * The rocks do change their density, due to thermal contraction, and this is a significant driver of subduction. --Kent G. Budge (talk) 16:27, 21 July 2022 (UTC)

Inaccuracies in the edits of 16 March 2007
This is just to address a couple of the inaccuracies present in the edits of 16 March 2007 that I reverted. (1) The Oceanic crust and the SIMA are not the same, they are related. The SIMA can be solid or molten, the crust is solid. (2) The lava that solidifies at the mid-oceanic ridges may not come from as deep as the asthenosphere. It is considered by some authors much more likely that the circulating heat from the asthenosphere melts the rock above it. (3) The lava does not pour out at the crests. The crests are an erosional feature that are pushed upwards by magma domes. Additionally, the magma can solidify without being extruded as lava. (4) The edit added send magma into the rock cycle, which is inaccurate as the differentiation between lighter and heavier elements in the Earth has been going on for a long, long time. The magma has been in the rock cycle since very early in the process. (5) The edit added [such magma] are the origins for all the Earth's materials. This statement is both overbroad and incorrect. One simple counter-example is limestone. I don't think that I need to go on any further. --Bejnar 03:00, 17 March 2007 (UTC)

Conrad Discontinuity
Om February 2008 Cmapm added a sentence about the Conrad Discontinuity, indicating that it was the sial/sima boundary. This was what some thought before deep drilling below the Kola Peninsula found no such switchover. No one knows what the Conrad Discontinuity represents. It doesn't signal a change in rock type; neither is there a fault or boundary of any kind. It is important to find out what is wrong here, because much of modeling of the unseen structure of the earth's crust depends upon a realistic interpretation of seismic records. Monastersky, Richard (1989) "Inner Space" Science News, 136: p.266. Is there an update in the last 18 years? Isostasy predicts that there will still be lighter sial at the depth of the Conrad Discontinuity, but it is possible that there is a phase change due to pressure effects at that depth. --Bejnar (talk) 00:40, 19 February 2008 (UTC)
 * It sometimes appears in scientific papers up to this day (I've found one paper of 2007, mentioning it, in springerlink), but is quite a mystical thing I'd say:) with too few sources. I seriously doubt if we can find enough inf. for the article on this, just for a short stub.Cmapm (talk) 09:29, 19 February 2008 (UTC)

Implications of geologically young oceanic crust
The age of the oldest parts of the oceanic crust as ~200 Ma is not a matter of dispute. The hypothesis that this implies continuous expansion of the earth, approximately doubling in the last 200 Ma, is the view of a small minority of earth scientists (see Expanding earth theory). This view denies that subduction exists or claims that the amount of material consumed is far less than that created at the mid-ocean ridges. At the most a link to the Expanding earth theory page could be included in the 'life cycle' section, to do more would be giving this undue weight. Mikenorton (talk) 09:16, 3 October 2008 (UTC)

Merger from sima (geology)

 * Oppose I oppose the merger from sima (geology) into oceanic crust as the two are not the same. See my comments above, and my comment on merging sial into continental crust at Talk:Sial.--Bejnar (talk) 06:42, 28 November 2008 (UTC)
 * Oppose The merger was my proposal, and I withdraw it on the weight of Bejnar's arguments. However, the articles as they stand now are inaequate as they do not convey the distinction and contain duplicated material. --Jorge Stolfi (talk) 10:50, 28 November 2008 (UTC)
 * I agree about the need for improvement. The problem with duplicated material comes from a desire to edit, something not to be disparaged, that seems to possess (among others) students who attend their first geology lecture. They may not understand everything, but by golly, they want it all in the first article that they visit. "Semper vigiles". --Bejnar (talk) 16:35, 28 November 2008 (UTC)


 * Oppose As it says on the Sial page "Sial" is a geochemical term rather than a plate tectonic term." So if sial gets it's own page then so should sima. I like the distinction. Logrythm (talk) 10:01, 11 December 2008 (UTC)
 * Oppose I like the terms Sial, Sima and Nife and the like - it makes it easier to remember what they mean. silicon and magnesium, silicon and aluminum, nickle and iron. OsamaBinLogin (talk) 19:57, 19 October 2011 (UTC)

Composition and formation
I suggest to modify the 'Composition' section to 'Composition and formation'. I mostly suggest modification of the layer 3 (the lower oceanic crust), with addition of the most appropriate references.

