where do rocks enter the earths core?
you see, many geologists believe that as the Earth cooled, the heavier and denser materials sank to the centre. and the lighter materials rose to the top, I'm pretty sure Inge lehmann discovered that.
as for your other question, how do they come up to the surface?
when the ground thaws, space is left under the stone, which fills with dirt, so it rests a little higher. over a period of time, this repeated freezing, expanding, and upward push fills underneath. eventually shoves the rock to the surface.
hope this helped, oh and just saying, that discovery was identified by James Hutton.