Geoscience Reference
In-Depth Information
Figure 9.16.
The gradient
of the mantle Bouguer
anomaly (MBA) and the
axial topographic relief
plotted against the
half-spreading rate. The
major difference between
ridges with a median
valley and those without
suggests that an axial
high is indicative of
efficient transport of melt
along the ridge axis.
(After Wang and Cochran
(1995).)
abruptly at 58
◦
40
N. On the East Pacific Rise there is an axial high where a
magma chamber is present but where no magma chamber is evident (at 23
◦
N),
there is instead an 18-km-wide, 1-km-deep axial valley.
Side-scan sonar and swath bathymetry yield detailed images of the fabric of the
seabed (Figs. 9.15(b), 9.19 and 9.31 and Plates 18, 20 and 21). On slow-spreading
ridges the median valley generally has a 5-12-km-wide inner valley floor within
which the volcanic activity takes place. Within each segment the width and depth
of the median valley is thermally/magmatically controlled, with the valley being
shallower and narrower at segment centres where the magma supply is greatest.
The magma supply may be a direct feature of the upwelling process or it may be
that melt migrates laterally along segments from the centre. Often small individual
volcanic eruptions build a linear volcanic ridge, an axial volcanic ridge (AVR).
