The Hubbard Glacier is the largest tidewater glacier in North America. A tidewater glacier is one that flows into the ocean. The glacier is approximately 7 miles wide at its foot and 76 miles long. These measurements are constantly changing as the Hubbard Glacier continues to grow and move forward.
The Hubbard Glacier is an active glacier with two major surges in the last 30 years, at one point advancing more than 7 feet a day. These surges were large enough to threaten the nearby coastal town of Yakutat, and they almost blocked the entrance to Russell Fiord, notes Alaska.org. Large chunks of the glacier break off into the ocean on a regular basis. This process is known as calving. Most tidewater glaciers calve above sea level, causing huge splashes as the new icebergs strike the water. If the water is deep enough, glaciers can calve under water.
Like land glaciers, tidewater glaciers push mounds of debris in front of them, but these mounds occur under the sea. The underwater mound is called a moraine shoal, and the moraine shoal provides support and protects the glacier from the action of ocean tides. If the glacier starts to retreat, this support is removed, and the tides cause excess calving, further hastening the glacier's retreat. Once the glacier becomes stable, the cycle repeats and the glacier continues to cycle through periods of advancement followed by retreat.