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在这里,我们给核—幔边界层的热状态提出了一个定性的物理论说,设想出一个线性超绝热的温度分布,研究了地幔底部D“层的对流不稳定性。边界层的稳定性主要由核—幔边界上的地幔粘滞度ηmc的数值来确定;当ηmc是～10~(22)p时,边界层是稳定的,而当ηmc是～10~(20)p时,该层则是不稳定的。借助于经过修正的关于上升热流在热的边界层上形成的Howard理论,可以估计热的不稳定层的形成时间(τ_D”)。这个时间等于～10~8年,该层的宽度为～10~2km。因此,许多部分的过热流体由于不稳定性而离开不稳定层,其特征宽度也是～10~2km。按照我们理论的一般结论,我们提出:以其不均匀的特征尺度为～(10—100)km的不稳定层之特殊热结构,去阐明D“层的一切异常性质,即平稳的或折合的地震速度、折合力学性质及力学不均匀性(地震波散射的原因)。 D”层的热状态不是稳定的,当热由传导迁移时,相对平静的长周期由于地幔底部形成局部不稳定的热层(或热点)而告终,然后是不稳定的热层的很短的衰变期,接着而来的又是下一次的长久的传导期,如此循环不已。
Here, we present a qualitative physical theory of the thermal state of the nuclear-mantle boundary layer, and assume a linear super-adiabatic temperature distribution and study the convective instability of the D “layer at the bottom of the mantle. The stability of the boundary layer It is mainly determined by the value of ηmc in the mantle at the core-mantle boundary. The boundary layer is stable when ηmc is ~ 10 ~ (22) p, and when ηmc is ~ 10 ~ (20) p, This layer is unstable, with the help of the modified Howard theory on the hot boundary layer with respect to ascending heat flow, the thermal instability layer formation time (τ_D ”) can be estimated. This time is equal to ~ 10 ~ 8 years and the width of this layer is ~ 10 ~ 2km. As a result, many parts of the superheated fluid leave the unstable layer due to instability, and its characteristic width is ~ 10 ~ 2 km. According to the general conclusion of our theory, we propose that all the anomalous properties of the D "layer, ie, the stable or the equivalent, are given by the special thermal structure of the unstable layer with its inhomogeneous characteristic scale of ~ (10-100) km The seismic velocity, mechanical properties and mechanical inhomogeneities (the cause of seismic wave scattering) are not stable, and when the heat is transferred by conduction, the relatively quiet long period is locally unstable due to the bottom of the mantle The hot layer (or hot spot) ends up with a very short decay of the unstable thermal layer, followed by the next long conduction period, so endless.