Hi Sharon - Bogoslof's magma and eruption is definitely the result of plate tectonics. In Alaska, the Pacific Plate is being subducted under the North American Plate at a rate of about 2.5 to 3 inches per year.

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: "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.

"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 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." Regarding Bogoslof's specific magma chamber and prognosis for eruption, our best guide to what Bogoslof might do right now is what has happened in the past - previous eruptions of Bogoslof have lasted for months to years. Larger amounts of material expelled doesn't necessarily mean a longer eruption - the largest eruption of the 20th century (Novarupta-Katmai 1912) took only 3 and a half days. Some volcanoes have very long lived eruptions - like Kilauea, in Hawaii - which has been erupting from the Pu‘u ‘Ō‘ō vent since 1983! Thank you, Cheryl ----- Sharon Eoff wrote: I have a question. Is there any way of finding out what is under the volcano? Is there a big magma chamber that is erupting like, for a lack of a better analogue, a bowl of beans? Could it be caused from plate subduction or convergent??? Why is it erupting like it is and not settling down like Mt. St. Helens?? Thank you for your time. Respectfully Sharon -- Cheryl Cameron Geologist, Alaska Volcano Observatory Alaska Division of Geological & Geophysical Surveys voice: 907-451-5012 fax: 907-451-5050