The Earth's plates move very slowly, so the effects are difficult to notice except at the plate boundaries. Modern measuring techniques can detect the movements however.
Earth's plates are constantly in motion because of convection currents in the mantle. How this energy translates to individual plates is still not understood. (see the related question)
Yes tho, my book says something like that I how I see here, yes.
Moving air or wind is difficult to see. The motion of atoms can not bee seen with the unaided eye. One can not see the motion of tectonic plates. In these examples, the phenomena are not visible but one can measure them.
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The actual motion of stars is very hard to see because stars are very far away.
Smoke, it's hard to see, but it's there at any given time
They can be caused by either motion of the tectonic plates. See the related question below about what causes earthquakes more more detail.
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If something is "constantly in motion", then it's moving. At that level, the question is nonsense.There are a couple of reasons they might APPEAR not to move, depending on exactly what you're talking about when you say "constantly in motion".If you mean that they're constantly in motion because the Earth itself is whizzing through space, then they don't appear to move because you're comparing them to other objects on Earth which are ALSO whizzing through space.If you mean they're constantly in motion in the sense of tectonic plates, the motion is so slow that it's hard to notice. What makes it doubly hard is finding something to compare it to, because plates are often large, and any object sitting on the plate will be moving with it. It's easiest to notice motion at the plate boundaries.If you mean they're constantly in motion in the sense that the individual atoms and molecules that make them up are moving, you don't notice it because they don't move very FAR, they just sort of wiggle in place, over such small distances that you can't see them.
Earthquakes are one very obvious example. But there are places where a piece of ground which was once at the same level is now lower than the other, in remote regions where there is no human activity.
On the surface and under the oceans of the Earth. They vary in thickness from around 2 miles to 80 miles. Every part of the Earth's surface is a tectonic plate. The entire crust is made of these plates. The movement of these plates against each other are what causes earthquakes, many of the worlds volcanoes, and tsunamis. See the links below for information and maps of the worlds tectonic plates.
See the following link for a complete listing of Plates and their locations: http://bulbapedia.bulbagarden.net/wiki/Plates