Earth Materials. John O'Brien. Читать онлайн. Newlib. NEWLIB.NET

Автор: John O'Brien
Издательство: John Wiley & Sons Limited
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Жанр произведения: География
Год издания: 0
isbn: 9781119512219
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oceanic crust cause it to be less buoyant than continental crust, so that it occupies areas of lower elevation on Earth's surface. As a result, most oceanic crust of normal thickness is below sea level and covered by sea water to a depth of several thousand meters. Oceanic crust consists principally of basic igneous rocks such as basalt and gabbro composed largely of the minerals pyroxene and calcic plagioclase. These dark‐colored, mafic igneous rocks comprise layers 2 and 3 of oceanic crust and are commonly topped with sediments that comprise layer 1 (Table 1.1). An idealized profile of typical ocean crust consists of these three main layers, each of which can be subdivided into sublayers which are briefly discussed later in this chapter.

      Oceanic crust is young relative to the age of the Earth (~4.55 Ga = 4550 Ma). The oldest ocean crust in the major ocean basins, less than 190 million years old (190 Ma), occurs along the western and eastern borders of the Atlantic Ocean and in the Western Pacific Ocean. Recently, still older oceanic crust that may be 340 Ma has been discovered in the eastern Mediterranean Sea (Granot 2016). Still older oceanic crust has largely been destroyed by subduction, but fragments of such crust are preserved on land in the form of ophiolites. Ophiolites contain slices of ocean crust thrust onto continental margins and provide evidence for the existence of Precambrian oceanic crust. The age of the oldest true ophiolites of Precambrian age remains controversial (Chapter 18).

Properties Oceanic crust Continental crust
Composition Dark colored, mafic rocks enriched in MgO, FeO, and CaO Complex; many lighter colored felsic rocks
Enriched in K2O, Na2O, and SiO2
Averages ~50% SiO2 Averages ~60% SiO2
Density Higher; less buoyant Lower; more buoyant
Average 2.9–3.1 g/cm3 Average 2.6–2.9 g/cm3
Thickness Thinner; average 5–7 km thickness Thicker; average 30 km thickness
Up to 15 km under islands Up to 80 km under mountains
Elevation Low surface elevation; mostly submerged below sea level Higher surface elevations; mostly emergent above sea level
Age Up to 190 Ma for in‐place crust Up to more than 4000 Ma
~3.5% of Earth history 85–90% of Earth history

       Continental crust

      Continental crust has a much more variable composition than oceanic crust. Continental crust can be generalized as “granitic” in composition, and is enriched in K2O, Na2O, and SiO2 relative to average crust. Although igneous and metamorphic rocks of granitic composition are fairly common in the upper portion of continental crust, lower portions contain more rocks of intermediate dioritic and even basic gabbroic composition. Granites and related rocks tend to be light colored, lower density felsic rocks rich in quartz and potassium and sodium feldspars. Continental crust is generally much thicker than oceanic crust; depth to the Moho averages 30–40 km. Under areas of very high elevation, such as the Himalayas, its thickness approaches 85 km. The greater thickness and lower density of continental crust make it more buoyant than oceanic crust. As a result, the top of continental crust is generally located at higher elevations and the surfaces of continents with normal crustal thicknesses are above sea level. The distribution of Earth's land and sea is largely dictated by the distribution of continental and oceanic crust. Only the thinnest portions of continental crust, most frequently along thinned continental margins and in rifts, have surfaces below sea level.

      1.4.2 Earth's Mantle

       Upper Mantle and Transition Zone

      The uppermost part of the mantle and the crust together constitute the relatively rigid lithosphere which is strong enough to rupture in response to Earth stresses. Because the lithosphere can rupture in response to stress, it is the site of most earthquakes and is broken into large fragments called plates, as discussed later in this chapter.