Volcanism and Other Igneous Processes

1. Volcanism and Other Igneous Processes 1

2. On Sunday, May 18, 1980, the largest volcanic eruption to occur in North American historic times transformed a picturesque volcano into a decapitated remnant. On this date in southwestern Washington State, Mount St. Helens erupted with tremendous force. • What happened?? • Approximately 1 km3 of ash erupted. • Summit decreased by 1,350 feet. • Claimed 59 lives • Ash propelled 11 miles into the atmosphere. • Ash covered surrounding areas of Yakima, Tri-cities, and • northern Oregon for 3 days – Noon felt like night. • Feb. 1981- highest birth rate in Portland and surrounding areas –TRUE FACT • Advice from the authorities: • If there is another major eruption, put your head between your • legs and kiss your ash goodbye! Before After Mt. St. Helens 2

3. The “buzzword” is VISCOSITY What is viscosity? Viscosity = how well a material flows more viscous – flows very slowly (high viscosity) less viscous – flows quickly (low viscosity) Does glass have viscosity? 5

4. Why do volcanoes have different eruptive styles??? • high viscosity • high SiO2 • felsic • “pasty” • explosive 4 • Factors influencing eruptions • dependant on the magma’s viscosity • high viscosity –”pasty” explosive • low viscosity –”fluid” flows easily • Factors influencing viscosity • Temperature of magma • T viscosity = fluid flow • T viscosity = pasty flow A • low viscosity • low SiO2 • mafic • “fluid” • non-explosive • Chemical composition • SiO2 content (high or low) mafic composition: (50% SiO2)=“fluid” flow intermediate comp.: (60% SiO2) felsic composition: (70% SiO2) =“pasty” flow B

5. Dissolved gasses – influencing the movement of magma (volatiles – water, CO2, SO2….) Silica content and volatiles erupt two types of materials: Gas charged expands 100 times its volume Gas charged lava expands 100 times its volume lava fountains very explosive lava flows fluidly Volatiles migrate upwards with difficulty volatiles easily migrate upward 6 magma low in SiO2 magma high in SiO2

6. sulfur dioxide SO2 > 1% 5% Carbon dioxide water vapor 15% 70% 5% volatiles • Dissolved gasses (volatiles) • 1-6% of total magma wt. • contributes to atmosphere Magma chamber 7

7. Types of Basaltic Lava Flows (low silica (SiO2) content) Pahoehoe Aa • very fluid, thin, • broad sheets • flows 10-300 km/hr • (30-900 ft/hr) • high volatile gas content • smooth “skin,” ropey • type flow • very “pasty,” sticky, • thick, cool flows • flows 5-50 m/hr • (15-150 ft/hr) • low volatile gas content • rough, blocky, sharp, • angular type flow 9

8. Pyroclastic materials Bombs Ash Volcanic Bombs Nuee-Ardente Lahars Lahars mud flows Ash • Ryholitic magmas • high silica • very explosive • thick, pasty • high viscosity • pyroclastic ejections Nuee-Ardente 10

9. Three types of Volcanoes • Volcano type is dependant on • SiO2 content. Shield Composite (stratovolcano) Explain the differences. Cinder cone 14

10. Shield Volcano - Hawaii Broad, low angle flanks • Shield Volcanoes • Hawaiian Islands, Iceland, Galapagos Islands • commonly rise from the deep ocean floor • formed by the accumulation of fluid basaltic flows • low silica content (basaltic composition) • low viscosity • less than 1% pyroclastic debris • non-explosive eruptions • pahoehoe flows • aa flows 15

11. Stratovolcano Nuee Ardente gas cloud • Composite Cones (stratovolcanoes, stratacomposite) • Western U.S. coast, Western South American coast, Japan • typically form in the ocean along continent convergent boundaries • found along the ring of fire Steep high angle flanks Pyroclastics • formed from layering deposits of • ash, lava, and pyroclastic flows • high silica content (70%)- (Rhyolitic • composition) • high viscosity flows • abundant pyroclastic activity • deadly airborne debris • explosive eruptions – very hazardous 16

12. The Ring of Fire Cascade Mt. Range Stratovolcanoes 17

13. Old lahars 18

14. Cinder Cones • Exist all over the earth’s surface (by the 1000’s) • Located in volcanic fields (Flagstaff, AZ– about 600) Very high, steep angled flanks 30-40 degrees Averages 100 ft – 1000 ft high • Formed by gas rich basaltic flows (low viscosity, low silica) • producing small sized material. Common • rock scoria and volcanic glass • Single eruptive episode lasting • a short time • Composed of scoria and loose • pyroclastic material 19

