Chapter 18

1. Chapter 18 Volcanism

2. Section 18.1Volcanoes • Objectives: • Describe how plate tectonics influences the formation of volcanoes • Locate major zones of volcanism • Identify the parts of a volcano • Differentiate b/w volcanic landforms • Define: • Volcanism • Hot spot • Flood basalt • Fissure • Conduit • Vent • Crater • Caldera • Shield volcano • Cinder cone • Composite volcano

3. I. Zones of Volcanism • Volcanoes fueled by magma • Magma forms  rises to surface b/c less dense than surrounding mantle & crust • Magma at surface = lava • Volcanism – describes all processes associated w/ discharge of magma, hot fluids, and gases Hawaii’s Kilaueo Volcano

4. 20 volcanoes will erupt in as long as it takes to read 1 section • Will erupt in 60 places this year • Distribution is not random • Volcano location = patterns • Form at plate boundaries • Mostly convergent & divergent • Only about 5% of magma erupts far from plate boundaries Hawaiian Island volcano’s = far from plate boundaries

5. A. Convergent Volcanism • Tectonic plates collide • Can form subduction zones – slabs of oceanic crust descends into mantle  magma forms  magma moves up b/c less dense  magma mixes with rock, minerals, sediment from overlying plate • Most volcanoes on land result from oceanic-continental subduction • Characterized by explosive eruptions Mount Saint Helens - 7.22.80

6. B. 2 Major Belts • Major belt = mostly convergent boundaries • Cirucum-Pacific Belt (Pacific Ring of Fire) • Corresponds to outline of Pacific Plate • Along western coast of N & S America • Across Aleutian Islands • Down eastern coast of Asia • Cascade Range (W. US), Mount Pinatubo (Philipines) • Mediterranean Belt – smaller – corresponds to boundaries b/w Eurasian, African, Arabian Plates • Mount Etna, Mount Vesuvius (Italy)

7. C. Divergent Volcanism • Divergent – move apart – new ocean floor produced as magma rises to fill gap • Lava take form of giant pillows = pillow lava • Not explosive • Effusions of large amounts of lava • 2/3 of Earth’s volcanism occurs underwater at ocean ridges

8. D. Hot Spots • Far from plate boundaries • Hot spots – unusually hot regions of Earth’s mantle – high-temperature plumes of magma rise to surface

9. 1. Hot Spot Volcanoes • Best known volcanoes formed as result of hot spots under ocean • Hawaiian islands – located over plume of magma • Rising magma melted through crust  formed volcanoes • Hot spot formed by magma plume remained stationary while Pacific Plate slow moved northwest • Over time  plume left a trail of volcanic islands on floor of Pacific Ocean • Volcanoes on oldest island (Kauai) = inactive b/c no longer sits above stationary hot spot • Even older volcanoes – to NW – no longer above sea level • Most active volcano in world = Kilauea – on Big Island of Hawaii – currently located over hot spot • Loihi – forming on seafloor SE of Big Island of Hawaii – might eventually rise above ocean surface to form new island

10. 2. Hot Spots and Plate Motion • Chains of volcanoes that form over stationary hot spots  give info about plate motion • Rate and direction of plate motion can be calculated • Hawaiian islands – at one end of Hawaiian-Emperor volcanic chain • Meiji – oldest seamount – at other end of chain (80 myo) • Bend in chain (@ Daikakuji Seamount) = change in direction of Pacific Plate (43 mya)

11. 3. Flood Basalts • Flood basalt – form when lava flows out of long cracks in Earth’s crust • Hot spots occur beneath continental crust • Fissures – cracks • Over 100s/1000s of years  fissure eruptions can form flat plains = plateaus • When lava flows across surface  water vapor & gases escape

12. 4. Columbia River Basalts • Located in NW US • Contain 170,000 km3 of basalt • Could fill Lake Superior 15 times • Small in comparison to Deccan Traps

13. 5. Deccan Traps • 65 mya – India • Huge flood basalt eruption created enormous plateau = Deccan Traps • 512,000 km3 – cover NY state w/ 4km thick • Caused global change in climate that might have influenced extinction of dinosaurs

14. II. Anatomy of a Volcano • Conduit – tubelike structure that carries lava to surface • Vent – where lava emerges at surface • As emerges from vent  lava cools & solidifies • Over time  layers can accumulate to form mountain = volcano • Crater – top of volcano, around vent, bowl-shaped depression • Connected to magma chamber by conduit • Usually < 1km in diameter

