Igneous Activity

• Origin of magma
  • Magma originates when essentially solid rock, located in the crust and upper mantle, melts
    
  • Factors:
    
    • Role of heat
      
      • Earth’s natural temperature increases with depth (geothermal gradient) is not sufficient to melt rock at the lower crust and upper mantle
        
      • Additional heat is generated by
        
        • Friction in subduction zones
          
        • Crustal rocks heated during subduction
          
        • Rising, hot mantle rocks
          
    • Role of pressure
      
      • Increase in confining pressure causes an increase in melting temperature
        
      • Drop in confining pressure can cause decompression melting
        
      • Lowers the melting temperature
        
      • Occurs when rock ascends
        
    • Role of volatiles
      
      • Primarily water
        
      • Cause rock to melt at a lower temperature
        
      • Play an important role in subducting ocean plates
        
    • Partial melting
      
      • Igneous rocks are mixtures of minerals
        
      • Melting occurs over a range of temperatures
        
      • Produces a magma with a higher silica content than the original rock
        
        
• Where do volcanoes form?
  • Volcanoes form at:
    
  • Hot Spots (10% of all volcanic activity)
    
  • Spreading Centers (80% of all volcanic activity)
    
  • Convergent Plate Boundaries (10% of all volcanic activity)
    
    • Ocean–Continental
      
    • Ocean – Ocean
  • Plate tectonics and igneous activity
    
    • Global distribution of igneous activity is not random
      
      • Most volcanoes are located on the margins of the ocean basins (intermediate, andesitic composition)
        
      • Second group is confined to the deep ocean basins (basaltic lavas)
        
      • Third group includes those found in the interiors of continents
        
    • Plate motions provide the mechanism by which mantle rocks melt to form magma
      
      • Convergent plate boundaries
        
        • Descending plate partially melts & magma slowly rises upward
          
        • Rising magma can form
          
        • Volcanic island arcs in an ocean (Aleutian Islands)
          
        • Continental volcanic arcs (Andes Mountains)
          
      • Divergent plate boundaries
        
        • Produces the greatest volume of volcanic rock
          
        • Lithosphere pulls apart
          
        • Less pressure on underlying rocks
          
        • Partial melting occurs
          
        • Large quantities of fluid basaltic magma are produced
    • Intraplate igneous activity
      
      • Activity within a rigid plate
        
      • Plumes of hot mantle material rise
        
      • Form localized volcanic regions called hot spots the Hawaiian Islands and the Columbia River Plateau
        
        
• Volcanic Eruptions
  • Factors that determine the violence of an eruption
    
    • Composition of the magma
      
    • Temperature of the magma
      
    • Dissolved gases in the magma
  • The above three factors actually control the viscosity of a given magma which in turn controls the nature of an eruption
    
  • Viscosity is a measure of a material's resistance to flow
    
  • Factors affecting viscosity
    
    • Temperature
      
      • hotter magmas are less viscous
        
    • Composition (silica content)
      
      • High silica – high viscosity (e.g., rhyolitic lava)
        
      • Low silica – more fluid (e.g., basaltic lava
        
    • Dissolved gases (volatiles)
      
      • Mainly water vapor and carbon dioxide
        
      • Gases expand near the surface
        
      • Provide the force to extrude lava
        
      • Violence of an eruption = how easily gases escape
        
  • Plate-Tectonic Setting of Volcanoes Revisited
    
    • Why more volcanic activity at spreading centers?
      
      • Low SiO2 content
        
      • High temperature
        
      • Low pressure as plates pull apart
      • Fluid basaltic lavas generally produce quiet eruptions
        
    • Why less volcanic activity at subduction zones?
      
