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Tidewater Glaciers

Erin (Mercé) Mueller

Physical Geology

Autumn 2008

 

Tidewater Glaciers, A Spectator Sport

 

When people think of glaciers they often think of icebergs, gigantic sheets of ice on land that melt very slowly over time, Canada or even the bottled water. However, glaciers come in various forms, one of the most interesting and popular kinds, are tidewater glaciers. 1, 6

 

Tidewater glaciers are often outlet glaciers of ice flows or ice caps, but that’s not always exclusively the case. Sometimes they can form independently on their own with no connection to larger ice caps or fields. Tidewater glaciers can be found all over the world, and were common in the Pleistocene 3 age. Now they can be found mostly in Antarctica, Norway, Greenland, most Artic Islands and southern South America. In the United States tidewater glaciers are only found in south and southeastern Alaska.

 

Tidewater glaciers have a several unique features that can be found in no other glaciers, behaviorally and physically. One, is the fact that they terminus in a body of water, be it saltwater or freshwater. In the correct circumstances this causes ‘calving’ 2, or the birthing of icebergs as they break off of the glacier into the water. The second and most defining feature is referred to as a ‘tidewater glacier cycle.’ This cycle stretches over centuries and is caused by altering periods of advance, stability and retreating. It is also the reason for tidewater glacier’s relative immunity to climate changes, while other glaciers are suffering from the effects of it. 1,3,6

 

 

 

Picture copyright E. Mueller 2005-2008

Tidewater glacier. Aialik Glacier, Kenai Fjords National Park, Alaska

 

 

During the advancing stage of a tidewater glacier cycle, the glacier usually deposits the sediments and rocks that it’s plucked, or lifted, during its descent to the water.  This forms a terminus or moraine shoal, alternatively a terminal moraine in front of the glacier. The shoal does two things; it stabilizes the glacier for more advancing and protects it from tidal water. This keeps the calving rate low. As long as the shoal is there the glacier is relatively safe from climate changes.  Despite the relative safety from climate changes, the glacier has to contend with the changes and conditions in the fjord it has terminated in.1,3,6  When a tidewater glacier terminates into water the snout, or terminus, of it usually cracks and breaks forming many crevasses in the glacier, this allows the glacier to easily calve when it hits the retreating stage of the tidewater glacier cycle.

 

 

 

 

Picture copyright E. Mueller 2005-2008

Icebergs. Kenai Fjords National Park, Alaska

 

 

Tidewater glaciers are most popular when they are in their retreating stage, for this is when the calving, or breaking off of icebergs can be seen. Usually with spectacular effects large chunks of ice shatter, break and slide off the glaciers ice face into the water with a large splash. This makes them a scenic view from a cruise ship. The retreating of the glacier is due to mostly non-climate conditions; if the fjord widens, this causes the ice to gain surface area, causing strain on the snout of the glacier, this strain, along with the possible loss of shoal support due to the expansion, causes less stability and can herald a retreat in the glacier. Once the glacier has left the shoal it has created and started its retreat it is subjected to the deep water, which has filled in the fjord that in many cases the advancing glacier has deepened; producing the calving to breathtaking effect, much to the delight of onlookers, making tidewater glaciers a spectator sport. 3,6

 

 

Picture copyright E. Mueller 2005-2008

Preparing to calve. Aialik Glacier, Kenai Fjords National Park, Alaska

 

 

In the midst of the ongoing cycles of advancing and retreating the glacier will go through another stage, this time of stability. This is often reached when the glacier has hit a perfect equilibrium. Calving will continue but at a much lower rate, as the glacier retains a ‘happy medium’.4  At this point the glacier will stop moving either forward, or back. The little calving that happens works almost as a counterweight in the delicate balance of the glacier.

 

The tidewater glacier cycle is an ongoing cycle that takes hundreds of years or centuries for it to occur. There are some exceptions however, that give scientists a look at the cycle on a smaller scale. The Hubbard Glacier, the longest tidewater glacier in the United States and bearing a six mile ice face, located in the Alaskan Yukon advances and retreats seasonally due to snow and glacial melt. 8 Allowing glaciologists a unique look at the cycle on a yearly basis.

 

 

 

Picture copyright E. Mueller 2005-2008

Carving out the rock. Kenai Fjords National Park, Alaska

 

Unlike other forms of glaciers, tidewater glaciers don’t leave as many tangible geological patterns to view once they have retreated. Glacial polish, smoothing and scratching can be observed on the rocks that the glacier has flowed over but little else is blatantly obvious, as it is mostly covered by water. Geologically speaking, tidewater glaciers are a massive aspect in the shaping of coastlines in the farthest northern and southern areas of the world. They’re known for cutting u-shaped valley’s into already present ravines as they flow, or sometimes in flat plains, to the ocean. Once the glacier has retreated, and in most cases melted, these valleys having filled with water, are called fjords. Geological features of fjords are steep cliffs and a shallow mouth at the entrance of the ocean, caused by the terminus shoal the glacier has left behind. The depth of fjords promotes the removal of oxygen from water and sediment, due to the lack of water mixing and moving. In some cases a fjard will be formed. A fjard is a fjord, but on a much smaller scale, generally termed to be a “glacially carved embayment that is drowned by the sea” 5 

 

 

 

Resources and References

 

1) What types of Glaciers are there? Online. http://nsidc.org/glaciers/questions/types.html

 

2) Glaciers and Icefields. Online. http://www.fs.fed.us/r10/tongass/forest_facts/resources/geology/icefields.htm#glacier

 

3) NASA. Online. http://disc.sci.gsfc.nasa.gov/geomorphology/GEO_9/GEO_PLATE_G-11.shtml

 

4) Calving Glaciers. Online.  http://www.wooster.edu/geology/tr/mkennedy.html

 

5) Somes Sound, Mount Desert Island, Maine. Online. http://www.state.me.us/doc/nrimc/mgs/explore/marine/sites/nov98.htm

 

6) Tidewater glacier cycle. Online. http://en.wikipedia.org/wiki/Tidewater_glacier_cycle

 

7) Kenai Fjords National Park. Online. http://www.kenai.fjords.national-park.com/map.htm

 

8) US Forest Service – Tongass National Forest. Online. http://www.fs.fed.us/r10/tongass/hubbard/index.shtml