There sure is.
Drag the iPhoto Library folder to the external drive (make sure it is formatted for Macs)
Launch iPhoto holding down the Option key
Choose to open another library
Navigate to the library on the external and choose it.
When you launch iPhoto holding down the Option key this is the window you will get.
Create a new library or choose a library to open
Choose to create a new library if that is what you want to do, or choose a library to open by clicking the "choose" button.
If you click the "create library" iPhoto will open with a brand new library for you to import images to. If you click "choose" then you will get this window.
Navigate to where the library is on your computer and then choose it.
Since you copied the library to the external, you still have the one in the Pictures folder. You can delete that folder after you check and make sure the one iPhoto opened on the external is working fine.
Also since your library is now on the external how are you going to back it up?
You can always make a duplicate of it, but that will not help you if the external goes kaput.