Images

Scientists monitor data in the California Volcano Observatory operations room, Menlo Park, California.

Table of ash fall severities based on accumulated thickness.

Heavy ashfall (20-30 mm) from Tungurahua, Ecuador, 2006.

Collapsed roofs after severe ashfall from Galunggung Volcano, West Java, Indonesia.

Minor ashfall on hay field, Augustine eruption, Homer area, Alaska, 1986.

Dusting of Klyuchevskoy ash on camera bag, 2006.

Trace of Augustine ash on car hood, Homer, Alaska, January 2006.

Moderate ash on corn crops from eruption of Tungurahua, Ecuador, 2007.

Aerial view of Mount St. Helens, taken from the southwest.

Oceanic crust forms by eruptions along the Juan de Fuca ridge. As the Juan de Fuca plate drifts eastward, it cools, becomes more dense, and eventually dives under the less dense continental plate at the Cascadia trench.

Magma forms above the subducting slab of oceanic crust and accumulates at the base of Earth's rigid crust before collecting in a storage zone 13 km (8 mi) beneath the volcano prior to eruption.

Surveyor Frank Dodge's 1894 cross-section of Halema‘uma‘u overlaid on his 1892 cross-section. The 1892 lava lake was measured at 73 m (240 ft) below the rim of Halema‘uma‘u pit and by early-1894, the lava lake had filled the pit and frequently overflowed onto the caldera floor.

Photograph taken on March 20, 1894 looking up at the Halemaʻumaʻu lava lake perched atop a low dome on the floor of Kaluapele (Kīlauea caldera).

The painting by Jules Tavernier, depicting the 1880-81 lava flow from Mauna Loa as it enters a stream near Hilo, Hawaii on July 20, 1881, provided courtesy of the National Park Service, Hawaii Volcanoes National Park (catalog number HAVO 806).

Strong shaking during the 1868 Hayward Fault earthquake caused the second story of the Alameda County Courthouse in San Leandro to collapse (photo courtesy of the Bancroft Library, University of California). The inset photo shows the courthouse before the quake