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Transcript
Lava and Volcanoes Flows and Vents Topics • Explosivity of an Eruption • Lava • Volcanoes • Hawai’i Shield Volcanoes • Volcanic Features • Flank Collapse and Tsunamis Topics • Explosivity of an Eruption • Lava • Volcanoes • Hawai’i Shield Volcanoes • Volcanic Features • Flank Collapse and Tsunamis Explosivity of an Eruption Why some volcanoes explode • Magma can be erupted as lava or tephra • Tephra Pu’u ’O’o Explosivity Explosivity • The explosivity of an eruption depends primarily on two factors: Explosivity • The explosivity of an eruption depends primarily on two factors: 1. Gas content Explosivity • The explosivity of an eruption depends primarily on two factors: 1. Gas content 2. Viscosity of the magma Relationship Relationship • In general, the explosivity increases as the Relationship • In general, the explosivity increases as the gas content increases Relationship • In general, the explosivity increases as the gas content increases viscosity increases Viscosity Viscosity • The viscosity of a magma varies with Viscosity • The viscosity of a magma varies with composition Viscosity • The viscosity of a magma varies with composition temperature Relationship Relationship • The viscosity of a magma increases as the Relationship • The viscosity of a magma increases as the silica content increases Relationship • The viscosity of a magma increases as the silica content increases temperature decreases Felsic Volcanoes Felsic Volcanoes • Felsic volcanoes tend to erupt explosively because they Felsic Volcanoes • Felsic volcanoes tend to erupt explosively because they have a higher silica content Felsic Volcanoes • Felsic volcanoes tend to erupt explosively because they have a higher silica content erupt at lower temperatures (700-900oC) • More viscous felsic magmas inhibit the escape of gas • Gases build up until they are released cataclysmically • Felsic volcanoes generally form at subduction zones Volcanic Explosivity Index Mount Saint Helens Mount Saint Helens May 18, 1980 Volcanic Dome Volcanic Dome • Round, steep-sided dome built by very viscous magma Volcanic Dome • Round, steep-sided dome built by very viscous magma • Generally forms from a silica-rich magma Volcanic Dome • Round, steep-sided dome built by very viscous magma • Generally forms from a silica-rich magma • Such magmas typically are too viscous to flow far from the vent before cooling and crystallizing Volcanic Dome • Round, steep-sided dome built by very viscous magma • Generally forms from a silica-rich magma • Such magmas typically are too viscous to flow far from the vent before cooling and crystallizing • Domes might be made of one or more flows Mafic Volcanoes Mafic Volcanoes • The volcanoes in Hawai’i are mafic Mafic Volcanoes • The volcanoes in Hawai’i are mafic • Mafic volcanoes generally erupt less explosively because they Mafic Volcanoes • The volcanoes in Hawai’i are mafic • Mafic volcanoes generally erupt less explosively because they have a lower silica content Mafic Volcanoes • The volcanoes in Hawai’i are mafic • Mafic volcanoes generally erupt less explosively because they have a lower silica content erupt at higher temperatures (>1000oC) • Gases escape more readily from the less viscous mafic magma Next Lava
