What is the average composition of the oceanic crust

Earth has three layers: the crust, the mantle, and the core. While continental crust is thick and light-colored, oceanic crust is thin and very dark. Oceanic crust is only about miles thick, but continental crust is around 25 miles thick. The crust is the thinnest layer of the Earth. It has an average thickness of about 18 miles 30km below land, and around 6 miles 10km below the oceans. The crust is a thin but important zone where dry, hot rock from the deep Earth reacts with the water and oxygen of the surface, making new kinds of minerals and rocks.

Answer: If this event continues for millions of years, the cracks will become wider and the two land masses will be more far away from each other. The creation of new crust between the land masses also widen. A crust is the outermost layer of a planet. The crust of the Earth is composed of a great variety of igneous, metamorphic, and sedimentary rocks. Volumetrically insignificant part of all the minerals contain no oxygen.

Almost every common rock type contains oxygen. Only sulfide ore bodies and evaporite beds are almost free of oxygen, but they are volumetrically relatively insignificant. Silicon has its very own large group of minerals known as silicates. Silicon and oxygen are the two most common chemical elements in the crust that also happen to like each others company very much.

There is not a single common non-silicate mineral that contains silicon — silicon always combines with oxygen. Silicate minerals are the building blocks of most common rock types basalt , granite , schist, gneiss , sandstone, etc.

Carbonate rocks limestone, dolomite rock and evaporites gypsum rock, rock salt are notable exceptions. They do not contain silicon if they are pure. Opaque ore minerals oxides and sulfides are frequent minor components of most rock types. They are also free of silicon.

Very widespread in silicate minerals feldspar , clay minerals, mica. Aluminum hydroxides boehmite, diaspore, gibbsite are economically important as aluminum ore minerals. Hence, aluminum is extremely widespread as well. It is usually not very concentrated in silicate minerals, though. Aluminum has been extracted from silicate rocks very rarely. Bauxite which is aluminum-rich laterite formed in humid hot areas contains aluminum hydroxides and is primarily mined for aluminum. Aluminum in bauxite is a residue of chemical weathering of silicate rocks.

Iron is a widespread element in minerals. Notable iron-rich silicate minerals are pyroxenes, amphiboles , olivine , black mica biotite , garnet , etc. Iron is also an important element in sedimentary rocks. It is just like aluminum hard to dissolve and carry away with water. Iron is common in lateritic soil and forms rust-colored iron oxide mineral hematite. Hematite is responsible for the red coloration of many minerals and rock types.

Iron oxide magnetite is common as an accessory mineral in metamorphic and igneous rocks. Iron sulfide pyrite is the most common sulfide mineral. Iron also occurs in carbonates siderite , ankerite , clay minerals glauconite , chlorite. Iron is a strong chromophore element, it gives dark coloration to its host minerals. This is why most pyroxenes and amphiboles are black. Iron is actually the single most abundant chemical element in the whole of Earth, but most of it is in the core.

Basalt, gabbro , amphibolite, greenschist , etc. There is a large number of rock types that contain significant amount of iron, but most of the iron mined comes from metamorphosed sedimentary rocks known as BIF banded iron formation. Calcium is also very widespread. The most important pyroxenes and amphiboles augite and hornblende contain calcium. Calcium occurs in many other silicate minerals like garnet, epidote , wollastonite , titanite, etc.

Calcium is a constituent of calcite which is very important mineral chiefly in sedimentary environments. Calcium phosphate apatite is a common mineral as well. Gypsum is a major evaporite mineral that is chemically hydrated calcium sulfate.

Calcium fluoride is known as mineral fluorite. Occurs equally successfully in igneous, sedimentary, and metamorphic rocks. Especially well-known calcium-bearing rock type is limestone. Its metamorphosed equivalent is marble. Marble is composed of calcite just like limestone. Calcite is a remarkable mineral. Even igneous rock composed of pure calcite exists. It is known as carbonatite , but it is very rare when compared with limestone and marble.

Calcium tends to be part of minerals like plagioclase, pyroxenes and amphiboles in igneous rocks. Major calcium-bearing metamorphic rock is amphibolite metamorphosed basalt, calcium is hosted by hornblende and plagioclase. Phosphorite is another important calcium-bearing sedimentary rock calcium is hosted by phosphate mineral apatite. Calcium also occurs in evaporites as a mineral gypsum. Sodium is widespread in silicate minerals.

