I could've sworn it was never shown how he actually caused it to start moving.
http://www.mangareader.net/one-piece/632/16
EDIT: I just realized that on the page, VDD actually says that he threw it.
He also says on that page that he just touched it. There's no way that he is tall enough to lift the whole ark out of the ground and throw it… it looks like it was buried pretty deep - see all of the dirt that's falling off?
If you need a size comparison, just look at wadatsumi (sp?).