Knik Glacier Icebergs EarthCache
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The Knik Glacier is located about 50 miles east of Anchorage on the northern end of the Chugach Mountains. The ice field averages over 25 miles long and over 5 miles across, making it one of the largest glaciers in south-central Alaska. Ground zero will bring you to a fantastic viewing spot of Knik Glacier, and equally impressive, the icebergs that have formed in the Knik River from this glacier! There are several ways to get here – the jet boat tour is highly recommended and you will not be disappointed! The terrain rating is based on the fact that you will either need a watercraft or long ATV ride to get here. It is worth the trip!
An iceberg is ice that broke off from glaciers or shelf ice and is floating in open water. An iceberg begins on land in the form of a glacier. Glaciers form on land as a result of snow accumulating over thousands of years. Layers of snow and ice press on previous accumulations until they are 60 to 70 meters thick and glacial ice is formed. Therefore, icebergs are made of fresh water. Glaciers flow, or creep, outward under their own weight. When the edge of a glacier ‘flows’ into water, the pieces that break off are what we call icebergs. This is called ‘calving’.
Iceberg drift speed depends upon many factors including iceberg size and shape, currents, waves and wind. Almost 90% of an iceberg is below the water line. This figure is approximate as all icebergs are different, and is based on the mass of the iceberg.
To be classified as an iceberg, the height of the ice must be greater than 16 feet (five meters) above sea level and the thickness must be 98-164 feet (30-50 meters) and the ice must cover an area of at least 5,382 square feet (500 square meters).
As glaciers creep over land, meltwater fills the crevasses and later freezes, creating clear, bubble-free ice. This shows up as bluish streaks in icebergs because of the light scattering characteristics of pure ice. Sometimes airborne dust from volcanic eruptions, or the wind, falls on a glacier and becomes trapped inside, forming a noticeably darkened brown or black layer. But because most volcanoes are south of glaciers and winds from the south rarely mix with Arctic air masses, there are very few pollutants in the ice.
There are smaller pieces of ice known as “bergy bits” and “growlers.” Bergy bits and growlers can originate from glaciers or shelf ice, and may also be the result of a large iceberg that has broken up. A bergy bit is a medium to large fragment of ice. Its height is generally greater than three feet (one meter) but less than 16 feet (five meters) above sea level and its area is normally about 1,076-3,229 square feet (100-300 square meters). Growlers are smaller fragments of ice and are roughly the size of a truck or grand piano. They extend less than three feet (one meter) above the sea surface and occupy an area of about 215 square feet (20 square meters).
Icebergs are also classified by shape, most commonly being either tabular or non-tabular. Tabular icebergs have steep sides and a flat top. Non-tabular icebergs have different shapes, with domes and spires. A fantastic variety of shapes result from both the formation and deterioration processes of icebergs. Like snowflakes, no two icebergs are exactly the same, but there are certain basic categories of shapes that are used for iceberg observations.
TABULAR: Steep sides with a flat top - like a huge tablet. These are very solid and often break away from ice sheets or ice shelves. Many show horizontal banding from the layers of ice that built up as the sheet formed.
DOME: An iceberg with a rounded top.
BLOCKY: A flat-topped, block-shaped icebergs with steep vertical sides.
WEDGE: As the name implies, it's like a wedge. The top narrows to a pyramid-like point.
DRYDOCK: An iceberg which is eroded to form a little U-shaped harbor-like enclosure - like a drydock.
PINNACLE: An iceberg with one or more spires.
In addition to shape, icebergs are also classified as to size. See the chart below for more information.
Size Category Height Length
Growler Less than 1 meter (3.3 ft.) Less than 5 meters (16 ft.)
Bergy Bit 1–5 meters (3.3–16 ft.) 5–15 meters (16–49 ft.)
Small 5–15 meters (16–49 ft.) 15–60 meters (49–200 ft.)
Medium 15–45 meters (49–148 ft.) 60–120 meters (200–390 ft.)
Large 45–75 meters (148–246 ft.) 120–200 meters (390–660 ft.)
Very Large Over 75 meters (246 ft.) Over 200 meters (660 ft.)
As this is an earth cache, there is no physical container or log to sign…it is all about the journey and the destination. However, in order to log your visit, please email the following answers:
1. Do you see coloring in the icebergs? Based on the information above, where do you feel this came from?
2. Facing the glacier, please comment on the “shape” of the icebergs you see, based on the six basic categories listed above.
3. Categorize the most of the icebergs you see by size, as per the table above.
While photos cannot be required any longer, I certainly cannot imagine anyone visiting here without taking multitudes of photos. Please post and share these.
Sources:
NOAA (visit link)
Athropolis (visit link)
Wikipedia (visit link)
IcebergFinder.com (www.icebergfinder.com)
Additional Hints
(Decrypt)
RAWBL GUR IVFVG!