I'm not for sure, but The mantle has convections that are believed to be responsible for the movement on the techtonic plates.
Answer:
It is a funny way of pointing out that Stanley and Elya are clearly not living at the same time.
Explanation:
This is the excerpt from Louis Sachar's novel "Holes". He is famous children and young adults author.
Now, regarding this excerpt, it is quite obvious that Elya is Stanley's great-great-grandfather and, of course, there was no way Elya, at the time, could have known that he's going to have a great-great-grandson, let alone some detailed information.
This makes that line rather unnecessary. But it also can suggest that its only purpose is comedic effect by stating an obvious thing.
