Plate tectonics is not a physical law (like the laws of physics for example). Roughly speaking it's an observation and explanation, based on evidence of where things were and are now, of how continents have moved and how things are still moving. Continental drift, creation of mountains, etc. Like there was at one time Pangaea where all the currently known continents were all in one piece and now we have them split out. Like the San Andreas fault being where two plates meet and the Pacific plate is shoving up against (and under as I recall) the North American continental plate. So it's a theory in that it is an explanation of how things were, are and work. Not an immutable physical law.

Heres a US Geo Survey site with a sort of brief primer on plate tectonics http://pubs.usgs.gov/publications/text/dynamic.html

thats a good link.

Plate tectonics as a whole is the theory of crustal plate movement and the mechanisms that cause it. The actual movement of the plates is an observed characteristic. The confidence in the evidence in favor of plate tectonics as a valid theory is quite high.

The crust of the earth averages about 30 miles thick, although it can be quite thin in places. It 'floats' on the mantle, which is the outer layer of the earths core, which consists of what we call magma, which is primarily molten rock. The mantle surrounds the outer core, which is composed of molten rock and metals, mostly iron and nickel. The outer core surrounds the inner core, which is molten iron and nickel, compressed to near solidity by the pressure.

The crust of the earth is broken into several pieces, or plates, and these rub up against one another at the edges. Where the plates meet, one of several things happen. One plate can be forced under another plate, called subduction. Usually this is accompanied by fault lines and upheaval (the pacific plate is subducting (moving under) the north american plate. Or both plates can buckle upward, which generally results in the larger and higher mountain ranges (the himalaya mountains are an example of this type). Or the two plates can move apart from each other, with the gap being filled by magma (the mid atlantic ridge is a result of this movement).

Crustal movement will not change the size of the earth, as the mass and volume remain relatively constant.

As for space borne dust, 10 tons a day is about 90.9 * 10^2 kilograms. The mass of the earth is about 5.97 * 10^24 kilograms. The amount of new mass as a percentage of the mass of the earth is about 0.0000000000000000000152 %. This would have to continue for about 6,578,947,368,421,052,631 days to make a 1% increase in mass.

The Earth can't be getting bigger. Where would all the extra mass come from?

It is possible to expand a body without changing its mass, isn't it? At least I think that's case on a chemical level.. increased entropy.

Also, I wonder if a shift in the magnetic poles would or could in fact cause Expansion and or Contraction leading to tectonic shift?

IPlural: that's an intressting question, why do you think the process of subduction could influence the Magnetic field?
Such a shift in the Magnetic field would most likely influence Cetaceans and their "sonar" system the most.

If the center of the earth were to cool and solidify, the diameter of the earth would probably shrink some small degree, since the radiation pressure from the heat would no longer be pushing outward.

That's interesting, I always thought that the core of earth was solid with a mantle of liquid magna surrounding it.

The core of the earth is molten liquid iron and nickel (with small amounts of other metals). It is under sufficient pressure to be effectively solid, but it still radiates heat, which pushes out on the other layers. If the core were to cool completely and become truly solid, the magnetic field would disappear (there might be a remnant of magnetized metal, but it would have nothing like the current level of intensity).

The magnetic field (according to current theories) is created from the turbulence and movement of the molten metal in the inner and outer cores.
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Many biologist would not aggre with you. If only we would actually know all these things, instead of just being comfortable about it. Remember, for hundreds of years humanity was very comfortable with thinking that earth was a disc in the centre of the universe

That's why I said, it just shows us how much we still have to learn.

We certainly have quite a bit left to learn, but that does not necessarily invalidate what we already do know.

There are quite a few things in the geological record that indicate that the gravitational force of earth was not significantly different from now. There apparently has been some variation, but that variation is currently estimated to be within a few percentage points at most.

Just as (to stay on topic) the evidence in favor of the accuracy of the plate tectonics model has a high level of confidence, and there is no clear evidence that the size of the earth has varied in any significant sense in the geological record.

As a scientist, I'm always open to new evidence and theories, but they'll generally have to subsume the observed and experimentally vetted body of evidence (in much the same manner that Einstein's laws of motion subsumed Newton's laws of motion).

nobody actually knows what the core is. The one assumption I am familiar with is a large molten core of iron. I can't possibly see how the earth would expand. I could understand it contracting. The gravity of the earth is more likely to pull the particles of earth closer together then for any of the accountable reasons for it to expand. I can only think of two possible reasons for the earth to exand: the first is the inner core getting hotter (the whole earth for that matter) via global warming or some other oddity of nature, the other way is if gravity lost some of its hold and the spin of the earth pulled it apart (but that would result in the destruction of the earth and the lessening of gravity is improbable).