Lecture Outlines Natural Disasters, 7 th edition

1. Lecture OutlinesNatural Disasters, 7th edition

2. Volcanic Eruptions: Plate Tectonics and Magma

3. Vesuvius, 79 C.E. • Cities of Pompeii and Herculaneum buried by massive eruption which blew out about half of Mt. Vesuvius • Similar to 1991 eruption of Mt. Pinatubo in Philippines • Clouds of hot gas (850oC), ash and pumice enveloped city • Many tried to escape near sea, but were buried by pyroclastic flows Figure 8.1 Figure 8.3

4. Vesuvius, 79 C.E. • Vesuvius was inactive for 700 years before 79 CE eruption • People lost fear and moved in closer to volcano • After 79 CE, eruptions in 203, 472, 512, 685, 993, 1036, 1049, 1138-1139 • 500 years of quiet, then 1631 eruption killed 4,000 people • 18 cycles of activity between 1631 and 1944, nothing since then • 3 million people live within danger of Vesuvius today; 1 million people on slopes of volcano

5. The Hazards of Studying Volcanoes • Eruptive phases are often separated by centuries of inactivity, luring people to live in vicinity (rich volcanic soil) • 400,000 people live on flanks of Galeras Volcano in Colombia • Many people killed each year by volcanoes, sometimes including volcanologists • Volcanoes may be active over millions of years, with centuries of inactivity

6. How We Understand Volcanic Eruptions • Understand volcanoes in context of plate tectonics • Variations in magma’s chemical composition, ability to flow, gas content and volume determines whether eruptions are peaceful or explosive

7. Plate-Tectonic Setting of Volcanoes • 90% of volcanism is associated with plate boundaries • 80% at spreading centers • About 10% at subduction zones • Remaining 10% of volcanism occurs above hot spots Figure 8.5

8. Plate-Tectonic Setting of Volcanoes • Subductioncarries oceanic plate (with water-rich sediments) into hotter mantle, where water lowers melting temperature of rock • Rising magma melts continental crust it passes through, changing composition of magma Figure 8.6

9. Plate-Tectonic Setting of Volcanoes • No volcanism associated with transform faults or continent-continent collisions • Oceanic volcanoes are peaceful • Subduction-zone volcanoes are explosive and dangerous • Subduction zones last tens of millions of years • Volcanoes may be active any time, with centuries of quiet Figure 8.6

10. Chemical Composition of Magmas • Of 92 naturally occurring elements: • Eight make up more than 98% of Earth’s crust • Twelve make up 99.23% of Earth’s crust • Oxygen and silicon are by far most abundant • Typically join up as SiO4 tetrahedron, that ties up with positively charge atoms to form minerals Figure 8.7

11. Chemical Composition of Magmas • Mineral formation in magma: crystallization • Order of crystallization of different minerals in magma can be determined: • Iron and magnesium link with aluminum and SiO4 to form olivine, pyroxene, amphibole and biotite families • Calcium combines with aluminum and SiO4 until calcium replaced by sodium, to form plagioclase feldspar family; calcium and sodium are later replaced by potassium, to form potassium feldspar and muscovite families; finally only Si and O remain, forming quartz

12. Chemical Composition of Magmas Figure 8.8

13. Chemical Composition of Magmas • Elements combine to form minerals • Minerals combine to form rocks • Different compositions of magma result in different igneous rocks • If magma cools slowly and solidifies beneath surface plutonic rocks • If magma erupts and cools quickly at surface  volcanic rocks

14. Viscosity, Temperature and Water Content of Magmas • Viscosity: internal resistance to flow • Lower viscosity  more fluid behavior • Water, melted ice-cream • Higher viscosity  thicker • Honey, toothpaste • Viscosity determined by: • Higher temperature lower viscosity • More silicon and oxygen tetrahedra  higher viscosity • More mineral crystals  higher viscosity • Magma contains dissolved gases: volatiles • Solubility increases as pressure increases and temperature decreases

