The Nature of Volcanic Eruptions Chapter 10, Section 1

Factors Affecting Eruptions The primary factors that determine whether a volcano erupts violently or quietly include magma composition, magma temperature, and the amount of dissolved gases in the magma Viscosity – a substance’s resistance to flow Something that is more viscous flows more slowly; as a lava flow cools, it becomes more viscous as the lava slows down The more silica there is in a lava, the more viscous that lava is During explosive eruptions, the gases trapped in magma provide the force to eject molten rock from the vent, an opening to the surface

Factors Affecting Eruptions

Volcanic Material The less silica content there is in a basaltic lava, the faster it will flow, lava flows are common in Hawaii Magma contains varied amounts of dissolved gases held in the molten rock by confining pressure, it only is about 1 to 6 percent of the total weight of the magma It is important to study the composition of volcanic gases, because they are the gases that formed the atmosphere in the past Pyroclastic material – name given to particles produced in volcanic eruptions The fragments ejected during eruptions range in size from very fine dust and volcanic ash (less than 2 millimeters) to pieces that weigh several tons (blocks and lava bombs)

Volcanic Material

Concept Check What is a volcanic bomb? A large streamlined chunk of pyroclastic material that is larger than 64 mm in diameter.

Anatomy of a Volcano The three main volcanic types are shield volcanoes, cinder cones, and composite cones Volcanic activity often starts with a fissure, or crack, develops and magma is forced through it Volcano – a mountain built by repeated eruptions of lava or pyroclastic material often separated by long inactive periods Crater – steep-walled depression located at the top of many volcanoes The form of the volcano is largely determined by the composition of the magma

Anatomy of a Volcano

Shield Volcanoes Shield Volcano – produced by the accumulation of fluid basaltic lavas Shape – broad, slightly domed structure that resembles a warrior’s shield Most have grown from the ocean-floor to form islands (Hawaii and Iceland)

Shield Volcanoes

Cinder Cones Cinder Cone – built by ejected lava fragments the size of cinders, which harden in the air Product of relatively gas-rich basaltic magma Shape – determined by the steep-sided slope that loose pyroclastic material maintains as it comes to rest Usually the product of a single eruption that lasts only a few weeks, rarely a few years The magma in the pipe solidifies after the eruption, and the volcano doesn’t erupt again Cinder cones are small, ~30-300 meters and less than 700 meters in height There are thousands of cinder cones around the world

Cinder Cones

Composite Cones Composite Cone (Stratovolcano) – large, nearly symmetrical structure composed of layers of both lava and pyroclastic deposits The silica-rich magmas typical of composite cones generate viscous lavas that can only travel short distances May generate the most explosive eruptions that eject huge quantities of pyroclastic materials About 50 of these have erupted in the U.S. in the last 200 years The most dangerous results of one of these eruptions is a pyroclastic flow (hot gases and rock fragments), coming at a speed of 200 km/hr Lahars – destructive mudflow created when volcanic debris becomes saturated with water and rapidly moves down the volcano, often following stream valleys

Composite Cones

Concept Check What is a lahar? A mudflow down the slope of a volcano.

Profiles of Volcanic Landforms

Other Volcanic Landforms Caldera – a large depression in a volcano Forms by: (1) collapse of the top of a composite volcano after an explosive eruption, or (2) collapse of the top of a shield volcano after the magma chamber is drained Most volcanoes are fed magma through conduits, called pipes, connecting a magma chamber to the surface When the volcano has been eroded, especially cinder cones, the harder solidified magma will remain and become a volcanic neck Lava Plateau – the greatest volume of volcanic material doesn’t build volcanoes, but is extruded through fissures and flows over a large area

Formation of Crater Lake, Oregon

Volcanic Areas in the Northwestern United States

Assignment Read Chapter 10 Chapter 10 Assessment