15. Cinder Cones 20

16. I see extensive lava flows, but where are the volcanoes? • Fissure type eruptions and lava plateaus • very fluid basaltic lava erupted from fractures in the earth’s • crust • lava fountains along “linear” fractures spreading out over • wide areas • extrudes voluminous amounts of low silica basaltic lava • single flows can travel 100’s of kilometers • Columbia River basalts • 11 m.y. flows • very extensive –single flows from • Idaho to Portland, Ore. • 1 mile thick in southwest • Washington My study Area Linear cracks (fissures) 21

17. Divergent plate volcanism Plates separate resulting from basaltic magma ascending into fractures. Shield type volcanoes form ridges and mountains below the ocean. • Very fluid eruptions • Less than 50% SiO2 content • Shield type volcanoes • Basalt rocks 23

18. Ocean – Ocean plate convergence • Very fluid eruptions • Less than 50% SiO2 • Shield type volcanoes • Basalt rocks Oceanic plate subducts beneath oceanic plate. Melting subducted plate ascends upward forming shield type volcanoes in the form of island arc systems “mountainous arcs” that rise above the ocean floor. – Japan, Aleutian Islands 24

19. Baker Ocean to Continent convergence Rainier Pacific Plate St. Helens Adams Oceanic plate is subducted beneath continental plate. Melting plate ascends upward mixing with continental material. Hood Jefferson Three Sisters Newberry Volcano Crater Lake North American Plate • High SiO2 – High viscosity • explosive volcanoes • “pasty” lava flows • composite type volcanoes • andesite/rhyolite rocks McLaughlin Medicine Lake Volcano Shasta Lassen Peak 25

20. Why and How Rocks Melt Granite/Shale (common rocks) typically begin to melt at 8000C and turn to liquid at 12000C. Why a temperature range? The range of temperature for complete melting of a rock is due to various individual mineral melting point characteristics. Amphibole (hornblende) melts at around 9000C. Feldspar (orthoclase) melts at around 7000C. Quartz melts at around 6000C. 27

21. What factors influence the melting points of rocks? • Temperature: • geothermal gradient • Pressure: • influence of pressure at depth • pressure/temperature relationship • Presence of water in the rocks: • water in the subduction zone • how water influences the melting point 28

22. Temperature inside the earth Geothermal gradient 0 500 1000 1500 2000 • the rate at which • temperature increases • with depth • Continent gradient • In thicker crust, • gradient increases. • average 7oC/km rate • temperature increases • gently 100 5,000 Depth (km) 200 Pressure (mpa) 10,000 300 • Oceanic gradient • Below the ocean floor, • temperature increases • rapidly. • average 130C/km 15,000 400 29

23. How does pressure influence the melting point of rock? Pressure inside the earth with depth. • Pressure/Temperature increase with depth. • Mantle (mesosphere) reaches temperatures • beyond rock melting point, but the mantle • remains “solid!” • Increased pressures raise melting points. • At a depth of 100 km, pressure is 35,000 • times greater than at sea level. • Many minerals will begin melting at • temperatures above melting points at • sea level. 30

24. How does the presence of water influence the rock’s melting point? 31 • The presence of water or water vapor • the rock’s melting point. • water present in subduction zones The introduction of water decreases melting points and creates melted plate material that moves upward. Magma stays liquid due to the decrease in pressure as magma rises. (decompression melting) Introduction of water

25. Which minerals form first, second, third…….etc.? Bowen’s reaction series Gabbro Diorite Granite 35

26. What types of features are formed when magma cools below the surface? Intrusive features 37

27. Dikes Sill Tabular Batholith Tabular intrusive bodies forming below the earth’s surface 38

28. Intrusive Bodies: Batholith: intrusive body GREATER than 40 mi2 Stock: intrusive body LESS than 40 mi2 Dike: intrusive body cutting across strata (disconcordant) Sill: intrusive/extrusive body parallel to strata (concordant) Laccolith: “mushroom-shaped” intrusive body forming a dome-like structure 39

29. Intrusive Bodies Sill Loccolith Dike Stock Batholith 40

30. Melting magma rises and mixes with continental material (high SiO2) and solidifies beneath the surface. Sierra Nevada Batholith Granite/Diorite 41

31. I>clicker question • What characteristics below characterize a • shield type volcanoes? • a. high SiO2 high viscosity • b. high SiO2 , low viscosity • c. low SiO2, high viscosity • d. low SiO2, low viscosity 2. How does the presence of water influence the melting of rocks? a. lowers the melting point temp b. raises the melting point temp c. there is no influence d. cools the rock temperature