Global Tectonics (part 1)
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth. Strictly speaking, it would also involve the study of the lithosphere of other planets, but for the purposes of this study, we will restrict ourselves to the Earth.
The currently accepted theory of how the lithosphere behaves is plate tectonics. There are plenty of excellent books available that recount the history of plate tectonics, and explain how scientists believe it works. It is not my intention to duplicate these books, and certainly not to try to improve on them. Rather, I propose to look at the whole question of the lithosphere and to ask questions such as: Why is the concept of plate tectonics so widely accepted? and: are there any reasonably credible alternative theories?
There is a tendency among geologists to consider proposed alternatives to plate tectonics as the work of crackpots, but one must not lose sight of the fact that when Alfred Wegener proposed the theory of Continental Drift, geological academia in general condemned it as a singularly crackpot idea. However, a few decades and a change of name turned it into the leading contender for an explanation of the way in which the world works. Apart from the fact that Wegener was a meteorologist and therefore had no right to be telling geologists their business; there was the problem of a power source. I suspect that few, if any, geologists really thought that Wegener was actually suggesting that solid continents ploughed through the even more dense underlying rock. Geologists were quite familiar with the idea that rock could act as a brittle solid under some conditions, such as sudden impact or rapid compression; whereas the same rock would act as a plastic solid if subjected to steady, persistent pressure over a long period. The concept of isostatic equilibrium was well understood. Continental material, formerly, often referred to as “sial” was less dense than the underlying, more basic rock, known as “sima”. Thus a column containing both sial and sima would have to have a greater vertical extent than a similar column containing only sima, in order to achieve isostatic balance. If material at the surface was moved, the underlying rock could move vertically, or even be displaced horizontally to maintain this balance. Even the accumulation or melting of great thicknesses of ice could bring about vertical movement of the underlying rock. Indeed, Scandinavia, Greenland and other areas are still rising as a result of the removal of ice cover at the end of the last ice age.
The initial stumbling block was therefore not an inability to appreciate that solid rock could flow; it was rather the apparent lack of any force capable of causing such mass movement in a horizontal plain. Gravity provided the force necessary for isostasy, but what could move vast continents about on the Earth’s surface? The problem must have been compounded by the fact that continental masses were known to have “roots” which logically should anchor them.
Improvements in the study of seismic waves and the way in which they propagated through the Earth led to greater understanding of the nature of the Earth at depth. This led geologists to think that convection within the mantle might be possible, and that this might provide the necessary driving force for tectonic movement.
One has to ask how reasonable it might be to suggest that convection within the mantle could be possible. The following quote from (
http://www.platetectonics.com/book/page_4.asp) certainly seems to argue that it is.
“One idea that might explain the ability of the asthenosphere to flow is the idea of convection currents. When mantle rocks near the radioactive core are heated, they become less dense than the cooler, upper mantle rocks. These warmer rocks rise while the cooler rocks sink, creating slow, vertical currents within the mantle”.
Whilst this kind of simplified explanation may be all that is sought by the majority of people who might read this, it is, with some justification, open to the criticism, not least, because it suggests neat cells spanning the full depth of the mantle. In spite of their one time popularity, such cells are probably unlikely.
Let us look first at temperature: We find that the temperature at the bottom of the mantle is about 3,740K, and the temperature at the top is about 930K. Increased temperature leads to decreased density, so one might expect the mantle to be constantly convecting, like a pot of simmering soup. However, gravity causes density to increase with depth, so we must look also at relative densities. We find that the density of the material at the bottom of the mantle is about 5,560 kg/m3, while at the top it is about 3,370 kg/m3. This seems to suggest that material at the bottom should stay at the bottom, and material at the top should stay at the top. However, things are rarely that simple.
In part 2 I propose to look more closely at the reasons why this simple convection picture might not be "that simple".
