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Excert from an essay I had to write for my degree about the Earth's crust.. It needs tidying up before being used in the main article.
Continental crust is divided into 2 layers, the update per and lower sections. The upper section is the only layer that can be sampled directly, through many samples of rocks from this layer it has been estimated that it is of a ?rock type between granodiorite and diorite? (Kearey & Vine, 1996). As it is too deep it has not been possible to sample the lower section of the continental crust using bore holes and such like. Instead the velocities of P waves, caused by seismic activity can be calculated, and from this the composition estimated. Early studies calculated the composition of this layer to be basaltic. However more recent studies have found that it is not possible for this lower layer to be basalt. This is because it has been shown that the pressure is around 2100MPa and the temperature is about 1100ºC(Green & Ringwood, 1967). Under these extreme conditions basalt would be expected to transform to ?ecologite via the intermediate stages of garnet granulite?. However these findings are disproved, as the velocity for P-waves in ecologite is around 8 km s-1, while the studied velocities in the lower crust varied from 6.5 to 7.6 km s-1 (Kearey & Vine, 1996). There is still much on going research and debate into the composition of the lower layer.
The line that divides the upper and lower sections of continental crust is called the Conrad discontinuity. This was detected due to the changes in the velocities of P-waves following seismic activity.
On average the overall depth of continental crust is considerably thicker than that of oceanic crust, being around 30 ?70km thick. It is thickest in high mountain ranges, such as the Himalayas and the Andes, this is due to the folding that took place during the time the mountains were formed. Due to its overall chemical composition it is less dense than oceanic crust.
Continental crust is formed along converging plate boundaries, such as the boundary between the Pacific and Indian plates. It is formed by any collision that involves two plates, whether oceanic or continental. This is because continental crust is less dense than oceanic crust, and therefore always goes over the top of oceanic crust at a collision zone.
Although some recycling and regeneration occurs of continental crust, it is of a significantly slower rate than that of oceanic crust. The most common form of recycling occurs at the margins between the two crust types, rocks that make up the continental crust are eroded, and carried off by the subduction of the oceanic crust. It is suggested that oceanic crust may act like sandpaper and break off bits of the continental crust, thereby recycling the continental crust.
Oceanic crust is a lot thinner than Continental crust, being only between 7 and 10km thick. It has been suggested that there are 3 distinct layers in oceanic crust, these have numbered 1, 2 and 3, with layer 1 being the top layer. Layer 1 has been investigated extensively due to its ease of access, it is only on average around 0.4km deep, and in some places, particularly around ocean ridges, does not exist. Layer 1 is made up of various sediments and clays. The second layer, Layer 2, varies between 1 and 2.5km thick, it has been sampled, by dredging at ocean ridges, where Layer 1 is not present. The results of these samplings have shown that it is igneous in origin and consists mainly of basalts.
The main depth of oceanic crust is taken up by Layer 3, this layer is believed to be mainly gabbroic in composition (Kearey & Vine, 1996). These are rocks which are similar to basalts. They were formed at depths where cooling and crystallisation have occurred slowly, this has created the coarse grains that they have (Whittow, 2000).
Oceanic crust is significantly younger than continental crust, this may be explained by subduction, as subduction continually recycles and recreates oceanic crust. Typically it is unknown to find oceanic crust older than around 180 - 200 million years old.
Reference: Kearey & Vine, 1996 Global Tectonics 2nd ed. Blackwell Science, Oxford --Cyr 10:51, 3 May 2005 (UTC)
- Feel free to add pertinent data and the references to the article, also some may be useful in Continental crust and Oceanic crust as these are both stubs. Vsmith 15:22, 3 May 2005 (UTC)
Omphacite 18:43, 9 September 2007 (UTC)
The table lists Calcium Oxide (CaO) as one of the major components of the earth's crust. Should this be Calcium Carbonate (CaCO3) instead, which is limestone? I don't think CaO exists in great quantities (if at all) naturally, since it reacts with water to form Ca(OH)2 and absorbs carbon diaoxide to form CaCO3.
Tcooke 05:01, 3 April 2007 (UTC)
- Crustal composition is usually listed in terms of elements as oxides and the ref for this appears to be the 1911 Brittanica so may be somewhat dated. As for the calcium carbonate bit, limestone occurs as a part of the relatively thin sedimentary veneer on the typically igneous crust. The calcium within the more common igneous rocks occurs in the form of various calcium silicate minerals. So calcium silicate would predominate, however, as stated prev. the traditional way to list chemical compositions of rocks is by element oxide. After all that, I'd say we probably need to find a more current estimate or listing of crustal abundances. Vsmith 10:56, 3 April 2007 (UTC)
suggested major changes - comments?
The article partly duplicates and partly disagrees with separate entries for continental crust and oceanic crust. Perhaps only a brief summary of the Earth's crust should be in this entry, based on the present introduction.
Short additions would be useful for crusts of Mercury, Venus, our Moon, and Mars.
Such changes would drastically change this entry. Comments?
