[Armin glows a little under the praise as if it was directed at him. But then, what is a book but the extension of a nerd soul?]

Eren really picked something incredible.

[But then, giving gifts is easier when they are flush with wealth and have so much to choose from. That is what it feels like being in this world, at least.]

[He allows Ford to look for a few moments more, before he reaches over and turns a few more pages, ending up in the part of the Atlas that describes the mountain ranges of Mt Mortar. He points at a small passage that roughly sums up the history of the area.]

Here, I don't understand this very well. What does it mean by tectonic activity?