- Slides: 24
Mount St. Helens
Formation of volcanoes • Under certain conditions, small amounts of mantle rock can melt, forming magma. • The magma rises upward through the crust, erupting at the surface as a volcano. • Magma rises because it is less dense than the solid rock around it.
How a volcano erupts • Similar to soda bottle after it has been shaken • Magma is under pressure and contains dissolved gases (CO 2 and water vapor)
How a volcano erupts • As magma approaches surface, the lower pressure allows gases in magma to expand rapidly. • Eruption occurs when gases bubble out through a crack in crust, propelling magma to the surface.
Structure of a volcano Crater Magma collects in a pocket called a magma chamber before an eruption. Vent A vent is an opening in the ground where magma escapes. Pipe A crater is a bowl shaped pit at to the surface. theistop of mostvertical volcanoes. A pipe a narrow, Often ONE central vent at top channel of through which magma the side. volcano, sometimes rises to Earth’s surface. Magma Chamber Tracy Saxby, Integration and Application Network, University of Maryland Center for Environmental Science (ian. umces. edu/imagelibrary/).
Caldera • After eruption, magma chamber and main vent may empty of magma…hollow shell. • Collapses inward, creating a huge depression called caldera. VHP Photo Glossary: Caldera. Aerial view of Aniakchak Caldera, Alaska; . . . volcanoes. usgs. gov
Eruptions • Volcanoes erupt explosively or quietly, depending on the characteristics of magma. • Viscosity = the resistance to flow. High viscosity = thick, resists flowing Low viscosity = thin, flows easily
Viscosity of Magma • 3 factors that determine viscosity of magma: – Temperature – Water content – Silica content
Temperature • Higher temperature = lower viscosity • Magma will flow more easily
Water Content • Greater water content = lower viscosity • The presence of water in magma helps it flow more easily.
Silica Content • High silica content = high viscosity • Silica is made up of silicon and oxygen that have strong bonds. • The silica in magma acts like glue, preventing the magma from flowing easily.
Quiet Eruptions Very HOT, low silica magma Lava flow = stream of low viscosity lava Lava flows can travel great distances Pahoehoe = HOT, fast moving lava, ropelike surface • Aa = cooler, slow moving lava, crumbly appearance • •
Explosive Eruptions • High silica magma • Thick magma can clog pipe, causing enormous pressure to build up. Trapped steam adds to pressure. • EXPLODES – lava and hot gases are hurled outward The June 12, 1991 eruption column from Mount Pinatubo taken from the east side of Clark Air Base. U. S. Geological Survey Photograph taken on June 12, w 1991, by Dave Harlo
Lava from Explosive Eruption • Lava cools quickly shattering into pieces • Range in size: – Dust and ash – Cinders (pebble sized) – Bombs (several cm to size of small car) Ash and volcanic rocks from the Mount Merapi volcano cover deserted houses in Central Java. Photograph by C. Heliker on January 26, 1988 Tarko Sudiarno/AFP/ Getty Images
Location • Most volcanoes occur… – Along plate boundaries – Hot spots
Plate Boundaries • Volcanoes occur most often at convergent plate boundaries where oceanic plate is subducting into the mantle. • Example: Ring of Fire volcano. oregonstate. edu
Hot Spots • Hot spot = region where hot rock extends from deep within the mantle to the surface. • Hot spot stays in place while plate moves over it…results in a chain of volcanoes over time. volcano. oregonstate. edu
Three Major Types of Volcanoes • Shield volcano • Cinder cone • Composite volcano
Shield Volcano • • Low viscosity lava Quiet eruptions Wide, flat volcano Example: Hawaii Photograph by D. Little (date unknown). View of the NNW flank of Mauna Loa Volcano from the south side of Mauna Kea Volcano, Hawai`i; both are shield volcanoes.
Cinder Cone • • Explosive eruption Composed of ash and cinders Steep sides, small Example: Mt. Etna, Italy Photograph by J. Lowenstern
Composite Volcano (Stratovolcano) • Explosive eruptions that produce a combination of lava and ash • Steep sides, tall • Example: Mt. Shasta, California Mount Shasta and Shastina, California. USGS Photograph taken by Lyn Topinka, 1984
Other igneous features • Sometimes magma does not reach the surface, but hardens in the crust. • Batholith = largest of igneous intrusions, often form core of mountain range (Sierra Nevada) • Sill = hardened lava squeezed into a crack that is parallel to existing rock layers. • Dike = hardened lava squeezed into a crack that cuts across rock layers. http: //www. tulane. edu/~sanelson/geol 212/intro&textures. htm
Other igneous features • Volcanic neck = magma hardens in a volcano’s pipe By occecid Devils Tower (Lakota: Mato Tipila, which means “Bear Tower”) is a monolithic igneous intrusion or volcanic neck located in the Black Hills near Hulett and Sundance in Crook County, northeastern