Volcanoes and Volcanism Nature of Volcanoes - Status

1. Volcanoes and VolcanismNature of Volcanoes - Status • Active – Currently erupting or within last few hundred yrs. • K’ilauea – Hawaii (current) • Stromboli – Mediterranean (current) • Lassen – Northwest U.S. (1917) • All continents except Australia • All major ocean basins • Indonesia – 76 • Japan – 60 • United States -53

2. Active Volcanism • Lava fountain seen at night from eruption on Mt. K’ilauea, Island of Hawaii

3. Active Volcanism • Lava flow reaching the ocean on Island of Hawaii

4. Nature of Volcanoes • Dormant – erupted in last few thousand yrs. and is likely to do so again • Shallow heat source • Surface bulge • Increased seismic activity

5. Nature of Volcanoes • Extinct – has not erupted for 10’s of thousands of years and is not likely to erupt again • Shiprock • Hopi Buttes • Picacho Peak

6. Eruption Potential Characteristics • Gas content • Gas remains dissolved at depth due to pressure • 1-9% of magma • H2O water vapor • CO2 carbon dioxide • N2 nitrogen • SO2 sulfur dioxide – smell • Viscosity – resistance to flow • Temperature • > heat/temp : < viscosity • Silica content • > silica : > viscosity

7. Types of Lava • Basalt (Bonito Lava Flow at Sunset Crater or the Hawaiian Islands) • Mafic (dark) • High temp • Low silica content • Low viscosity • Releases gas easily • Fluid eruptions • Pillow lavas – subaqueous • Aa – blocky and rough • Pahoehoe – smooth and ropey • Scoria – vesicular basalt

8. Types of Basalt Flows • AA – lava flow : Hawaii • Pahoehoe – lava flow : Hawaii

9. Intermediate (Andesitic) Lava Andesite (San Francisco Peaks, Mt. Rainier, Mt. St. Helens)Intermediate in colorModerate silica contentModerate viscosityAlternate flows and pryoclastic eruptionsDangerous violent eruptions

10. Composite Cone/StratovolcanoAndesitic lahar (mudflow) Mt. St. Helens Andesite (San Francisco Peaks, Mt. Rainier, Mt. St. Helens) Intermediate in color Moderate silica content Moderate viscosity Alternate flows and pryoclastic eruptions Dangerous violent eruptions

11. Types of Lava • Rhyolitic (Yosemite, Crater Lake) • Felsic (light) • Lower temps • Higher silica content • High viscosity • Resists gas release • Short, stubby flows • Violent, particulate eruptions • Ashflows, tephras, etc • pumice

12. Pyroclastic FlowsRhyolitic ash flows from Explosion and collapse of Mt. Mazama7,700 ya creating Crater Lake

13. Pyroclastics • Explosive ejecting solid material and gas into the atmosphere • Volcanic blocks – country rock inclusions • Tephra • Dust • Ash • Cinders • Bombs • Pyroclastic flows – (nuee ardente) • Volcanic mudflows - lahar

14. Pyroclastic Flows • Pumice Pyroclastic flow : Mt. St. Helens

15. Secondary Effects • Acid rain – sulfur dioxide + water = sulfuric acid • Climate – Toba Eruption (p. 127) • Biologic • Economic

16. Eruptive Styles / Landforms • Effusive • Quiet, fluid events • Lava flows • Central vents • Cones and shields • Fissure eruptions • Flood basalts – land • New ocean crust - subaqueous • Pyroclastic • Explosive, violent events • Tephras • Domes • Ashflows

17. Types of Volcanoes • Two main components • Cone – steep-sided hill formed from sequence of eruptions • Crater – (caldera) depression surrounding vent usually caused by collapse after eruption leaves vacant chamber beneath • Central Vents • Shield – large cone with broad, gentle slopes formed by quiet, low-viscosity eruptions (Mauna Kea/Mauna Loa) • Strato-volcano – large steep-flanked cone, composed of numerous types of eruptive materials (San Francisco Mtn.) • Cinder – Usually small steep-walled cone composed of tephra deposits

18. Prediction • Seismic swarms – movement of magma below a volcano • Crustal deformation – tilting and bulging of surface • Glacial and snow melting – increase in subsurface geothermal gradient • Changes in water tables levels and composition (i.e. gasses in solution)

19. Shield Volcano • Belknap Shield Volcano, Oregon – Shield Volcanoes are built almost entirely of fluid lava flows. Flow after flow pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of flat, domical shape, with a profile much like that a a warrior's shield. They are built up slowly by the accretion of thousands of flows of highly fluid basaltic (from basalt, a hard, dense dark volcanic rock) lava that spread widely over great distances, and then cool as thin, gently dipping sheets.

20. Composite/Stratovolcano • Mt. Rainier, Washington – Composite cones are formed by repeated eruptions of various types, i.e. flows, pyroclastic flows, ash-falls, etc. usually composed of more silicic lavas, that build-up over time. Eventually, this layering forms a fairly steep-sided cone, of complex form and composition.

21. Cinder Cone • Sunset Crater, Arizona Cinder Cones are the simplest type of volcano. They are built from particles and blobs of congealed lava ejected from a single vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a circular or oval cone. Most cinder cones have a bowl-shaped crater at the summit and rarely rise more than a thousand feet or so above their surroundings. Cinder cones are numerous in western North America as well as throughout other volcanic terrains of the world.

22. Volcanic Dome • Novarupta Dome, Katmai Vicinity, Alaska Volcanic domes are mounds that form when viscous lava is erupted slowly and piles up over the vent, rather than moving away as a lava flow. The sides of most domes are very steep and typically are mantled with unstable rock debris formed during or shortly after dome emplacement. Most domes are composed of silica-rich lava which may contain enough pressurized gas to cause explosions during dome extrusion.

23. Volcanic Crater – Crater Lake The largest and most explosive volcanic eruptions eject tens to hundreds of cubic kilometers of magma onto the Earth's surface. When such a large volume of magma is removed from beneath a volcano, the ground subsides or collapses into the emptied space, to form a huge depression called a caldera. Some calderas are more than 25 kilometers in diameter and several kilometers deep.

24. Types of Volcanoes • Fissure Eruptions – eruptive events that occur along a system of linear fractures or vents • Flood Basalts • Large volume eruptions that form lava plateaus extending over thousands of square kms and are hundreds to thousands of feet thick (Snake River Plain) • Submarine or mid-ocean ridges • Pillow basalts

25. Subaqueous EruptionsPillow Basalts • Lava, erupting onto a shallow sea floor or flowing into the sea from land, may cool so rapidly that it shatters into sand and rubble. The result is the production of huge amounts of fragmental volcanic debris. The famous "black sand" beaches of Hawaii were created virtually instantaneously by the violent interaction between hot lava and sea water. On the other hand, recent observations made from deep-diving submersibles have shown that some submarine eruptions produce flows and other volcanic structures remarkable similar to those formed on land. ...

26. Pillow Basalts • Pillow basalts along I-17 just south of the Bloody Basin exit on the right side of the highway. These were formed by subaqueous extrusion in lakebeds