There are different ways to create an iceberg. You are probably thinking of a hard chunk of ice breaking off into the ocean, but probably a more common way is glaciers running into the ocean. A glacier is basically a "river" of snow and ice slowly flowing down a mountain. It would generally not be very compacted.
Fun fact: smaller chunks (less than 15m) are called "bergy bits".
Fun fact: smaller chunks (less than 15m) are called "bergy bits".