Environmental Engineering Reference
In-Depth Information
MID-OCEAN RIDGES
The
thermal welt
or dome over a rising mantle plume stretches, thins and weakens the
lithosphere. The consequent fall in
overburden pressure
lowers the melting point of
asthenosphere, which rises faster than it cools. Rock-forming processes are dealt with in
detail in Chapter 12. It is sufficient here to appreciate that the fractionation of different
rocks is most intense in the lithosphere and that a more buoyant
gabbro
-basalt mixture
segregates and accelerates away from its denser, parent asthenosphere
peridotite
at
depths of 15-25 km to form subsurface magma reservoirs. Magma creates new layered
oceanic crust where it penetrates the lithosphere and inevitably leaves behind
depleted
peridotite. Gabbro cools to form a subsurface
intrusive
layer 4-6 km thick, whilst the
basalt continues to the surface, forming an
extrusive
layer of lava 1-2·5 km thick.
Volcanic activity is also associated with mid-ocean ridges and is seen best where the
ridge surfaces in the Atlantic Ocean at the volcanic islands of Iceland, the Azores,
Ascension and Tristan da Cunha.
The focus of this activity forms a topographic rise or
ridge
1-3 km high in the sea bed
(Figure 10.6). Extension faulting along its axis heart triggers shallow seismic activity and
may create a central rift in slow-spreading ridges. Since the asthenosphere feeds the
magma reservoir and thereby the continuous formation of oceanic lithosphere, during the
lifetime of the cycle the ridge system eventually achieves widths of 10
3-4
km. Why do we
speak of oceanic crust and mid-ocean ridges when the process starts by continental
rifting? Heat accumulation takes
Figure 10.6
Vertical section through a mid-ocean ridge,
drawn at right-angles to the ridge axis.
Source: After Kearey and Vine (1996).
30-80 Ma to reach the point of rifting, during which time light continental lithosphere is
replaced by rising, denser asthenosphere in the plume. Isostatic adjustments cause this
new crust to 'float' at a lower level on the mantle as the continent rifts apart, flooding
