It depends. Do you have any sort of Replication going on? Then the logreader agent will be marking transactions when they are moved to the distribution database.

The main source of writes on a transaction log are changes to database pages (not just on the commit, by the way). If you are updating 10 million rows, and the update takes 10 minutes, then the log entries (before and after pages) are written to the transaction log all throughout the 10 minutes. if the server is restarted in the middle of the 10 minutes, then none of the update records in the log will be marked as committed, and any applicable changes will be rolled back. If the server is restarted after the commit is written to the log, then the after images will be rolled forward.

Make sense?