Page 62 - Global Tectonics

THE INTERIOR OF THE EARTH 49

Figure 2.35 Section from San Francisco, California to Lamar, Colorado based on seismic refraction data (redrawn
from Pakiser, 1963, by permission of the American Geophysical Union. Copyright © 1963 American Geophysical Union).

that deforms by flow. This concept has assumed fun- 2000
damental importance since it was realized that the sub-
divisions of the Earth controlling plate tectonic 0.05% H 2 O
1600 Dry mantle solidus 0.1% H 2 O Mantle adiabat
movements must be based on rheology, rather than
composition.

The lithosphere is defined as the strong, outermost 1200
layer of the Earth that deforms in an essentially elastic Temperature (°C) A B C D
manner. It is made up of the crust and uppermost 800
mantle. The lithosphere is underlain by the astheno-
sphere, which is a much weaker layer and reacts to stress

in a fluid manner. The lithosphere is divided into plates, 400
of which the crustal component can be oceanic and/or
continental, and the relative movements of plates take 100 200 300 400 500
place upon the asthenosphere. Depth (km)
However, having made these relatively simple defi ni-
Figure 2.36 Variation of temperature with depth
tions, examination of the several properties that might
beneath continental and oceanic regions. A, ocean ridge;
be expected to characterize these layers reveals that they
B, ocean basin; C, continental platform; D, Archean
lead to different ideas of their thickness. The properties
Shield (redrawn from Condie, 2005b, with permission
considered are thermal, seismic, elastic, seismogenic,
from Elsevier Academic Press).
and temporal.
Temperature is believed to be the main phenome-
non that controls the strength of subsurface material.
Hydrostatic pressure increases with depth in an almost et al., 1973). Beneath ocean ridges, where temperature
linear manner, and so the melting point of rocks also gradients are high, the asthenosphere must occur at
increases with depth. Melting will occur when the tem- shallow depth. Indeed, since it is actually created in the
perature curve intersects the melting curve (solidus) for crestal region (Section 6.10), the lithosphere there is
the material present at depth (Fig. 2.36). The astheno- particularly thin. The gradient decreases towards the
sphere is believed to represent the location in the mantle deep ocean basins, and the lithosphere thickens in this
where the melting point is most closely approached. direction, the increase correlating with the depth of
This layer is certainly not completely molten, as it trans- water as the lithosphere subsides as a result of contrac-
mits S waves, but it is possible that a small amount of tion on cooling (Section 6.4). The mean lithosphere
melt is present. The depth at which the asthenosphere thickness on this basis beneath oceans is probably 60–
occurs depends upon the geothermal gradient and the 70 km. Beneath continents a substantial portion of the

melting temperature of the mantle materials (Le Pichon observed heat flow is produced within the crust (Section