Geoscience Reference
In-Depth Information
The crust in the vicinity of Iceland, which is situated over a plume, is much
thicker than normal. The crustal thickness beneath central Iceland averages some
35 km, decreasing to
20 km on the Reykjanes Peninsula in southwest Iceland
and 10-13 km beneath the Reykjanes Ridge. In order to generate this amount
of melt, the potential temperature of the mantle must be considerably higher
than normal, up to about 1480
◦
C beneath Iceland and 1360-1400
◦
C beneath the
Reykjanes Ridge.
∼
9.4 The shallow structure of mid-ocean ridges
9.4.1 Topography
The axial topography of mid-ocean ridges varies widely (Figs. 9.1 and 9.15).
The topography and the formation of the oceanic crust are controlled by the
magmatic supply, tectonic strain and cooling by hydrothermal circulation but the
spreading rate is the most important overall control on the topography. The crest
of the slow-spreading Mid-Atlantic Ridge is very rugged, with faulted blocks
and a narrow axial
median valley
(20-30 km wide and 1-2 km deep overall). The
very-slow-spreading Southwest Indian Ridge is also very rugged, in places the
median valley is
2kmdeep, with the bathymetric depth at the axis over 4 km (c.f.
Eq. (7.72)). The slowest-spreading ridge, the Gakkel Ridge, is very rugged, with
large scarps along a 2-km-deep continuous median valley and a bathymetric depth
at the axis of
>
∼
5 km. In contrast, the axial regions of the faster-spreading Pacific-
Antarctic Ridge and the East Pacific Rise are much smoother and generally lack
the median valley, having instead an axial high some 1-2 km wide. Globally
the changeover from median valley to no valley is at half-spreading rates of 3-
4 cm/yr
−
1
(Fig. 9.16). However, the presence of a median valley is not controlled
purely by the spreading rate, with slow-spreading ridges having median valleys
and fast-spreading ridges lacking them: there are exceptions. For example, much
of the slow-spreading Reykjanes Ridge does not have a median valley, and in
some locations the fast-spreading East Pacific Rise has a shallow axial valley.
These differences in axial topography can be modelled by the ductile (i.e., non-
brittle) extension of a viscous or plastic lithosphere, which thickens with distance
from the ridge axis, as well as the effect of accumulation of magma beneath the
plate. For slow-spreading ridges, the depth of the rift valley primarily depends
on the strength of the model lithosphere and the rate at which it thickens. On
fast-spreading ridges the lithosphere is weaker and cannot support significant
stress. This means that the seabed acts as a free surface and the topography is
due mainly to the buoyancy of the magma. The Reykjanes Ridge, even though
it is a slow-spreading ridge, does not have a median valley because the overall
effect of the hotter underlying mantle close to the Iceland plume means that the
lithosphere is hotter and hence weaker than normal. On the Reykjanes Ridge the
change from a 700-m-deep median valley in the south to an axial high happens
