The oceanic crust

The oceanic crust (Francheteau, 1983) is in isostatic equilibrium with the continental crust according to the Airy mechanism (Section 2.11.2), and is consequently much thinner. Seismic refraction studies have confirmed this and show that oceanic crust is typically 67 km thick beneath an average water depth of 4.5 km. Thicker oceanic crust occurs where the magma supply rate is anomalously high due to higher than normal temperatures in the upper mantle. Conversely, thinner than normal crust forms where upper mantle temperatures are anomalously low, typically because of a very low rate of formation (Section 6.10).

Table 2.2 Oceanic crustal structure (after Bott, 1982).

P velocity (km s-1)

Average thickness (km)

Water

1.5

4.5

Layer 1

1.6-2.5

0.4

Layer 2

3.4-6.2

1.4

Layer 3

6.4-7.0

5.0

Moho

Upper mantle

7.4-8.6

The earliest refraction surveys produced time-distance data of relatively low accuracy that, on simple inversion using plane-layered models, indicated the presence of three principal layers. The velocities and thicknesses of these layers are shown in Table 2.2. More recent refraction studies, employing much more sophisticated equipment and interpretational procedures (Kennett B.L.N., 1977), have shown that further subdivision of the main layers is possible (Harrison & Bonatti, 1981) and that, rather than a structure in which velocities increase downwards in discrete jumps, there appears to be a progressive velocity increase with depth (Kennett & Orcutt, 1976; Spudich & Orcutt, 1980). Figure 2.17 compares the velocity structure of the oceanic crust as determined by early and more recent investigations.

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How To Have A Perfect Boating Experience

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