      • High SiO2 content
        
      • Lower temperatures
        
      • Higher pressures
      • Highly viscous lavas produce more explosive eruptions
        
        
• Extrusive Igneous Activity
  • Volcanoes
    
    • General features
      
      • Conduit, caries gas-rich magma to the surface
        
      • Vent, the surface opening (connected to the magma chamber via a pipe)
        
      • Crater, steep-walled depression at the summit
        
      • Caldera (a summit depression greater than 1 km diameter)
        
      • Parasitic cones
        
      • Fumaroles
  • Types of volcanoes
    
    • Shield volcano
      
      • Low Viscosity, Low Volatiles, Large Volume
        
      • Broad, slightly domed
        
      • Primarily made of basaltic (fluid) lava
        
      • Generally large size
        
      • Hawaiian Islands
        
    • Cinder cone
      
      • Low Viscosity, Medium Volatiles, Small Volume
        
      • Built from ejected lava fragments
        
      • Steep slope angle
        
      • Rather small size
        
      • Frequently occur in groups
        
      • Sunset Crater, Flagstaff, Arizona
        
    • Composite cone (or stratovolcano)
      
      • High Viscosity, High Volatiles, Large Volume
        
      • Most are adjacent to the Pacific Ocean
        
      • Large in size
        
      • Interbedded lavas and pyroclastics
        
      • Mt. St. Helens – a typical composite volcano
        
        • Mt. St. Helens following the 1980 eruptionMt. St. Helens
          
        • Mt. St. Helens, October 1, 2004
          
      • Often produce nuée ardente
        
      • May produce a lahar - volcanic mudflow
        
      • Eruptions in the Cascades Ranges
        
    • A size comparison of the three types of volcanoes
      
  • Other volcanic landforms
    
    • Calderas
      
      • High Viscosity, High Volatiles, Very Large Volume
        
      • Steep walled depression at the summit formed by collapse
        
      • Size exceeds one kilometer in diameter
        
      • Formation of Crater Lake
        
    • Fissure eruptions and lava plateaus
      
      • Low Viscosity, Low Volatiles, Very Large Volume
        
      • Basaltic lava extruded from crustal fractures
        
      • Incredibly large volumes of lava pour out of fissures over 2-3 million years
        
      • Can affect global climate
        
      • Formation of Flood Basalts
        
      • The Columbia River basalts
        
    • Lava Domes
      
      • Bulbous mass of congealed lava
        
      • Most are associated with explosive eruptions of gas-rich magma
      • One is currently developing in Mt. St. Helens
        
    • Volcanic pipes and necks
      
      • Pipes are short conduits that connect a magma chamber to the surface
        
      • Volcanic necks (e.g., Ship Rock, New Mexico) are resistant vents left standing after erosion has removed the volcanic cone
        
        
• How Volcanoes Cause Damage
  • Lava Flow Eruption
    
    • Kapoho, Hawaii example
      
      • The January 1960 flank eruption followed the December 1959 summit eruption
        
      • seismic swarms indicated the eruption was coming
        
      • Diversion walls were built but also destroyed by the lava
        
      • Kapoho, a farming village, was buried http://hvo.wr.usgs.gov/kilauea/history/1960Jan13/
        
    • Generally slow moving and not a threat to cities
      
      • Basaltic lavas are much more fluid than andesitic lavas
        
      • Types of basaltic flows
        
        • Pahoehoe lava (resembles a twisted or ropey texture)
          
        • Aa lava (rough, jagged blocky texture)
          
    • Fissure eruptions and lava plateaus
      
      • Fluid basaltic lava extruded from crustal fractures called fissures
        
      • e.g., Columbia River Plateau
        
  • Gases
    
    • One to 5 percent of magma by weight
      
    • Mainly water vapor and carbon dioxide How Volcanoes Cause Damage
      
  • Cameroon
    
    • August of 1986 Lake Nyos
      
      • 1 km of CO2 released
        
      • ~1700 people killed up to 26 km away from the lake
        
    • August of 1984
      
      • smaller gas burst from Lake Monoun
        
      • 37 people killed
        
  • Pyroclastic materials
    
    • Ash and dust – fine, glassy fragments
      
    • Pumice – from "frothy" lava
      
    • Cinders – "pea-sized"
      
    • Lapilli – "walnut" size
      
    • Particles larger than lapilli
      
    • Blocks
      
    • Bombs
      
  • Explosions and Ashflows
    
    • Ashflows are mixtures of hot gas and ash that move very quickly along the ground
      
    • Pompeii was destroyed by Ashflows
      
    • Mt St. Helens
      
      • Eruption began in late March and climaxed May 18, 1980
        
      • A dome pushed into the north flank over steepening it
        
      • A landslide followed and caused a huge explosion
        
      • 0.5 cubic miles of rock fell into Spirit Lake causing mudflows
        
      • Ashflow traveling 150 miles/hr traveled 18 miles devastating 215 sq miles (Temperature = 300°C )
        
