An iceberg is a large mass of free-floating ice that has broken away from a glacier. Beautiful and dangerous, icebergs wander over the ocean surface until they melt. Most icebergs come from the glaciers of Greenland or from the massive ice sheets of Antarctica. A few icebergs originate from smaller Alaskan glaciers. Snow produces the glaciers and ice sheets so, ultimately, icebergs originate from snow. In contrast, "sea ice" originates from freezing salt water. When fragments break off of a glacier, icebergs are formed in a process called calving.

Iceberg. Photograph by Commander Richard Behn, NOAA Corps. National Oceanic and Atmospheric Administration.

Icebergs consist of freshwater ice, pieces of debris, and trapped bubbles of air. The combination of ice and air bubbles causes sunlight shining on the icebergs to refract, coloring the ice spectacular shades of blue, green, and white. Color may also indicate age; blue icebergs are...