Sentence:  When an iceberg breaks off from a large ice shelf along an existing fissure, it looks like a large, flat rectangle. Typically, only 10 percent of an iceberg is visible above the water. As it <WSD>calved</WSD>, the iceberg may have been smooth and flat underneath, but ocean currents would have quickly changed it." 
Meaning of the word marked with <WSD> tags:  to break off