15. Calderas - >1km across up to 50 km in diameter • Often form after magma chamber empties from major eruption • Summit (side) of volcano collapses • Water sometimes fills caldera = scenic lakes • Crater Lake (S. Oregon) – formed when Mount Mazma collapsed

16. III. Types of Volcanoes • Appearance depends on 2 factors • Type of material that forms volcano • Type of eruptions that occur • 3 types of volcanoes can form • Differ in shape, size, composition

17. A. Shield Volcanoes • Shield volcano – mountain w/ broad, gently sloping sides & nearly circular base • Form when layers of lava accumulate during nonexplosive eruptions • Largest type • Mauna Loa

18. B. Cinder Cones • When eruptions eject small pieces of magma into the air • Material called tephra • Cinder cones form as tephra falls back to earth and piles up around vent • Steep sides • Generally small= <500m high • Lassen Volcanic Park cinder cone = 700m • Often form on or very near larger volcanoes

19. C. Composite Volcanoes • Formed of layers of hardened chunks of lava from violent eruptions alternating with layers of lava that oozed downslope before solidifying • Cone-shaped w/ concave slopes • Much larger than cinder cones • Explosive nature = potentially dangerous to humans and environment • Mount Augustine (Alaska) & several in Cascade Range – Mt. St. Helens (W. US)

20. Section 18.2Eruptions • Objectives: • Explain how magma type influences volcanic activity • Describe the role of pressure and dissolved gases in eruptions • Recognize classifications of material ejected by eruptions • Define: • Viscosity • Tephra • Pyroclastic flow

21. I. Making Magma • Activity of a volcano depends on composition of magma • Lava can be thin/runny or thick/lumpy

22. A. Temperature • Most rocks begin to melt b/w 800 & 1200 degrees Celsius • Found in curst and upper mantle • Temp increases w/ depth • Temp, pressure, and presence of water affect formation of magma

23. B. Pressure • Pressure increases w/ depth b/c weight of overlying rocks • As pressure increases  temp at which a substance melts also increases • Albite: • @ surface w/ no water melts at 1100 • @ 12km = 1150 • @ 100km = 1440 • Effects of pressure explain why most rocks in Earth’s lower crust and upper mantle do not melt

24. II. Composition of Magma • Determines volcano’s explosivity • How it erupts • How its lava flows • Factors of influence • Magma’s interaction w/ overlying crust • Temp • Pressure • Amt dissolved gas • Amt of silica!!! • Understanding factors allows scientists to predict explosivity of volcanic eruptions

25. A. Dissolved Gases • Amt of gases in magma incr = explosivity incr • Like gas in soda  gas in magma gives volcano “bang” • Water vapor, CO2, SO2, hydrogen sulfide • Water vapor = most common – determines where magma forms • Presence of water = lower melting temp • Causes mantle to melt • Forms volcanoes & fuels eruptions

26. B. Viscosity • Viscosity – physical property that describes resistance of material to flow • Temp & silica content affect viscosity • Cooler magma = higher viscosity = resists flowing • High silica = thick & sticky – traps gases = explosive eruptions • Low silica = thin/runny – nonexplosive eruptions

27. III. Types of Magma • Silica content determines • Explosivity • Viscosity • Type of volcanic rock formed after lava cools

28. A. Basaltic Magma • Rock in upper mantle melts • Has same silica content as rock basalt (<50%) • Magma rises and reacts very little w/ overlying continental crust • Low silica content = low viscosity = dissolved gases escape easily = quiet eruption • Kilauea • Mauna loa • Surtsey (S. Iceland) (1963 formed)

29. B. Andesitic Magma • 50-60% silica content • Found along oceanic-continental subduction zones • Sources of andesitic magma: • Oceanic crust • Oceanic sediments • Higher silica = intermediate viscosity = intermediate explosivity • Colima volcano (mexico), Tambora (Indonesia)

30. C. Rhyolitic Magma • Molten material rises & mixes w/ overlying continental crust rich in silica and water • >60% silica = high viscosity = large volume of gas trapped = very explosive • Yellowstone National Park (dormant volcanoes) • Most recent eruption = 640,000 ya  so powerful, it released 1,000km3 of volcanic material into air

