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Geology of Glacier Bay National Park & Preserve

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Great Pacific Glacier, Glacier Bay.

Great Pacific Glacier, Glacier Bay. (Photo credit: Wikipedia)

Glacier Bay National Park & Preserve was not surprisingly shaped by immense energies, and we’re not just talking about the glaciers here. The area of the park is in an active collision zone, wedged between the North American plate and the Pacific plate. For more than 100 million years (yes, that long) the North American plate has been diving over the Pacific plate at approximately the same speed as it takes to grow your finger nails. That might not seem like very much to you or I, but just think how long your finger nails would be if you didn’t file them for 100 million years!

If you look at a map of Glacier Bay you could very well notice that there are four separate geological terranes, mostly in a northwest – southeast pattern. These are evidence of the leading edge of the North American plate, comprising of pieces of the sea floor and other bits and pieces which have broken off. The most outwardly terrane and the continental margin right now are in the process of closing up the gap, the frequent earthquakes evidence of the plate motion. The two plates are being fiercely forced together resulting in some of the rocks being pushed upwards forming mountain chains. The ones which are forced downwards get melted and turn into molten rock which periodically oozes volcanically before cooling and welding the Glacier Bay – often described as one of the most complicated jig-saw puzzles in the world.

The Fairweather mountain range was formed by such a process, which makes up a large part of the western Glacier Bay National Park. There are numerous peaks reaching more than 10,000 feet, and at 15,300 feet Mount Fairweather is the highest. This is actually the highest range of coastal mountains anywhere in the world.

Snows have accumulated in the in the uplands for millions of years eventually morphing into glacial ice. These glaciers have periodically slid down the mountains due to climate change. In the most recent of the Great Ice Ages, only 20,000 years ago – which is kind of like last week in geological terms – the entire Glacier Bay was covered by an ice-sheet, all except the very highest peaks anyway. In those days you would have been able to walk all the way from Glacier Bay to Cape Cod without ever leaving the ice – makes you think doesn’t it?

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