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Harriman Expedition Retraced

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Kristine J. Crossen

Aleutian Volcanoes and Plate Tectonics on the Pacific Rim

A 2,500 km long line of volcanoes, with 80 volcanic mountains and 41 historically active (since 1760 AD) volcanoes produce an arc stretching along the southern edge of the Bering Sea and onto the Alaska Peninsula. This volcanic arc delineates the boundary between the Pacific and North American tectonic plates. A subduction zone, composed of the deep Aleutian Trench coupled with a landward line of volcanoes, creates a series of offshore islands (the Aleutians) as well as a line of volcanoes along the edge of the continent (the Aleutian Range on the Alaska Peninsula). This zone marks the interface where the denser Pacific oceanic plate subducts beneath the less dense continental North American plate. As the subducted plate descends, the heat from the earth's interior causes the plate to begin melting, and buoyant liquid magma rises to the surface forming a line of volcanoes.


augustine

Augustine Volcano showing the typical cone shape, erupting ash cloud, and pyroclastic flows on the snow covered slopes (Photo by U.S. Geological Survey).
Click image for a larger view.

Along the northern and central portion of the arc, the subduction rate is 6-8 cm/yr where the two plates have opposing relative motion. But along the extreme southern end of the arc, the two plates slip past each other and volcanic activity is reduced. Eastward from the current arc, and separated from it by several hundred miles, lie the Wrangell Mountains. This currently inactive volcanic area, stretching to the Canadian border, was produced when the Yakutat terrane (a sliver of crust) was subducted and accreted to southern Alaska between 22 million and 1 million years ago.

Subduction diagram

Subduction of the Pacific Plate beneath the North American Plate creating the Aleutian trench and volcanic island arc (Credit: Alaska Volcano Observatory).
Click image for a larger view.

Subduction zone volcanoes commonly exhibit a symmetric shape with steeper slopes near the vent and a broad gentle base. These stratovolcanoes produce a wide range of volcanic products including numerous andesitic rocks, gray to green to red in color, that often contain visible white feldspar crystals. The magma generated by subduction zone volcanoes is less fluid than that of Hawaiian or Icelandic volcanoes, so it does not flow easily from the vent. Instead, this sticky lava clogs the volcano's throat and causes explosive eruptions when magma again tries to move to the surface. Spectacular ash clouds and pyroclastic flows of hot ash and gases race down the slopes when an eruption occurs. If this hot debris is deposited on snow and ice, it causes melting and generates lahars (volcanic mudflows) that may deposit debris miles away from the volcanic vents.

The largest eruption in the last 100 years was the 1911 Mount Katmai event. The Valley of 10,000 Smokes was formed when a 35 cubic km pyroclastic flow filled a former river valley. A two hundred foot thick deposit of hot ash heated the stream and ground water under the pyroclastic flow, changing the water to steam, and producing the "smokes". These have since died out. The event emptied the magma chamber beneath Mt. Katmai, causing its summit to collapse and forming a caldera, now filled with a warm blue crater lake.

Katami

Katami Caldera with crater lake, formed after the 1911 eruption (Photo by K. Crossen).
Click image for a larger view.

Aleutian volcanoes erupt sporatically, and have not been studied long enough to predict a recurrence rate. Pavlov, Akutan, and Shishaldin volcanoes are the most active in the arc. Augustine Volcano, near Homer, is the most active Cook Inlet volcano, with eruptions approximately every 11 years.


valley

Valley of 10,00 Smokes filled with pyroclastic flow material with Novarupta vent on extreme bottom right (Photo by Alaska Volcano Observatory).
Click image for a larger view.

Most Aleutian volcanoes are located in remote areas, and their eruptions thus affect few inhabitants living on their slopes (unlike volcanoes in the Phillipines, Italy or Central America). However, the greatest hazards are generated by ash clouds. A KLM jet sustained millions of dollars of damage when it unknowingly flew through an undetected ash cloud from the 1989 Mt. Spurr eruption. The Alaska Volcano Observatory closely monitors Aleutian volcanic activity and to help assess current eruptions and predict future ones.


cauldera

Pyroclasitic flow form Aniakchak Cauldera, showing the white pumice pebbles mixed with gray sand. (Photo by K. Crossen).
Click image for a larger view.


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For information on the Harriman Retraced Expedition e-mail: harriman2001@science.smith.edu

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