Geology/general geology(i think)
i heard that the earth is not exactly a rigid body- my question is whether the outer shell of the earth changes its shape (even very small changes or waves) and if so what is the magnitude of these changes, wave length etc.
Interesting question. It turns out that the surface of the Earth (the Crust) does indeed change shape. But, perhaps, not as you might imagine.
Earthquakes can cause "ground waves" that look much like the waves on water, they can also push sections up, down, or sideways. Earthquakes can, temporarily, liquify the ground or create large fractures. Water, wind, rain, and ice all erode the surface changing or removing structures. Volcanoes both add material and create structures. That doesn't even speak of human intervention and actions.
But the rocks that make up the crust have a plasticity all their own. Every rock will flow if enough pressure and temperature is applied. Even solid rock will change shape if it's buried, deep enough, on a hillside. The relentless pressure and gravity will deform the rock. However, this process, like most geological processes, takes thousands of years, or more.
There is another force that causes even greater movement of the crust; Plate Tectonics. This topic is far too complex for this venue but I can give you an idea what's going on. The surface of the Earth is made up of more than 20 crustal plates. They are rigid and slide around on a very thick layer of rock made plastic by the great heat of the Earth's interior. The mechanism for that movement is not fully understood but heat currents in the rock below the crust are thought to be the prime mover. As the plates are moved, they slide under others or push up against each other and bend. Those that slide under can generate volcanism while those that bend are the mountains we see. There is much more but if you visit these sites you'll get a better understanding:
The surface does move in a variety ways and for a variety of reasons.
Hope this helps.