O

Figure 2.8. Pattern of global heat flux variations complete to spherical harmonic degree 12. After Pollack et al. (1993).

For a color version of this figure, see plate section.

0* 30' 60' 90' 120' 150' 160* -150' -120' -B0" -60' -30' 0'

0* 30' 60' 90' 120' 150' 160* -150' -120' -B0" -60' -30' 0'

0' 30' 60" 90' 120' 150" ISO' -150' -120" -90' -60' -30* 0"

Figure 2.9. Global distribution of volcanoes active in the Quaternary.

0' 30' 60" 90' 120' 150" ISO' -150' -120" -90' -60' -30* 0"

Figure 2.9. Global distribution of volcanoes active in the Quaternary.

Volcanic Line

Volcanic Line

Figure 2.10. Accretion of a lithospheric plate at an ocean ridge (accretional plate margin) and its subduction at an ocean trench (subduction zone). The asthenosphere, which lies beneath the lithosphere, and the volcanic line above the subducting lithosphere are also shown. The plate migrates away from the ridge crest at the spreading velocity u. Since there can be relative motion between the ocean ridge and ocean trench, the velocity of subduction can, in general, be greater or less than u.

Figure 2.10. Accretion of a lithospheric plate at an ocean ridge (accretional plate margin) and its subduction at an ocean trench (subduction zone). The asthenosphere, which lies beneath the lithosphere, and the volcanic line above the subducting lithosphere are also shown. The plate migrates away from the ridge crest at the spreading velocity u. Since there can be relative motion between the ocean ridge and ocean trench, the velocity of subduction can, in general, be greater or less than u.

contraction. As a result, the lithosphere becomes gravitationally unstable with respect to the warmer asthenosphere beneath. At an ocean trench, the lithosphere bends and sinks into the interior of the Earth because of its negative buoyancy.

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