31. IV. Explosive Eruptions • When lava = too viscous to flow freely from vent  pressure builds up  volcano explodes • Tephra – erupted materials • Pieces of lava solidified during eruption • Pieces of crust carried by magma before eruption • Classified by size • <2mm = ash – can rise 40km into air (threat to aircraft & weather) • Largest = blocks (1m high  size of car) • Mount Pinatubo (Philippines) eruption: • Plume of ash = 24km high • Sulfuric acid particles in atmosphere for 2 years • Blocked sun’s rays & lowered global temp

32. V. Pyroclastic Flows • Rapidly moving clouds of tephra mixed with hot, suffocating gases • Internal temp >700 degrees Celsius • Violent volcanic eruptions can send clouds of ash and other tephra down a slope at speeds >80km/h • Mayon Volcano (Mexico 2000) • Mount Pelee (Martanique island 1902) - >29,000 people suffocated or burned to death

33. Section 18.3Intrusive Activity • Objectives: • Compare and contrast features formed from magma that solidifies near the surface with those that solidify deep underground • Classify the different types of intrusive rock bodies • Describe how geologic processes result in intrusive rocks that appear at Earth’s surface • Define: • Pluton • Batholith • Stock • Laccolith • Sill • Dike

34. I. Plutons • Most volcanism = below surface; not all magma emerges at surface • On the way up  magma can interact w/ crust in several ways • Can force overlying rock apart and enter newly formed fissures • Can cause blocks of rock to break off and sink into magma  rocks melt • Can melt its way through rock into which it intrudes • As magma cools  it crystallizes

35. Over period of time of magma cooling  intrusive igneous rock bodies can form • Some = ribbon-like (few cm thick & 100s m long) • Others = massive (range in volume 1km3 – 100s km3) • Plutons – intrusive igneous rock bodies • can be exposed at surface as a result of uplift and erosion • Classified based on size, shape, relationship to surrounding rocks

36. A. Batholiths and Stocks • Batholiths – largest plutons – irregularly-shaped masses of coarse-grained igneous rocks • Cover at least 100 km2 • Take millions of years to form • Common in interior of major mountain chains • Commonly composed of granite (diorite, gabbro = less common) • Largest in N. Am = Coast Range Batholith (British Columbia) = 1500 km long • Stocks – irregularly shaped plutons that are similar to batholiths but smaller • Both cut across older rocks and generally form 5-30km beneath surface

37. B. Laccoliths • Magma intrudes into parallel rock layers close to surface  some rocks bow upward from intense pressure of magma body • Laccolith forms when magma solidifies • Laccolith – lens-shaped pluton w/ round top and flat bottom • Relatively small compared to batholiths and stocks (most = 16km wide) • Red and White Mountain, CO • Black Hills, SD • Judith Mountains, Montana

38. C. Sills • Sill – forms when magma intrudes parallel to layers of rock • Range from few cm - 100s m in thickness • Palisades Sill (near Hudson River, NYC) = 300m thick • Rock above sill eroded  effects sedimentary rocks it intruded • Lifts rock above it - Form close to surface b/c take great amt of force to lift entire layers of rock • Metamorphose surrounding rocks

39. D. Dikes • Dike – pluton that cuts across preexisting rocks • From when magma invades cracks in surrounding rock bodies • Range in size from few cm – several m wide & can be 10s of km long • Great Dike (Zimbabwe, Africa) – exception: 8km wide & 500 km long • Volcanic neck – when magma in volcano conduit solidifies • Ship Rock (NM) – has dikes extending from neck

40. 1. Textures • Sills and dikes – textures vary  mostly coarse-grained • Coarse-grained texture suggests that formed deep in crust  magma cooled slowly enough for large mineral grains to develop • Dikes and sills w/ fine-grained texture formed closer to surface  many crystals began growing at same time

41. II. Plutons and Tectonics • Many plutons form as result of mountain-building processes • Batholiths found at cores of many mountain ranges • Major mountain chains formed along continental-continental convergent plate boundaries • Some of collisions might have forced continental crust down into upper mantle  melted  intruded into overlying rocks  cooled to form batholiths

42. Plutons can also form from ocean plate convergence • Subduction zone develops  water from subducted plate causes overlying mantle to melt  plutons form when melted material rises but does not erupt at the surface • Sierra Nevada, CA = 5 episodes of this type of igneous activity • Yosemite National Park (granite cliffs) • Uplift & erosion brought them to surface