The very top layer on earth gets the most leathering because the rain, sleet, snow, ect. Mostly hits that top surface...
The other layers in earth dont get that much weathering because, well.... Like i said, everything mainly happens on the top layer.
~Hope this helped, anymore questions please ask! :)
I do not know, i just need points :/
I think you might find your answer here https://answers.yahoo.com/question/index?qid=20121101101721AAeZyHE