Geoscience Reference
In-Depth Information
Figure 9.8.
Principal ophiolite belts of the world. (Based on Coleman (1977).)
by the fact that the depth and temperature of melting in fast- and slow-spreading
ridges are similar, which means that the extent of melting is similar (Section 9.3.2).
It has been suggested that the oceanic crust gradually thickens slightly with age,
but there are insufficient results from seismic experiments to confirm this. The
Pacific oceanic crust is similar to, but rather more uniform than, the Atlantic
crust, due to its having a faster and more constant magma-supply rate. The very or
ultra-slow-spreading mid-ocean ridges (e.g., the Arctic Gakkel Ridge, half-rates
0.3-0.75 cm yr
−
1
; and the Southwest Indian Ridge, half-rate 0.57-0.85 cm yr
−
1
)
produce much thinner crust than normal as a result of there being less melting in
the mantle. To understand the structure and composition of the oceanic crust, it
is necessary to determine the structure of the mid-ocean ridges and the processes
occurring there where the crust is formed (Sections 9.3 and 9.4).
←−
distances less than about 6 km, is the reflection from the seabed. (From NAT Study
Group 1985.) (b) The reduced record section of a refraction profile shot over 140 Ma
oceanic crust in the western central Atlantic Ocean on the North American Plate. The
seismograms have been low-pass filtered at 15 Hz, corrected for varying sizes of
explosive charge and plottend with a gain that increases linearly with distance. P
3
and S
3
are the crustal refractions, P
m
P and S
m
S the reflections from the crust-mantle
boundary and P
n
and S
n
the upper-mantle refractions. The S phases are converted
shear waves (converted from P to S and then back from S to P at the sediment-crust
interface beneath the shot and receiver, respectively). (From Purdy (1983).)