Why is most of the earth's land mass located in the northern hemisphere?

Dear teragram-ga,
 
This aspect of geology is still not fully understood, but the current
theory is that the earth?s surface is split into separate plates which
are continually moving in relation to each other. Millions of years
ago, it is argued, it was the opposite situation with most of the land
mass above sea level in the southern hemisphere. We just happen to be
living in an age when most of the visible land mass above the sea
level is in the northern hemisphere.

I recommend that you read in its entirety these page from the US
Geological Survey which describe the Plate-Tectonics Theory. There are
numerous diagrams to assist your understanding.
http://pubs.usgs.gov/gip/dynamic/dynamic.html#anchor19309449

University of California Berkeley has some animations which will also help you.
http://www.ucmp.berkeley.edu/geology/tectonics.html


I hope this answers your question. If it does not, or the answer is
unclear, then please ask for clarification of this research before
rating the answer. I shall respond to the clarification request as
soon as I receive it.
Thank you
answerfinder

i would say this is more than the "current theory". It is established fact.