Part 1
The oceans


Chapter 1
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 [age]. 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:
Tz-Tm
Ts-Tm
= erfc



z

k t
 




(1)
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:

erfc



z

k t
 




0
and Tz approaches Tm. As z tends to 0 or t to then
erfc



z

k t
 




1
and Tz approaches Ts, providing




z

k t
 




< 2

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.