We are able to precisely detect various horizontal motions on Earth thanks to devices like GPS today. Geodetic stations around ridges move as the ridges generate new and new basalts. Wait. Do they really move?
In order to correctly interpret such motions in centimeters or millimeters, we need to look at the situation from a planetary view.
There is a group of satellites orbiting the Earth. They are able to localize a geodetic station. If the station was put on the north pole, the localization would indicate 90° N. If it was relocated to a new position, say 1000 meters from the pole, the GPS localization would give a new locality 1000 meters farther away from the previous one on the north pole. It is evident that a forced motion of the station from A to B indicates a change of its position. It is no problem to evaluate a distance between the points A and B and if needed, also the velocity, if the station was relocated for example over a period of one week.
What would happen, if the station was put inside a rock massif and was left to the mercy of Earth? The station will probably move again. Now, we have got two options how to explain it.
Earth with Constant Radius
The first option that crosses your mind would be that the station moves because of some forced motion (like in our previous example). This means that a certain area of Earth’s crust really moved – slided over Earth from A to B. Like a train going from one station to another station. Or like a person going to work and then back home. This is one of the foundation stones of plate tectonics and the measured motion is considered to be a proof of continental drift.
Earth with Variable Radius
Some of you wonder, how is it possible that I continue with a list of possible explanations. In addition, the paragraph will be divided into two.
A complement – if a body with constant mass changes its size, we may take it as a point. Thus the influence of volume changes on the GPS satellite trajectory won’t be taken into consideration from the physical point of view. In other words, it doesn’t matter whether the Earth changes its size or not, the satellite’s orbit and orbital period will be the same. There will be the pole (90°) according to the reference net all the time.
Expansion over the Whole Surface
An example of expansion over the whole surface is a balloon. When we blow up a balloon, all points on its surface move at a roughly constant velocity and direction. Since they have united directions, they don’t change their coordinates. The station at the pole would stay at the pole. We wouldn’t be able to measure any horizontal motion. The only motion would be the vertical one.
Expansion with Expansive Ruptures
The second option is to tear the balloon to pieces and change its size by adding new (balloon) material inside the ruptures. The added material doesn’t expand after addition. Now, only the area of the ruptures expands – surrounding parts are rigid and don’t expand.
This kind of expansion goes for Earth. It leads to changes of positions of geodetic stations. With this expansion, the station doesn’t stay on the pole. It moves away with respect to closest expansive ruptures (ridges). The motion occurs perpendicularly to the ridges.
Let’s take India. When we have a look at the measured vectors on India (picture above), we see that they lead somewhere to NE. As with the vectors in other parts of Eurasia. Now, there is a question whether we push India towards Eurasian mass like a high-tonnage icebreaker or whether we realize that there is a massive expansive rupture to the south from India. This rupture leads from Alaska to Europe through the Red Sea. And that the spread of this rupture makes those cm-shifts measured in Eurasia or also India.
If you stand on an idea of an icebreaker, then this idea from plate tectonics doesn’t make sense, well, we would assume that the Earth’s radius is constant. If we stand on the idea of spreading expansive rupture, the Earth’s radius will slightly change in reaction to measured horizontal motions.
We learned at school how one calculates the circumference of a circle, the circumference equals roughly 6.28 times the radius. In other words, the change in radius from 1 to 2 results in the change in circumference from 6.28 to 12.56. We see that the surface change is more evident than the radius change in absolute numbers. Thus our chances for measurement of radius increase are lower.