Science Heresy - September 2010
The Mantle Instability
The Internal Temperature of an Ideal Planet
The diagram above shows the temperature inside an idealised planet which is being uniformly heated internally by radioactivity and in which the thermal conductivity is uniform. Heat is conducted to the cool outside of the planet where it is radiated into space. The temperature inside the planet decreases according to the square of the distance from the centre and the temperature curve is an inverted parabola (Curve 1).
If there is high enough radioactivity or low enough conductivity, the temperature curve (Curve 2) will intersect the "solidus", i.e. the temperature-pressure boundary beyond which the rock becomes liquid (Curve 3). There will be a band or "spherical shell" of liquid magma between the two intersection points a shown in the diagram on the left.
The Distorting Effect of Convection
When the solid rock turns into liquid magma, convection comes into play which means that more heat is transported than would otherwise be the case. This causes the inside of the liquid shell to be cooled and the outside to be heated more than would be the case if conduction were operating alone. This is the reason for the little blips at "a" and "b" in the top diagram.
Thus the deep end of the convection current freezes into rock and increased melting occurs at the shallow end.
This extra freezing and melting means that the liquid carrying the convection current is unstable. It will migrate outwards toward the surface of the planet and the spherical shell may break up into upwardly migrating liquid blobs as shown on the left.
When a Blob Arrives at the Surface
When an upwardly migrating blob comes to the surface it forces the crust upwards and causes it to crack open. The blob is hotter than the surrounding rock and therefore lighter which is why it bulges upward.
The triangle of forces shown in the diagram on the left indicates that there will be a net horizontal force pushing the crust away from the convective blob. The crust subsides by sliding down the sides of the blob.
This is the origin of the seafloor spread away from mid-ocean ridges and any other crustal drift. There is no need to postulate convection currents underneath the plates themselves dragging the crust along.
The Mid-Oceanic Ridges
In reality the heat production rate and thermal conductivity will not be perfectly uniform nor even radially symmetrical. Nevertheless all that is required for a phase instability such as that described here above to occur is that there are regions in the interior of a planet in which the temperature curve intersects the solidus. Planets or planetoids in which this does not occur, such as the Moon or Mars, are tectonically quiet.
In the general case convective blobs will not ascend radially but follow the temperature gradient. Consequently they will tend to be deflected away from continental shields and towards thin oceanic crust where cooling is greatest. Hence convective blobs tend to break the crust and create bulges in mid-ocean forming mid-ocean ridges (MORs).
A false colour map of an MOR called the East Pacific Rise is shown on the left (red is shallower, blue is deeper - image due to NOAA). A map of the Earth's MORs and volcanic hotspots is shown below.
Note that the mechanism described here precludes the existence of "steady-state" convection currents, and that this resolves the longstanding paradox of the postulated "flows" of The Standard Model.
Convective liquid blobs ascend from the solid interior of the Earth in a quasi-random manner, rather like bubbles of vapour in a boiling saucepan only slowly, over geological time scales, consistent with the "random walk" explanation of Ice Ages in the "What Caused the Ice Ages?" article of this September issue.
The map below should be considered only as a snapshot of tectonic activity as it is occurring in the present day. Note its quasi-randomness.
Volcanic Hot Spots and Mid-Oceanic Ridges
Draft Paper on Mantle Instabilities (776 kB)