Geoscience Reference
In-Depth Information
Figure 10.3. Ranges of
laboratory measurements
of the P-wave velocity in
various rock types. (From
data in Press (1966).)
low velocities in the middle crust beneath rift zones are probably due to ele-
vated temperatures. Velocities increase with composition on going from felsic
to mafic to ultramafic; elevated temperatures cause a reduction in velocity and
high-grade metamorphism increases velocities. Thus velocities of 6.6-6.8 km s 1
in the lower crust may be typical of regions in which arc magmatism has been the
dominant mechanism of continental growth, whereas velocities of 7.0-7.2 km s 1
in the lower crust may be typical of regions in which mafic/ultramafic magmatic
underplating and/or high-grade metamorphism has dominated.
Figure 10.3 shows the ranges of laboratory measurements of the P-wave
velocity for various rock types. For example, not every basalt has a velocity of
6.0 km s 1 ,but velocities in the range 5.1-6.4 km s 1 are reasonable for basalts.
Laboratory measurements show that the P-wave velocity increases with pressure.
However, this does not necessarily mean that the P-wave velocity for a given rock
unit will increase with depth in the crust. The increase of temperature with depth
can either counteract or enhance the effect of increasing pressure, depending on
the physical properties (e.g., pores and fissures) of the particular rock unit.
The Moho is in some places observed to be a velocity gradient, in other places
it is a sharp boundary and in still others it is a thin laminated zone. The thickness
of this transition from crust to mantle can be estimated from the wavelength of
the seismic signals. Two kilometres is probably a maximum estimate of the thick-
ness of the transition. Geologically, however, the Moho represents the boundary
between the lower-crustal granulites and the ultrabasic upper mantle, which is
predominantly olivine and pyroxene.
The gross structure of the continental crust as determined from surface waves
was discussed in Section 4.1.3 and illustrated in Fig. 4.6;wefound the continental
lithosphere to be thicker than the oceanic lithosphere. The thermal structure of
the continents was discussed in Section 7.6, and we concluded that the oceanic
geotherms for lithosphere older than 70 Ma can equally well be applied to the
continental lithosphere (see Figs. 7.14 and 7.15).
10.1.3 The composition of the continental crust
The continental crust has been formed from mantle material over the lifetime
of the Earth by a series of melting, crystallization, metamorphic, erosional,
Search WWH ::




Custom Search