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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 more plastic than the lithosphere and flows slowly, rather than rupturing, when subjected to stress. The anomalously low rigidity of the LVZ has been explained by small amounts of partial melting (Anderson et al. 1971). This is supported by laboratory studies that suggest peridotite should be very near its melting temperature at these depths due to the high temperature. This is especially likely if it contains small amounts of water or water‐bearing minerals. Below the base of the low velocity zone (250–410 km), seismic wave velocities increase (Figure 1.2) indicating that the materials are more rigid solids. These materials are still part of the relatively weak asthenosphere which extends to the base of the transition zone at 660 km.

Seismic discontinuities marked by increases in seismic velocity occur within the upper mantle at depths of ~410 and ~660 km (Figure 1.2). This interval (~410–660 km) is called the transition zone between the upper and lower mantle. The sudden jumps in seismic velocity record sudden increases in rigidity and incompressibility. Laboratory studies suggest that the minerals in peridotite undergo transformations into new minerals at these depths. At approximately 410 km depth (pressures of ~14 GPa), olivine (Mg₂SiO₄) is transformed into more rigid, incompressible beta spinel (β‐spinel), also known as wadleysite (Mg₂SiO₄). Within the transition zone, wadleysite is transformed into the higher pressure mineral ringwoodite (Mg₂SiO₄). At approximately 660 km depth (~24 GPa), ringwoodite and garnet are converted to very rigid, incompressible perovskite [(Mg,Fe,Al)SiO₃], also known as bridgmanite (Tschauner et al. 2014) and oxide phases such as periclase (MgO). The mineral phase changes from olivine to wadleysite and from ringwoodite to perovskite are inferred to be largely responsible for the seismic wave velocity changes that occur at 410 and 660 km respectively (Ringwood 1975; Condie 1982; Anderson 1989). Inversions of pyroxene to garnet and garnet to minerals with ilmenite and perovskite structures may also be involved. The base of the transition zone at 660 km marks the base of the asthenosphere in contact with the underlying mesosphere or lower mantle (Figure 1.2).

Figure 1.2 Major layers and seismic (p‐wave) velocity changes within Earth; showing details of upper mantle layers. Colors are as for Figure 1.1.

Figure 1.3 World map showing the distribution of major plates separated by boundary segments that end in triple junctions.

Source: From USGS.