UNDERSTANDINGVOLCANOS.ppt

Volcanoes Composite cone (stratovolcano) Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens) Large, classic-shaped volcano (1000’s of ft. high and several miles wide at base) Mt. St. Helens – prior to the 1980 eruption Mt. St. Helens (after the 1980 eruption) Volcanoes Composite cone (stratovolcano) -continued Composed of interbedded lava flows and pyroclastic debris Most violent type of activity (e.g., Mt. Vesuvius) Volcanic Hazards Nuée ardente – A Fiery pyroclastic flow made of hot gases infused with ash and other debris Also known as “glowing avalanches” Move down the slopes of a volcano with velocities approaching 200 km/hour Volcanic Hazards Lahar – volcanic landslide or mudflow Mixture of volcanic debris and water Move down slopes of volcano and stream valleys with velocities of 30 to 60 miles/hour volcanic hazards Pyroclastic flow Felsic and intermediate magmas Consists of ash, pumice, and other debris Material ejected at high velocities Example: Yellowstone plateau Other volcanic landforms Caldera Steep-walled depressions at the summit Generally 1 km in diameter Produced by collapse Example: Crater Lake, Oregon Other volcanic landforms Fissure eruptions and lava plateaus Fluid basaltic lava extruded from crustal fractures called fissures Example: Columbia River Plateau Lava domes Bulbous mass of congealed lava Associated with explosive eruptions of gas-rich magma Other volcanic landforms Volcanic pipes and necks Pipes - short conduits that connect a magma chamber to the surface Volcanic necks (e.g., Ship Rock, New Mexico) - resistant vents left standing after erosion has removed the volcanic cone Shiprock, New Mexico Intrusive igneous activity Most magma is emplaced at depth in the Earth Once cooled and solidified, it is called a pluton Intrusive igneous activity Nature of plutons Shape - tabular (sheetlike) vs. massive Orientation with respect to the host (surrounding) rock Concordant vs. dis