  • Ashfall
    
    • Huge areas may be covered by volcanic ash
      
      • Crater Lake ash covers the entire Northwest
        
    • Damage to urban areas can be enormous
      
      • Crops are destroyed threatening the food supply
        
      • Public water contaminated
        
      • Buildings collapse under weight of ash
        
      • Air travel disrupted
        
  • Mudflows
    
    • Ways that volcanoes make mudflows
      
      • Burn vegetation
        
      • Erupt ash
        
      • Produce rain
        
      • Melt glaciers or displace lakes
        
  • Nuée Ardente (or pyroclastic flow)
    
  • Fiery pyroclastic flow made of hot gases infused with ash
    
  • Flows down sides of a volcano at speeds up to 200 km (125 miles) per hour
    
  • Mudflows
    
    • St. Helens example
      
      • Mudflow was caused by the displaced Spirit Lake
        
      • Mudflow went 60 miles to the Columbia River
        
      • 45 million cubic yards sediment entered Columbia River
        
    • Nevado del Ruiz, Columbia
      
      • 2 eruptions on Nov 13, 1985 melted the summit glaciers
        
      • Mudflows travelled in all directions from the summit
        
      • Mud traveling 30 mph and 50 feet deep buries Amero 30 miles away
        
      • 25,000 killed
        
    • Mt. Rainier
      
      • Mudflows threaten the towns and villages blow this dangerous volcano
        
      • Evacuation plans and drills are the key to survivalA nueé ardente on Mt. Saint Helens
        
      • A lahar along the Toutle River near Mt. St. Helens
        
  • Caldera Collapse
    
  • Believed to be caused by magma evacuating its chamber
    
  • Caldera eruptions in New Zealand could damage cities like Auckland
    
  • Famous (or infamous) collapsed calderas:
    
    • Crater Lake, Oregon
      
    • Yellowstone, Wyoming
      
    • Long Valley Caldera, California
      
    • Krakatoa, Indonesia
      
  • Crater Lake
    
    • About 6,850 years ago Mount Mazama erupted
      
    • Caldera collapsed and produced Crater Lake
      
    • Eruption released ~12 cubic miles (50 cubic km) of magma to the surface.
      
    • One of the largest eruptions in the last 10,000 years.
      
  • Yellowstone, Wyoming
    
    • A Hot Spot Volcano
      
    • Three very large eruptions in the last 2 million years
      
    • 2.0, 1.3, and 0.6 million years ago
      
    • Still active today
      
  • Long Valley Caldera, California
    
    • Volcanic activity began in the area ~3.6 million years ago
      
    • Catastrophic eruption ~730,000 years ago
      
    • Bishop Tuff
      
    • Mammoth Mountain formed along the southwest rim of Long Valley caldera from 200,000 to 50,000 years ago
      
