Geoscience Reference
In-Depth Information
is a mirror image of that at the ridge. Midway
between the ridge and the trench (M), the temper-
ature increases rapidly near the top and bottom
of the cell. Thus, the velocities decrease rapidly
in these regions.
The crystal orientation, if alignment with
flow is assumed, is with the shortest and slow-
est axis (
b
axis) perpendicular to the flow. Thus,
at the ridge and the trench where the flow is
near vertical, the
b
axis is horizontal, and mid-
way between where the flow is horizontal, the
b
axis is vertical. VPH and VSH between the upward
and downward flowing edges of the convection
cell are controlled by the velocities along the
a
axis and
c
axis. Thus, at midpoint, and wher-
ever flow is horizontal, SH
Fig. 20.8
S
velocity in the upper mantle along the
cross-section shown. Note low velocities at shallow depth
under the western Pacific, replaced by high velocities at
greater depth. The eastern Pacific is slow at all depths. The
Atlantic is fast below 400 km.
relation between velocity and depth. At the
ridge the temperature increases very rapidly with
depth near the surface; thus, the effects of tem-
perature dominate over those of pressure, and
velocities decrease. Deeper levels under the ridge
are almost isothermal; thus, the effect of pres-
sure dominates, and the velocities increase. At
the trench the temperature gradient is large near
the base of the cell and nearly isothermal at shal-
lower depths. Therefore, the velocity response
>
SV and PH
>
PV.
