The questions below are ones that we frequently receive from people interested in Alaska's volcanoes. Since some of these questions are
answered by information in this website (and other websites), we've provided links for you to follow to learn of the answer(s).
As the plate descends into the mantle (dense rock that underlies the earth's crust), it undergoes a series of chemical and physical changes
caused by increasing pressure and temperature. First, the water that is stored in subducted sediments and in the oceanic crust is released. Then, at greater depths, water-bearing minerals (such as hornblende) change into non-water-bearing minerals (such as pyroxene). Water given off by this process, along with dissolved impurities, rises into the overlying mantle. The addition of water to the mantle lowers its
melting point and is one of the primary processes that leads to the production of magma. Magma also forms as the mantle, stirred by the motion of the descending Pacific Plate, rises to a position beneath the volcanoes (see Formation of Subduction Zone Volcanoes).
The magma that results from these processes is less dense than the surrounding mantle and rises toward the surface of the earth. When it reaches the continental crust, which is less dense than mantle and the
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mantle-derived magma, it pools and begins to change in character. First it heats, then melts, and then mixes with the surrounding crust or country rock. As the magma cools, it begins to crystallize and the crystals that form differ in composition from the magma. This is important because the crystals separate from the liquid, which changes the magma's composition still further; it becomes richer in those chemical components not concentrated in the crystals. This process is called fractional crystallization, or fractionation.
The most fundamental change that results from the fractional crystallization of magma is the increase in silica. Throughout the fractionation process magma changes from the initial basalt to andesite and then to dacite. As the silica content of the magma increases, the magma continually becomes less and less dense until it reaches a point where it is lighter than the crust that surrounds it and then resumes its rise to the surface. Depending on the magma's
rate of ascent, it can continue to crystallize, fractionate, and assimilate with the surrounding crust producing, in extreme cases, rhyolite with up to 76 percent silica.
When the magmas finally reach the surface, if they are relatively poor in dissolved gases, they erupt non-explosively and form lava flows or domes. If they are rich in dissolved gases, they explode violently (like a shaken soda bottle) and form columns of volcanic ash that can reach more than 15 kilometers (45,000 feet) into the atmosphere.
The processes outlined above are a thumbnail sketch of the complicated processes that form the volcanoes of the Aleutian Arc. Current models suggest that the Wrangell volcanoes formed in a very similar way and are associated with a small sliver of the Pacific Plate that is thrust northeastward beneath central Alaska. Several of the Wrangell volcanoes are among the most voluminous andesite volcanoes in the world - several times the volume of Mt. Rainier.
There are two major types of volcanoes in Alaska not directly tied to the Aleutian subduction zone. The first type is a series of small craters (and one larger one, Mt. Edgecumbe, near Sitka) scattered throughout southeastern Alaska. These small volcanoes may result from the intense shearing along many strike-slip transform faults that are caused by the northward movement of the Pacific Plate. Deep crustal fractures such as these faults may allow magma to rise and volcanoes to form in areas where magma could not normally reach the surface.
The second type of non-subduction volcanoes form the basalt fields of western Alaska. These fields typically consist of many basaltic cinder cones and lava flows, each of which formed over the course of only a few weeks or months. It has been suggested that at least some of these fields formed where tension in the earth produced deep fractures and thinning within the crust. The origin of these fields is not well understood, but they are analogous to the cinder cone fields northwest of the Japanese volcanoes in Southeast Asia, eastern China, and Korea.
From Nye, C. J., Queen, Katherine, and McCarthy, A. M., 1998, Volcanoes of Alaska: Alaska Division of Geological & Geophysical Surveys Information Circular IC 0038, unpaged, 1 sheet, scale 1:4,000,000