Geoscience Reference
In-Depth Information
Figure 9.24.
Compressional waves
from a distant earthquake
are well transmitted to an
ocean-bottom
seismometer A, situated
on the East Pacific Rise
axis, but the shear waves
are attenuated. Both
compressional and shear
waves are well
transmitted to a
seismometer B, 10 km
from the ridge axis. These
observations suggest that
a narrow magma
chamber is present
beneath the ridge axis.
(From The crest of the
East Pacific Rise,
Macdonald and Luyendyk.
Copyright
c
1981 by
Scientific American, Inc.
All rights reserved.)
Figure 9.25.
A schematic
diagram of the northern
East Pacific Rise. The
checker- board pattern
beneath the axial zone
represents melt. Note the
difference between
on-axis eruptions of
basalt and off-axis
volcanism of pillow lavas
from fissures and faults.
The off-axis volcanism
tends to be more enriched
(E MORB) than the on-axis
volcanism (N MORB) and
is fed by separate
conduits. (From Perfit
et al
.(1994).)
