Excel 2008 : iterative average calculation for entire column

Hello. My problem. I want to calculate averages via iteration in each cell for a specified column.

Example: I’ve initially calculated average(a1:a10) in the first cell of the column, copied this and pasted it as a formula into the second cell of the same column which gives average(a2:a11). If I copy and paste for the entire column, it just give average(a3:a12), (a4:a13) and so on.I don’t want this. I want to copy average(a1:a10) from the first cell and compute average(a11:a20) in the next cell, average(a21:a30) in the next cell, and so on i.e for the entire column. How do I this?