Geoscience Reference
In-Depth Information
Figure 9.17.
Schematic
geological cross sections
of the axis of the
Mid-Atlantic Ridge. The
MARK cross sections are
at intervals of about 15 km
along the axis at 23
◦
N,
with MARK 2 being the
most northerly and MARK
5 the most southerly.
FAMOUS is at 37
◦
N,
AMAR at 36
◦
N and TAG at
26
◦
N. Arrows indicate the
ridge axis. The vertical
exaggeration is 3 : 1.
Depths beneath the sea
surface are in kilometres.
(From Karson
et al
.
(1987).)
teleseismic distances from the Gakkel Ridge in the Arctic Ocean in 1999. Over
200 earthquakes were recorded over a 7-month period, a sudden and huge increase
on the seismic activity recorded over previous decades (Fig. 9.18, Plate 19). Ini-
tially earthquakes had normal-faulting mechanisms, as is usual for an extensional
tectonic environment. Later the activity changed to focal mechanisms of a non-
double-couple type typical of caldera-collapse volcanic activity. It is possible that
the earthquakes were caused by a major dyke propagating along the ridge axis
through the cold, brittle crust. Subsequent side-scan sonar images confirmed the
presence of fresh lava flows. This is quite unlike seismic activity recorded from
slow-spreading ridges, for which earthquakes occurring in swarms are generally
too small to be detected teleseismically, are far fewer in number and have a shorter