Also, updates for the separate entries on continental crust and oceanic crust would be valuable. Omphacite 18:43, 9 September 2007 (UTC)
major changes begun, please participate
- Seems as 90+ percent of the linking articles are concerned with Earth's crust it seems that the emphasis should remain on earth with smaller sections for other planets. The section Perspective from the Moon is valid, but should follow the description of Earth's crust. Vsmith (talk) 21:30, 1 January 2008 (UTC)
A different order of the sections would be fine, but perhaps the text added to the introduction will make the present order seem better. Also, perhaps eventually the table on the composition of the continental crust might be moved to the entry on continental crust.Omphacite (talk) 22:15, 6 January 2008 (UTC)
- It says that the crust is a chemically and mechanically distinct layer. From what I understand, it's just chemically/mineralogically distinct, and the lithosphere defines the mechanical boundary. Awickert (talk) 21:28, 19 January 2009 (UTC)
The diagram which correctly says "Not to Scale" for the pie wedge somehow lost the first word "Not" for the cut through the sphere. Do the math. Proportionally, the crust averages thinner than an eggshell.
The earths crust
The crust of the Earth is composed of a great variety of igneous, metamorphic, and sedimentary rocks. The crust is underlain by the mantle. The upper part of the mantle is composed mostly of peridotite, a rock denser than rocks common in the overlying crust. The boundary between the crust and mantle is conventionally placed at the Mohorovičić discontinuity, a boundary defined by a contrast in seismic velocity. Earth's crust occupies less than 1% of Earth's volume.
The oceanic crust of the Earth is different from its continental crust. The oceanic crust is 5 km (3 mi) to 10 km (6 mi) thick and is composed primarily of basalt, diabase, and gabbro. The continental crust is typically from 30 km (20 mi) to 50 km (30 mi) thick, and is mostly composed of slightly less dense rocks than those of the oceanic crust. Some of these less dense rocks, such as granite, are common in the continental crust but rare to absent in the oceanic crust. Both the continental and oceanic crust "float" on the mantle. Because the continental crust is thicker, it extends both above and below the oceanic crust, much like a large iceberg floating next to smaller one. (The slightly lighter density of felsic continental rock compared to basaltic ocean rock also contributes to the higher relative elevation of the top of the continental crust.) Because the top of the continental crust is above that of the oceanic, water runs off the continents and collects above the oceanic crust. The continental crust and the oceanic crust are sometimes called sial and sima respectively. Due to the change in velocity of seismic waves it is believed that on continents at a certain depth sial becomes close in its physical properties to sima and the dividing line is called Conrad discontinuity.
The temperature of the crust increases with depth, reaching values typically in the range from about 200°C (392°F) to 400°C (752°F) at the boundary with the underlying mantle. The crust and underlying relatively rigid mantle make up the lithosphere. Because of convection in the underlying plastic (although non-molten) upper mantle and asthenosphere, the lithosphere is broken into tectonic plates that move. The temperature increases by as much as 30°C (about 50°F) for every kilometer locally in the upper part of the crust, but the geothermal gradient is smaller in deeper crust. 
Partly by analogy to what is known about our Moon, Earth is considered to have differentiated from an aggregate of planetesimals into its core, mantle and crust within about 100 million years of the formation of the planet, 4.6 billion years ago. The primordial crust was very thin, and was probably recycled by much more vigorous plate tectonics and destroyed by significant asteroid impacts, which were much more common in the early stages of the solar system.
The Earth has probably always had some form of basaltic crust, but the age of the oldest oceanic crust today is only about 200 million years. In contrast, the bulk of the continental crust is much older. The oldest continental crustal rocks on Earth have ages in the range from about 3.7 to 4.28 billion years  and have been found in the Narryer Gneiss Terrane in Western Australia, in the Acasta Gneiss in the Northwest Territories on the Canadian Shield, and on other cratonic regions such as those on the Fennoscandian Shield. A few zircons with ages as great as 4.3 billion years have been found in the Narryer Gneiss Terrane.
The average age of the current Earth's continental crust has been estimated to be about 2.0 billion years. Most crustal rocks formed before 2.5 billion years ago are located in cratons. Such old continental crust and the underlying mantle lithosphere are less dense than elsewhere in the earth and so are not readily destroyed by subduction. Formation of new continental crust is linked to periods of intense orogeny or mountain building; these periods coincide with the formation of the supercontinents such as Rodinia, Pangaea and Gondwana. The crust forms in part by aggregation of island arcs including granite and metamorphic fold belts, and it is preserved in part by depletion of the underlying mantle to form buoyant lithospheric mantle. —Preceding unsigned comment added by 18.104.22.168 (talk) 23:30, 17 February 2010 (UTC)
This is really popular now.. I read on youtube that it is the yearly average plus 110f per mile depth.. Perhaps this can be added. —Preceding unsigned comment added by 22.214.171.124 (talk) 22:10, 19 June 2010 (UTC)
- The article gives a figure of 30°C/km or roughly 90°F/mile, which is a bit less than your quote, which would be nearer 40°C/km, rather high for an average figure. Mikenorton (talk) 22:43, 19 June 2010 (UTC)
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