Although a complete section of oceanic crust has not yet been drilled, geologists have several pieces of evidence that help them understand the ocean floor. Estimations of composition are based on analyses of ophiolites (sections of oceanic crust that are thrust onto and preserved on the continents), comparisons of the seismic structure of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from the ocean floor by submersibles, dredging (especially from ridge crests and fracture zones) and drilling.[8] Oceanic crust is significantly simpler than continental crust and generally can be divided in three layers. According to mineral physics experiments, at lower mantle pressures, oceanic crust becomes denser than the surrounding mantle.[9]

- Layer 1 is on an average 0.4 km thick. It consists of unconsolidated or semiconsolidated sediments, usually thin or even not present near the mid-ocean ridges but thickens farther away from the ridge.[10] Near the continental margins sediment is terrigenous, meaning derived from the land, unlike deep sea sediments which are made of tiny shells of marine organisms, usually calcareous and siliceous, or it can be made of volcanic ash and terrigenous sediments transported by turbidity currents.[11]

Julien.leuthold (talk) 20:11, 23 March 2018 (UTC)

- Layer 2 could be divided into two parts: the layer 2A (0.5 km thick) uppermost volcanic layer of glassy to finely crystalline basalt usually in the form of pillow basalt, erupted on the seafloor. Layer 2B (1.5 km thick layer) is composed of vertical diabase dikes.[12] Those dykes feed volcanic eruptions.

- Layer 3 is formed by slow cooling of magma beneath the surface and consists of coarse grained gabbros, troctolite and cumulate ultramafic rocks. It constitutes over two-thirds of oceanic crust volume with almost 5 km thickness.[13] It constitutes the lower oceanic crust. It is difficult to access, and its study requires the use of submarines along faults or drilling. The lower oceanic crust certainly grows by injection of magma sourced from the underlying Earth mantle, forming magma chambers. With high magma emplacement rate (typically in fast-spreading ridges), the lower oceanic crust remains partially molten and can deform during horizontal extension of the crust. With low magma emplacement rate (typically in slow-spreading ridges), magma batches fully crystallise prior to emplacement of new magma. The mantle melts intruding the lower oceanic crust are most commonly modified by fractional crystallisation due to cooling, before rising to layer 2. However, reaction of liquid with pre-existing rocks and by partial melting of crustal rocks also affect the chemical composition of the lavas in layer 2.


 * Wikipedia policy regarding citations is to prefer secondary sources. Sources that have been cited many times can be regarded as authoritative. Wikipedia policy is also to discourage self-citation. Julian, your suggesting that your paper be cited is self-citation. Your paper is also a primary source, though I note that Oceanic crust has many of those. And, since your paper hasn't yet been cited (at least that is what google indicates), it can't yet be regarded as authoritative. I suggest you keep applying your evident expertise to build up the encyclopedia in other ways. Attic Salt (talk) 13:35, 24 March 2018 (UTC)


 * "Julian, your suggesting that your paper be cited is self-citation." This is not actually correct. Experts are encouraged to bring up their work on talk pages rather than just cite it in articles, so that other editors can evaluate their appropriateness in a disinterested manner. See WP:SELFCITE. --Kent G. Budge (talk) 16:31, 21 July 2022 (UTC)

Third paragraph needs revision
(Replicated here for convenience.)


 * The crust uppermost is the result of the cooling of magma derived from mantle material below the plate.

"The crust uppermost" is ungrammatical. "Uppermost" was inserted by a March 2018 edit which also added the mention of lower crust (portion after the sentence ending "flood basalt eruptions".) Yet simply changing the word order to "The uppermost crust" is not logically sound. If we seperate the oceanic crust into layers, the upper layer cannot be derived directly from the mantle because it is not in direct contact with the mantle: the lower layer(s) are in-between. If the lower layers are discontinuous, such that the mantle pierces them and does directly contact the upper crust, e.g. at mid-ocean ridges, this should be made clear. (I don't even know if this is true.)


 * The magma is injected into the spreading center, which consists mainly of a partly solidified crystal mush derived from earlier injections, forming magma lenses that are the source of the sheeted dikes that feed the overlying pillow lavas.[4] As the lavas cool they are, in most instances, modified chemically by seawater.[5] These eruptions occur mostly at mid-ocean ridges, but also at scattered hotspots, and also in rare but powerful occurrences known as flood basalt eruptions.


 * But most magma crystallises at depth, within the lower oceanic crust. There, newly intruded magma can mix and react with pre-existing crystal mush and rocks.[6]

As mentioned, these two sentences were added by a seperate editor. They are poorly integrated into the paragraph. The leading "But" makes it sound like it is contradicting what was previously stated. Yet the process as described sounds like a restatement of the same thing. ("a partly solidified crystal mush derived from earlier injections" vs. "mix and react with pre-existing crystal mush and rocks") The point of emphazing where "most magma crystalizes" sounds less like an attempt to inform and more like an attempt to jockey for prominence, i.e. asserting a specific topic (usually one's own specialty) within a field as being of greater importance than others. 2600:4040:A057:1900:945B:4B70:7DE5:8089 (talk) 02:04, 20 January 2023 (UTC)