The ocean lithosphere
The age of the oceanic crust, as reflected in the magnetic striping of the ocean
floor, increases with distance away from the mid-ocean ridges, indicating that the
ridges are the site of generation of new oceanic crust. The volcanic rocks extruded at
the surface of the ridges are exclusively basalt (mid-ocean ridge basalt or MORB)
which, together with there sub-volcanic intrusive equivalents - gabbros and sheeted
dykes, comprise the entire oceanic crust. The total thickness of the oceanic crust
generated by mafic igneous activity at the ridges is typically about
5-7 km. The structure of oceanic crust and parts of the subcrustal lithosphere can be directly
observed in some ancient orogenic belts where fragments of the oceanic lithosphere have been
obducted to form ophiolites during collision processes, for example the Semail
ophiolite in Oman.
Figure 1: Schematic structure of the ocean lithosphere. The ocean lithosphere
consists of about 6-7 km thick crust (heavy stiple), and the mantle lithosphere
(intermediate stiple) which thickens with age (and hence distance away from the ridge as
indicated by arrows). Light stiple show the region of decompression melting beneath the
ridge. Stream lines in the asthenosphere may be largely decoupled from the motion of the
overlying lithosphere (see Section 5.5), although asthenosphere must undergo
decompression immediately beneath the ridge.
1 Age, bathymetry and heatflow
In ocean lithosphere younger than about 80 Ma there is a remarkable correspondence
between age of the ocean crust, the depth to the sea floor (bathymetry) and the heat flow
through the lithosphere (Fig. 1); with bathymetric depth increasing, and
surface heatflow decreasing, with the Ö[Êage]. This correspondence between age,
bathymetry and heatflow is due to time dependent changes in the thickness of the
lithosphere. Two models for the ocean lithosphere have been proposed in order to account
for this relation: the half-space model and the thermal plate model.
1.0.1 The half-space model
The cooling of ocean lithosphere after formation at a ridge can be treated as a thermal
conduction problem (see Chapter 3) in which a non-steady state condition (the situation
at the ridge) gradually decays towards a thermally equilibrated state (as the ocean
lithOSphere slides away from the ridge). The analogy (Fig a) can be made
with the cooling of a semi-infinite half space, which is given by:
where Tz is the temperature at depth z, Ts is the
temperature at the surface interface of the semi-infinite half space (which in this case
is the temperature of ocean water and is taken to be 0oC), Tm is the
temperature of the half space in the initial condition and which is
maintained at infinite distance for all time (in our case the temperature
of the deep mantle, 1280oC), k is the thermal diffusivity
and t is time (the error function, erf, and its compliment, erfc,
arise commonly in analytical solutions to the heat equation and related differential equations which
employ similarity variables).
The behaviour of the error function, and hence Eqn 1, is illustrated in
Figure b. As z tends to ¥ or t to 0 then:
and Tz approaches Tm. As z tends to 0 or t to ¥
and Tz approaches Ts, providing
Figure 2: Schematic thermal structure of the ocean lithosphere treated as a problem of the cooling
of a semi-infinite half space. (a) shows the thermal structure at the
ridge (t0) where asthenosphere at temperature Tm is juxtaposed with ocean
waters at temperature Ts. The thermal structure at successive distances away
from the ridge where cooling of the initial temperature discontinuity in the
semi infinite half space has lead to thickening of the ocean lithosphere is
shown by the curves t1 and t2. (b) shows the error function
(erf) and complimentary error function (erfc = 1 - erf).
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On 7 Oct 2000, 11:27.