It is an important constituent of both alkali feldspar and plagioclase. Sodium-bearing pyroxenes are relatively rare. Sodium is somewhat more widespread in amphiboles but not as much as calcium.

Well-known sodium-bearing silicate mineral is tourmaline. Sodium is an important component of feldspathoids, but both feldspathoids and tourmaline group minerals are relatively rare.

Major sodium-bearing mineral in sedimentary environments is halite NaCl. Igneous and metamorphic rocks that contain feldspar. Much of sodium from weathered igneous and metamorphic rocks is dissolved in seawater.

Rock salt is the most important sodium-bearing sedimentary rock. Potassium and sodium are similar chemical elements both chemically and geologically. Potassium is an important constituent of alkali feldspars. Most alkali feldspars contain much more potassium than sodium and are therefore frequently referred to as K-feldspars. Biotite and muscovite are the most important micas and they both contain potassium. Most important potassium-bearing sedimentary mineral is sylvite KCl.

Alkali feldspars and micas are common rocks in silicate igneous and metamorphic rocks granite, gneiss, schist, etc. Much of potassium from weathered igneous and metamorphic rocks is dissolved in seawater. Sylvite is not as common evaporite as halite rock salt because it takes much higher evaporation rate to precipitate sylvite. Magnesium is very widespread in the mantle beneath the crust. Olivine and pyroxene are the most important Mg-bearing minerals there and these minerals are also constituents of some crustal rocks, especially dark-colored igneous rocks.

Amphiboles also contain magnesium but less than pyroxenes. Magnesium ion is similar to iron in size and can therefore easily replace iron in the lattice of minerals. This is the case in olivine, pyroxenes, amphiboles and even micas phlogopite is a Mg-rich variety of biotite.

Important Mg-rich minerals in metamorphic rocks are talc and serpentine. Magnesium in the sedimentary environment occurs chiefly in carbonates dolomite and magnesite. Lots of magnesium is dissolved in seawater. Magnesium is extracted from seawater. Important Mg-bearing igneous rocks are ultramafic rocks peridotite , pyroxenite.

Rocks that contain lots of pyroxenes like basalt and gabbro contain Mg also but to a lesser extent. Some such basalts do not have enough data to be classified but have been left in because of their close association with the usually older andesitic rocks, eg the Mango basalts of Fiji.

Should ignimbrites be included? Their huge volume and the lack of fractionated members has usually been taken to mean that ignimbrites originate from the partial melting of the continental crustal keel. In deep Himalayan valleys mobilised sediments of rhyolitic composition can be seen breaking through the overlying more basic layered metasediment towards the surface. While they appear at the surface as rhyolitic ignimbrites and flows, a basic residue is left at the continental base so the average composition of the crust has not been greatly changed if at all.

The samples of the Andean Arc include a hundred or more ignimbrites. With some dubiety these have been left in as at least some rhyolites may be formed by the partial melting of subducted oceanic basalt. The Aeolian Arc in the Mediterranean Sea ranges from normal calc-alkaline rocks in the islands of Salina, Alicudi etc to highly potassic leucitic members in both Vulcano and Stromboli.

This is likely to indicate that some crustal refusion has been involved and so the AeolianArc has been left out for now. We may give an average later but the Roman province will not be included in the crustal average at this point. It may well be asked, "Do arithmetic means of a diverse body of rock, often showing a curved relationships between elements have any real meaning? The evolution of the crust appears to parallel the evolution of the andesites, from a basalt or basaltic-andesite average composition in the barely emergent proto arcs towards andesite in the continental arcs.

The Cascades are, due to the large volume of associated basalts, not as mature as might have been expected. In all, the averages are more basic than we might have assumed and are in fact rather more basic than earlier estimates of average continental crust made 40 years ago. This is a difficult proposition as the majority of oceanic crustal samples collected are now aphyric glasses due to their lack of alteration.

This means that while all cumulative rocks are omitted, their higher fractionates may be present, so any average will be less magnesian than the true average. If we use only non-glass whole rocks then some at least will have a changed composition due to alteration.

As the range of partial melts is between 6 -