  • Krakatoa
    
    • Inactive for 200 years before 1883
      
    • Eruption began in May and climaxed on August 26 & 27
      
      • Lava, ash, and gas erupted
        
      • Ash covered neighboring islands
        
      • Suddenly 10 sq miles collapsed
        
      • A strong earthquake occurred
        
      • Sound could be heard 3000 miles away
        
      • Tsunami over 100 feet high killed 36,000 people in Java and Sumatra
        
        
• Volcanic Risks
  • Benefits of Volcanoes
    
    • Produce great amounts of new land
      
    • Frequently produce very fertile soils
      
    • Provide Geothermal Power
      
    • Recreation
      
  • Methods Of Forecasting Volcanic Eruptions
    
    • Geophysical methods
      
      • Seismology
        
        • Structure of the volcano can be determined
          
        • Movement of the magma can be traced
          
      • Gravity Surveys
        
        • Can recognize inflation and deflation
          
    • Geophysical methods
      
      • Precise surveying
        
        • Reveals changes in shape
          
        • Tiltmeter measurements reveal inflation and deflation
          
      • Heat flow studies
        
      • Geochemical methods
        
        • The Cl, F, and S composition of hot springs may change before eruptions
          
        • Has worked in Japan
          
      • Geochronology
        
        • Precise dating may reveal regular repeat intervals
          
  • Evaluation Of Volcanic Risk In California
    
    • Population changes since 1915
      
      • Last violent eruption was Mt Lassen in 1915
        
      • California population was 2,800,000
        
      • 1999 population was >34,000,000
        
      • Some of this population has extended into volcanically hazardous areas
        
    • The 3 Most Dangerous Areas in California
      
      • Mt Shasta especially around Weed on west side - ashflows
        
      • Mt Lassen - mudflows and rock avalanches
        
      • Long Valley - Mammoth Mtn area
        
        
• Intrusive Igneous Activity
  • Plutons are classified according to
    
    • Orientation with respect to the host (surrounding) rock
      
      • Discordant – cuts across existing structures
        
      • Concordant – parallel to features such as sedimentary strata
        
  • Types of igneous intrusive features
    
    • Dike, a tabular, discordant pluton
      
    • Sill, a tabular, concordant pluton
      
      • e.g., Palisades Sill, NY
        
      • Resemble buried lava flows
        
      • May exhibit columnar joints
        
    • Laccolith
      
      • Similar to a sill
        
      • A sill in the Salt River Canyon, Arizona
      • Lens shaped mass
        
      • Arches overlying strata upward
        
    • Batholith
      
      • Largest intrusive body
        
      • Often occur in groups Surface exposure 100+ square kilometers (smaller bodies are termed stocks)
        
      • Frequently form the cores of mountains
        
      • A batholith exposed by erosion
        
  • Emplacement of magma
    
    • Magma at depth is much less dense than the surrounding rock
      
    • Increased temperature and pressure causes solid rock to deform plastically
      
    • The more buoyant magma pushes aside the host rock and forcibly rises in the Earth as it deforms the “plastic” host rock
      
    • At shallower depths, the host rock is cooler and exhibits brittle deformation
      
    • Movement of magma here is accomplished by fractures in the host rock and stoping
      
    • Overall, the emplacement of magma is very similar to the emplacement and intrusion of salt domes

 

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LINKS

• California
  • Long Valley Caldera and Mono-Inyo Craters Volcanic Field, California
• Cameroon
  • Using Science to Solve Problems: The Killer Lakes of Cameroon
  • Volcanic Lakes and Gas Releases
• Columbia
  • CVO - Nevado del Ruiz, Colombia
  • Nevado del Ruiz, Colombia
• Greece
  • The Eruption of Thera
• Hawai'i
  • The 1960 Kapoho Eruption of Kilauea Volcano, Hawai`i
  • Kilauea Volcano
  • Volcanic and Seismic Hazards on the Island of Hawaii: Volcanic Hazards
• Kratatoa
  • THE EXPLOSION OF KRAKATOA MOUNTAIN
  • The Krakatoa Stories
  • Krakatau, Sundra strait
  • What happened in the Krakatoa eruption in the 1800's?
• Oregon
  • Crater Lake
• Washington
  • CVO - Mount Rainier, Washington
  • History of Landslides and Debris Flows at Mount Rainier, Washington
  • Mount Rainier, Washington
  • Mt. Rainier Mudflow (lahar) Warning System
  • Mount St. Helens, Washington - May 18, 1980 Eruption
  • NOAA Images - The Eruption of Mount Saint Helens
• Yellowstone
  • The Snake River Plain and the Yellowstone Hot Spot
• General
  • Deadliest Volcanic Eruptions Since 1500 A.D.
  • Potentially Active Volcanoes of the Western United States
  • Video Clips on VolcanoWorld
  • Volcanoes in the North and Central American Region
  • World Organization of Volcano Observatories (WOVO) - North America and the West Indies
    • Alaska Volcano Observatory
    • David Johnston Cascades Volcano Observatory
    • The Hawaiian Volcano Observatory
    • Instituto de Geofisica, Mexico
    • Centro Universitario de Investigacion en Ciencias de la Tierra (CUICT) - Colima, Mexico
    • Observatoire Volcanologique de la Soufriere, Guadeloupe
    • Observatoire Volcanologique de la Montagne Pele
    • Seismic Research Unit, University of the West Indies

Origin of magma | Where do volcanoes form? | Volcanic Eruptions | Extrusive Igneous Activity | How Volcanoes Cause Damage | Volcanic Risks | Intrusive Igneous Activity | Links | top