El Misti volcano in Peru is featured in this image photographed by an Expedition 21 crew member on the International Space Station. The symmetric conical shape of El Misti is typical of a stratovolcano — a type of volcano characterized by interlayered lavas and products of explosive eruptions, such as ash and pyroclastic flow deposits. Stratovolcanoes are usually located on the continental crust above a subducting tectonic plate. Magma feeding the stratovolcanoes of the Andes Mountains — including 5,822 meter-high El Misti — is associated with ongoing subduction of the Nazca Plate beneath the South American Plate. El Misti’s most recent — and relatively minor — eruption occurred in 1985. The city center of Arequipa, Peru lies only 17 kilometers away from the summit of El Misti; the gray urban area is bordered by green agricultural fields (right). With almost one million residents in 2009, it is the second city of Peru in terms of population. Much of the building stone for Arequipa, known locally as sillar, is quarried from nearby pyroclastic flow deposits that are white in color. Arequipa is known as "the White City" because of the prevalence of this building material. The Chili River extends northeastwards from the city center, and flows through a canyon (left) between El Misti volcano and Nevado Chachani to the north.
Mount Hood, Oregon is featured in this image photographed by an Expedition 20 crew member on the International Space Station. Mount Hood is located within the Cascade Range of the western United States, and is the highest peak (3,426 m) in Oregon. The Cascade Range is characterized by a line of volcanoes associated with a slab of oceanic crust that is subducting, or descending underneath, the westward moving continental crust of North America. Magma generated by the subduction process rises upward through the crust and feeds a line of active volcanoes that extends from northern California in the United States to southern British Columbia in Canada. While hot springs and steam vents are still active on Mount Hood, the last eruption from the volcano occurred in 1866. The volcano is considered dormant, but still actively monitored. Separate phases of eruptive activity produced pyroclastic flows and lahars — mudflows — that carried erupted materials down all of the major rivers draining the volcano. Gray volcanic deposits extend southwards along the banks of the White River (upper right), and form several prominent ridges along the southeast to southwest flanks of the volcano. The deposits contrast sharply with the green vegetated lower flanks of the volcano. The Mount Hood stratovolcano — a typically cone-shaped volcanic structure formed by interlayered lava flows and explosive eruption deposits — hosts twelve mapped glaciers along its upper flanks (center). Like other glaciers in the Pacific Northwest, the Hood glaciers have been receding due to global warming, and have lost an estimated 61 percent of their volume over the past century. The predicted loss of glacial meltwater under future warming scenarios will have significant effects on regional hydrology and water supplies.