Harrow and Hillingdon Geological Society

Alaska and the Yukon

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The Geology of Alaska and the Yukon

Jo Crocker

The west coast of North America that stretches through Alaska is where two tectonic plates meet; The Pacific and the North American. For the last 200 Ma the transform boundary between the two plates in Southern Alaska has resulted in several terranes being attached to form the Alaskan Range; high mountains from which the rivers flow inland eventually ending up in the Bering Straits. In the north the Pacific plate is subducting under the American plate forming volcanoes and producing Earthquakes

In 1896 gold was discovered in the Yukon Canada and by July of the following year the Klondike Gold Rush had started. By the time the ships had reached Alaska the winter had set in and snow covered the steep Chinook trail over the formidable Alaskan Range. The prospectors had to climb this trail several times to take their 1000lb loads of necessary goods over the top. When they reached the other side they had to spend all the months till May in the depths of winter building their rafts to enable them to sail down the Yukon river to the Klondike. When they did sail through the canyons, around the islands and reached Dawson city all the sites had been claimed. The town was built on the permafrost and the heat from the houses caused this to melt producing lob-sided buildings.

The gold is in placer deposits at the base of 40ft deep gravels and had to be dug out by hand, or later by machinery. The crescent-shaped piles of stones left behind makes a weird landscape of what was once tundra.

North Alaska has another major deposit - oil - on the Prudhoe Bay north slope of the coastlline. This has been piped hundreds of miles to Valdez on the Pacific Ocean, often above ground with great care to make sure the permafrost is not destroyed.

Anchorage was built close to the sea but unfortunately on unstable and unconsolidated soils. Consequently during the Good Friday earthquake of 1964 (8.6 on the Richter scale and lasting 4 minutes) these soils liquefied and there was massive loss of people and buildings; this land is still not buuilt upon.

Much further north on the peninsula Mount Katmai erupted in 1912 and ash and incandescent pumice flowed down into the river valley trappong water underneath. In 1916 the valley was first seen and the steam jets then seen issuing up through the tuff looked like the Valley of Ten Thousand Smokes. Today there is no steam but the valley is still eerily beautiful with Katmai in the far distance. Of course tourists visit this area for a more interesting feature - the wild bears!

As usual Jo gave us a very entertaining lecture with abundant geology to discuss.

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