The Moho as an Isobar
© Charles Chandler
I'm not satisfied that the Moho is an isobar. Saying that the oceanic crust is more dense (i.e., 2.9 g/cm3), and therefore is capable of producing the same isobar at a depth of 11 km (i.e., 4 km of water and 7 km of crust), compared to the continental crust (i.e., 2.7 g/cm3), which is so light that the isobar occurs at 35+ km, just isn't correct.
What if it's the 900 K isotherm, which is nearer the surface under the oceanic crust due to lesser thermal conductivity? It might still be the conduit for lateral telluric currents driven by tidal forces. The electric conductivity goes up sharply in the transition from solid to liquid. So as soon as this threshold is hit, electric currents become possible. And with the electric currents comes resistive heating.
But then where is the boundary for EDP? And what is the significance of it occuring at a depth that is different from the electric conductivity of the Moho? If EDP is occurring lower than the Moho, we would expect major currents through the molten magma, since everything below the Moho is molten, (?) and therefore is a good conductor. But if EDP occurs above the Moho, the currents will go through fractures in the solid rock, producing earthquakes and volcanoes.
If EDP and the Moho are not the same, it would make sense that some parts of the mantle are rigid. In other words, the Moho does not define the boundary between the rigid lithosphere and the plastic asthenosphere. EDP does, because where electrons have been expelled, the crystal lattice is weaker, and the rock can flow. But then there can be solid rock below the Moho, or it wouldn't be brittle. So the Moho is molten. In the oceanic crust, the Moho defines the transition between the lithosphere and the asthenosphere. And that's nearer to the surface. But in the continental crust, the lithosphere extends below the Moho.
Clearly, for there to be rigid lithosphere below the Moho, where the temperature in the Moho is sufficient for partial melting, the temperature cannot continue to go up with depth, or everything below the Moho would be molten, and it wouldn't have any rigidity. Therefore, the temperature goes down below the Moho, and only gets back up to the temperature necessary for partial melting in the asthenosphere. That's if the plasticity of the asthenosphere is even attributable to partial melting. It might be due to ionization.
So is the lithosphere~asthenosphere boundary an isobar? It doesn't look like it. With depths (apparently) varying from 5 to 250 km (if that's what it actually is), the bottom of the lithosphere cannot possibly be an isobar.
So they're saying that the lithosphere is a chemical differentiation. If the levels, depths, and densities check out, that's a reasonable explanation. And all of my isobar/isotherm stuff needs to be reworked. But then it also makes sense that the Moho doesn't follow the lithosphere, because it IS some sort of condition in whatever medium is there. I just don't understand why it would be deeper under the continental crust.

The Crust and Lithosphere


The Earth's tectonic plates constitute the lithosphere so no proper understanding of plate tectonics can be achieved without reference to the lithosphere, and this requires an understanding of its essential difference from the crust.

There are incorrect uses of both terms in text books - particularly common is the use of 'crustal', as opposed to 'lithospheric' plates - and these have contributed to widespread confusion and misunderstanding. The problem that teachers and, for that matter, authors of school text books have to face up to is that geologists need to employ two different concepts of layering within the outer part of the Earth to understand and explain geological processes - compositional layering (crust, mantle), and mechanical layering (lithosphere, asthenosphere).

What is the difference between the crust and lithosphere?

The crust (whether continental or oceanic) is the thin layer of distinctive chemical composition overlying the ultramafic upper mantle. The base of the crust is defined seismologically by the Mohorovicic discontinuity, or Moho. Oceanic and continental crust are formed by entirely different geological processes: the former is typically 6 - 7 km thick, the latter about 35 - 40 km.

The lithosphere is the rigid outer layer of the Earth required by plate tectonic theory. It differs from the underlying asthenosphere in terms of its mechanical (or rheological, ie, 'flow') properties rather than its chemical composition. Under the influence of the low-intensity, long-term stresses that drive plate tectonic motions, the lithosphere responds essentially as a rigid shell whilst the asthenosphere behaves as a highly viscous fluid.

The weaker mechanical properties of the asthenosphere are attributable to the fact that, within this part of the upper mantle, temperatures lie close to the melting temperature (with localised partial melting giving rise to magma generation). The base of the lithosphere is conventionally defined as the 1300 C isotherm since mantle rocks below this temperature are sufficiently cool to behave in a rigid manner.

The lithosphere includes the crust (whether continental or oceanic) and the uppermost part of the upper mantle. It thins to a few kilometres at ocean spreading centres, thickens to about 100 - 150 km under the older parts of ocean basins, and is up to 250 - 300 km thick under continental shield areas. Hence, whilst the crust is an integral part of the lithosphere, the lithosphere is mainly composed of mantle rocks. This is why authors sometimes state, loosely, that the lithosphere is the uppermost part of the mantle - they are choosing to disregard the thin veneer of crustal rocks.

Seismological evidence for the lithosphere and asthenosphere

The linear magnetic anomaly patterns in ocean basins were recognised in the early 1960s to be evidence for sea floor spreading and this paved the way for the development of plate tectonic theory, superseding the earlier theory of continental drift. The new theory clarified the requirement that there must be an outer rigid layer to the Earth (the lithosphere) decoupled from an underlying layer of lower strength (the asthenosphere).

The hypothesis that the Earth has an asthenosphere can be tested by searching experimentally for a layer with physical properties attributable to its low strength. Since the shear modulus of a material reduces as its melting temperature is approached the asthenosphere should retard the passage of earthquake S-waves, whose velocity is directly proportional to the shear modulus of the material through which it is travelling. The presence of a seismological low velocity layer (LVL) or zone (LVZ) near the top of the mantle thus provides evidence for the asthenosphere. The evidence is particularly convincing since S-waves, which are more sensitive to the prevailing shear modulus than P-waves, are slowed down to a greater extent than the latter. The low velocity zone is much better developed under ocean basins than under continental shield areas where it sometimes barely developed. Hence, oceanic lithosphere is much better defined seismologically than continental lithosphere.

Velocity-depth profiles through the Earth's upper mantle do not define the top and bottom of the zones of rigid and viscous behaviour precisely, however, because the zones must have transitional boundaries.
What if the Moho prefers the oceanic surface, because of its higher electric conductivity?
There is something that is fundamentally wrong with the whole concept of the Earth having an internal heat source, such as in the core, and that the temperature continues to increase with depth. It's that magma has a thermal expansion coefficient, wherein hotter magma is less dense, and therefore lighter. This precludes the formation of a solid crust at the top, no matter the temperature at the surface, and no matter the diffusivity. The reason is that there is nothing to support solids above less dense liquids. So the solids should sink, and the liquids should rise. If the crust is going to freeze, the whole thing has to freeze from the inside out. In fact, the density of the lithosphere is actually less than the density of the asthenosphere, proving chemical differentiation between the two.
The chemical differentiation in the crust can only be attributed to ET impacts, if the planet was once entirely molten. The reason is that mass separation in a fluid happens very rapidly, and even on the scale of the Earth, the sorting should have been done in less than a couple hundred years.

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