May 18, 1980

• Mount St. Helens
• Largest historic volcanic eruption in North America
• 59 deaths from blast, intense heat, suffocation, and mudflows
• Ejected nearly km3 of ash and rock
• Contrast this with the eruption of Kilauea

• Extrusive activity
• Volcano - mountain built up by accumulation of material ejected from a vent (opening in crust)

• Products of extrusive activity
• Lava
• Pyroclastics - particles ejected from volcanic vent; tephra
• dust -> ash -> cinders -> lapilli -> bombs or blocks

• Felsic (Rhyolitic; Granitic magma)
• Intermediate (Andesitic)
• Mafic (Basaltic)

Volcanic Gases

• Volcanic gases
• Water vapor, 70 - 95%
• Carbon dioxide (CO2)
• Sulfur dioxide (SO2)
• Others - nitrogen, hydrogen, carbon monoxide (CO), sulfur, and chlorine
• Origin of gases
• Water recycled groundwater and seawater
• Some gases may be juvenile (from mantle)

• Mt. St. Helens, Washington state (May 18, 1980)
• Mt. Pinatubo, The Philippines (1991)
• Popocatepetl, central Mexico (1997-2001)
• http://www.discovery.com/news/earthalert/earthalert.html

• Magma chamber
• Central vent or pipe
• Crater
• Pyroclastic layers (could also be lava layers or alternating lava and pyroclastics)

• Pyroclastic rocks (Review)
• Tuff, welded tuffs - mostly cemented ash
• Pumice - vesicular, glassy
• Volcanic breccia - large angular fragments cemented together

• Features of lavas
• Pahoehoe - ropy
• Aa - blocky
• Pillow basalt - bulbous surface; extrusion under water
• Columnar basalt - fractures associated with shrinking

• Determined by nature of material (e.g., proportion of lava to pyroclastics)
• Shield volcano - low, broad slope; built mostly of basaltic flows (Hawaiian Islands, Iceland)

• Composite cone (Stratovolcano) - Alternating lava and pyroclastics; classic cone-shape (Mt. Lassen, Mt. Fuji, Mt. Rainier, Mt. St. Helens)

• Volcanic dome - viscous magma fills crater; forms dome; short-lived (Ex: Mono Craters, CA)
• Cinder cone - steep slopes; layers of pyroclastics; <300 m high (Ex.: Paricutin, Mexico; Sunset Crater, Arizona)
• Caldera - volcano with unusually large crater; collapsed or blown volcano (Crater Lake, Jemez Caldera)

• Fissure eruptions - eruptions through deep fractures rather than through central vents
• flood basalts (Columbia Plateau)

• Ash-flows - eruptions of pyroclastics
• Lahars - catastrophic mudflows of volcanic debris
• Remnants of volcanoes
• Volcanic neck - basalt-filled remnant of vent
• Volcanic pipe - cylindrical dike; conduit
• Kimberlite pipe
• Diatreme - breccia-filled remnant of vent (Ship Rock)

• Divergent Plate Boundaries
• Greatest volume
• Basalt
• Fissure eruptions at MOR
• Convergent Plate Boundaries
• Ocean-ocean convergence
• Basalts and andesites (Ex: Japan, Philippines)
• Ocean-continent convergence
• Basaltic to felsic lava (Ex: Cascade Range, Parícutin)

• Intraplate Volcanism - Hot spots (Emperor Seamount Chain, Hawaii)

• Approaches
• Eruption history
• Radiometric dating of lava and/or fossilized wood
• Terms
• Active - erupting or expected to erupt
• Dormant - not erupting but will likely erupt in future
• Extinct - not expected to erupt
• Seismic activity
• Tilting
• Gas emissions
• Increased temperature of hot springs
• Success stories (Mt. St. Helens, Mt. Pinatubo)

• Volcanic dust blocks sunlight
• Eruption of Krakatoa lowered global temperatures a few degrees
• Volcanoes contribute CO₂, a greenhouse gas
• Unusual volcanic activity could cause warming

• Volcanoes of the World
• http://volcano.und.nodak.edu/vwdocs/volc_images/volc_images.html
• Volcanoes of North America
• http://volcano.und.nodak.edu/vwdocs/volc_images/north_america/north_america.html
• Mount Fuji
• http://volcano.und.nodak.edu/vwdocs/volc_images/img_fuji.html