Ocean crust thickness

Oceanic crust The mass of basaltic material, approximately 5-7 km thick, which underlies the ocean basins. 

The Earth s mantle is overlain with crust. The crust is divided into two main types the silicate-rich (felsic to intermediate) continental and the thinner mafic oceanic crusts. Oceanic crusts are located in the deeper parts of the ocean floor and have an average thickness of about 5 km. Continents and some shallow seas are typically underlain with continental crust. Continental crusts have an average thickness of about 40 km, but are up to 65 km thick in high mountainous areas (Press and Siever, 2001, 439 Figure 3.1). 

Continental crust Thtfelsic to intermediate outer rock layers of the Earth that constitute large portions of the continents. The maximum thickness of the continental crust is about 65 km. Compared with continental crust, oceanic crust is much thinner and more mafic. 

Crust (Earth) The outermost layer of the Earth, which includes the oceanic and continental crusts. The oceanic crust may be as thin as 5 km and the thickness of the continental crust may be as much as 65 km or so. 

Figure 2 The ocean crust reference section used in this paper, using a standard Penrose style ophiolite assemblage (Penrose Conference Participants, 1972) and a crustal thickness after White etal.(l992) and C. Z. Mutter and J. C. Mutter (1993). Various modeling parameters used are also indicated, including densities and fluxes, the approximate positions of drill holes discussed in this paper, and some typical extreme alteration environments found...

Earth s crust ranges in thickness from 10 to 50 km and contains at least trace amounts of 88 chemical elements. It can be subdivided into two distinctly different regimes the oceanic crust that underlies the oceanic basins and the continental crust. The two differ in composition — the oceanic crust being richer in iron, magnesium, and calcium, the continental crust being richer in silicon, aluminum, and alkali elements. The 88 natural elements are all present in both crusts, though in somewhat different concentrations. Nevertheless, only 12 elements, and the same 12 elements in each case, are... 

Using a subduction rate of about 3 km2/a and a slab thickness of 100 km Davies (1998) estimated that the mass flow into the mantle through subduction was about 1015 kg/a. Hence the mass flux into the mantle is about 1015 kg/a. The mismatch between this value and the mass flux out of the mantle arises because we are equating two different fluxes. One is the flux of ocean crust (flux out) the other is the flux of ocean crust plus oceanic lithosphere (flux in). 

If, as many suppose, the Archaean mantle had a higher potential temperature than the modern mantle, it is important to examine the implications of this for melt production during the early history of the Earth. The relationship between mantle potential temperature and melt thickness during adiabatic melting was outlined in Section 3.1.4.3 and may be briefly summarized by stating that as mantle potential temperature increases so will the melt production, as expressed in the depth of the melt column and the melt thickness. This is illustrated in Fig. 3.26, which shows how deeper, higher-temperature melting should lead to the formation of a thicker oceanic crust. 

In this model the mantle is differentiated, into an upper layer (1,600-2,000 km thick), and a compositionally distinct lower layer, enriched in iron and heat producing elements. This lower layer, making up 20-30% of the mantle, is thought to have originated either during the early differentiation of the Earth or by the burial of subducted oceanic crust. 

Present-day spreading rates allow us to estimate the volume of oceanic crust, which is subducted annually. Estimates are between 18 and 20 km3/ yr, the differences being in part due to different assumptions about the average thickness of the oceanic crust. If this rate of subduction has been constant over geological time, then the Earth s mantle should contain 5 % recycled oceanic crust (Helffrich Wood, 2001), although if this were concentrated in the upper part of the mantle then, of course, the proportion would be considerably greater. 

Plume/oceanic plateau models This model is a variant of the basalt lower crust melting model described above, but in this case basalt melting takes place at the base of thick basaltic crust. Crust of this type may form through imbrication and stacking of normal-thickness oceanic crust or may form as initially thick crust in an oceanic plateau as the product of mantle plume-related magma-tism (Chapter 3, Section 3.1.5). 

Figure 7. (a) Idealized oxygen isotope profile of altered, sediment-covered oceanic crust based on Gregory and Taylor s (1981) study of the Ibra section of the Oman ophiolite and data for marine sediments taken from data sources cited in the text. The vertical black bar marks the range in 5 0 t ical of mantle peridotites the vertical white bar marks the range in 5 0 typical of fresh oceanic basalts, (b) VMues of typical of various major rock types in the Earth s cmst. Data sources are listed in the text. These ranges emphasize typical, representative values and purposefully exclude extreme examples of many rock types. The isotopic composition of most mantle peridotites (Fig. 8) and all NMORBs (Fig. 9) spans a range equal to the thickness of the vertical black hne. 

Oceanic crust subducted calculated using subduction rate, trench length (Plank and Langmuir, 1998) and assuming a thickness of the oceanic crust of 7 km and a density of 2.89 g/cm. ... 

The crust is basically the akin of the earth. It only accounts for 5 percent of the eeurth s thickness. It comes in two forms continental and oceanic. The continental crust is rich in silica and alumina minerals. The oceanic crust is of a silica magnesia cco osition. Due to the processes of nature some of the continental crust is becoming oceanic and vice versa The crust s thinnest points can be found under the oceans, between three and five miles thick. The oceans form... 

The transition zone in the oceanic crust is represented by serpentine, the product of the Hess reaction of olivine with water. The serpentine forms in the lower oceanic crust, because of water percolation from the ocean through brittle -fissured basalts, and exhibits true plastic behavior, which prevents further migration of oceanic water into the peridotite mantle. Since the true plastic, that is, impermeable state of the serpentine is reached at pressures of 0.2-0.4 GPa and temperatures below 550°C, the thickness of the oceanic crust is about 11 km, if we account for the ocean water weight, Nikolaevskiy (1979), Lobkovsky (1988). 

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