Plate tectonics primer: subduction at the corner of Alaska, BC and the Yukon
Major mountain belts are typically formed by compression during convergence between two continental or oceanic plates that make up the earth’s crust. Along the west coast of North America, the Pacific plate is moving northwestward at about 5.5 cm per year. It dives beneath southwest Alaska, like a conveyor belt. Earth scientists call this subduction: a process that has been on-going since early in geological time, consuming oceanic crust at roughly the same rate as it is produced at spreading ridges such as the Mid-Atlantic ridge. It is the combination of the westward drifting North American continent (pushed by rifting in the Atlantic), and northward moving Pacific oceanic plate that causes “oblique subduction” beneath the Gulf of Alaska that has trapped a high-standing fragment of continental crust (the Yakutat microplate) above the subduction zone (see later section). Subduction of oceanic crust including the waterlogged sediments, leads to melting at about 80 km depth. This newly generated liquid rock (magma) rises through the over-riding plate to produce volcanoes – in the curved chain of Aleutian Islands, and the high peaks in the Wrangell Mountains of east-central Alaska. A volcanic eruption in the Wrangell Mountains about 1200 years ago, spread a tephra (called the White River Ash) across central Yukon. This white layer in the soil, and a prominent marker in lake-bottom and ice cores, is useful for determining the age of over- and underlying sediments.
Aleutian subduction zone beneath the Gulf of Alaska
Credit: S. Isreal, Yukon Geological Survey
Along the west coast, the Pacific oceanic plate slides northward, separated from the North American plate by a strike-slip fault known as the Queen Charlotte transform system. Most of the component faults lie offshore, but some, including the Fairweather Fault (it passes near Mount Fairweather (16,000 ft; the southwestern tip of northwestern BC), tracks inland in southeastern Alaska (see section on Earthquakes). Other northwest-trending faults further inland in Yukon and northern BC reflect former times when fragments of continents, islands and sea floor were shoved together by plate tectonics, resulting in the formation of distinct terranes.