Earth Materials. John O'Brien
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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).
Table 1.1 Characteristics of oceanic and continental crust: a comparison.
| 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.
Whereas modern oceans, with the exception of a small area in the Mediterranean Sea, are underlain by oceanic crust younger than 190 Ma, the oldest well‐documented continental crust includes 4.03 Ga rocks from the Northwest Territories of Canada (Stern and Bleeker 1998). Approximately 4 Ga rocks also occur in Greenland and Australia. Greenstone belts (Chapter 18) may date back as far as 4.28 Ga (O'Neill et al. 2008) which suggests that continental crust began forming within 300 million years of Earth's birth. Individual detrital zircon grains, derived from the erosion of older continental crust, occur in metamorphosed sedimentary rocks in Australia. These zircons have been dated at 4.4 Ga (Wilde et al. 2001) an age recently confirmed by Valley et al. (2014). These data suggest that continental crust may have existed no more than 150 Ma after Earth formed. The great age of some continental crust results from its relative buoyancy. In contrast to ocean crust, continental crust is largely preserved as its density is generally too low for it to be subducted on as large a scale. Table 1.1 summarizes the major differences between oceanic and continental crust.
1.4.2 Earth's Mantle
The mantle is thick (~2900 km) relative to the radius of Earth (~6370 km) and constitutes ~83% of Earth's total volume. The mantle is distinguished from the crust by being very rich in MgO (30–40%) and, to a lesser extent, in FeO. It contains an average of approximately 40–45% SiO2 which means it has an ultrabasic composition (Chapter 7). Some basic rocks such as eclogite occur in smaller proportions. In the upper mantle (depths to 400 km), the Mg‐rich silicate minerals olivine and pyroxene dominate; spinel, plagioclase and garnet are locally common. These minerals combine to produce generally dark colored ultramafic rocks such as peridotite, the dominate group of rocks in the upper mantle. Under the higher pressure conditions deeper in the mantle similar chemical components combine to produce dense minerals with tightly packed crystal structures. These high‐pressure minerals are produced by transformations that are largely indicated by changes high pressure in seismic wave velocity, which reveal that the mantle contains a number of sublayers (Figure 1.2) as discussed below.
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.
A discrete low velocity zone (LVZ) occurs within most areas of the upper mantle at depths of ~100–250 km below the surface. The top of low velocity zone marks the contact between the strong lithosphere and the underlying, weak asthenosphere (Figure 1.3). The asthenosphere is