15. Viscosity, Temperature and Water Content of Magmas • Consider three types of magma: basaltic, andesitic and rhyolitic • Basaltic magma has highest temperatures and lowest SiO2 content, so lowest viscosity (fluid flow) • Rhyolitic has lowest temperatures and highest SiO2 content, so highest viscosity (does not flow) • Basaltic makes up 80% of magma that reaches Earth’s surface, at spreading centers, because it forms from melting of mantle • Melted mantle at subduction zones rises through continental crust before reaching the surface, incorporating continental high SiO2 rock as it rises, to become andesitic or rhyolitic in composition before it erupts

16. Viscosity, Temperature and Water Content of Magmas

17. Viscosity, Temperature and Water Content of Magmas • Water is most abundant dissolved gas in magmas • As magma rises, pressure decreases, water becomes steam bubbles • Basaltic magma has lower water content  peaceful, safe eruptions • Rhyolitic magma has higher water content and high viscosity  many steam bubbles form and can not escape through thick magma, so explode out  violent, dangerous eruptions Figure 8.10 Figure 8.9

18. Plate-Tectonic Setting of Volcanoes Revisited • Spreading centers have abundant volcanism because: • Sit above hot asthenosphere • Asthenosphere has low SiO2 • Plates pull apart so asthenosphere rises and melts under low pressure, changing to high-temperature, low SiO2, low volatile, low viscosity basaltic magma that allows easy escape of gases peaceful eruptions

19. Plate-Tectonic Setting of Volcanoes Revisited • Subduction zones have violent eruptions because: • Magma is generated by partial melting of the subducting plate with water in it • Melts overlying crust to produce magmas of variable composition • Magma temperature • decreases while SiO2, • water content and • viscosity increase  • violent eruptions Figure 8.11

20. How a Volcano Erupts • Begins with heat at depth • Rock that is superheated (heated to above its melting temperature) will rise • As it rises, it is under less and less pressure so some of it melts (becomes magma) • Volume expansion leads eventually to eruption • Three things will cause rock to melt: • Lowering pressure • Raising temperature • Increasing water content • Lowering pressure is most common way to melt rock  decompression melting

21. How a Volcano Erupts • Magma at depth is under too much pressure for gas bubbles to form (gases stay dissolved in magma) • As magma rises toward surface, pressure decreases and gas bubbles form and expand, propelling the magma farther up • Eventually gas bubble volume may overwhelm magma, fragmenting it into pieces that explode out as a gas jet Figure 8.12

22. How a Volcano Erupts • Eruption Styles and the Role of Water Content • Concentration of water in magma largely determines peaceful or explosive eruption • Basaltic magma can erupt violently with enough water • Rhyolitic magma usually erupts violently because of high water content, high viscosity (secondary role) • Styles of volcanic eruptions • Nonexplosive Icelandic and Hawaiian • Somewhat explosive Strombolian • Explosive Vulcanian and Plinian Figure 8.14

23. How a Volcano Erupts • Some Volcanic Materials • Low-water content, low-viscosity magma  lava flows • High-water content, high-viscosity magma  pyroclastic debris

24. How a Volcano Erupts • Nonexplosive eruptions • Pahoehoe: smooth ropy rock from highly liquid lava • Aa: rough blocky rock from more viscous lava Figure 8.15 Figure 8.16

25. How a Volcano Erupts • Explosive eruptions • Pyroclastic debris: broken up fragments of magma and rock from violent gaseous explosions, classified by size • May be deposited as: • Air-fall layers (settled from ash cloud) • High-speed, gas-charged pyroclastic flow Figure 8.18 Figure 8.17a

26. How a Volcano Erupts • Explosive eruptions • Very quick cooling: • Obsidian: volcanic glass forms when magma cools very fast • Pumice: porous rock from cooled froth of magma and bubbles Figure 8.19

27. Side Note: How a Geyser Erupts • Geyser: eruption of water superheated by magma • Can only exist in areas of high heat flow underground • Water boils (becomes gas) at 100oC unless it is under pressure – no room for expansion to gas state • Water can be heated to higher than boiling temperature  superheated • When superheated water reaches point of lower pressure, it flashes to steam violently, and erupts out of the ground Figure 8.20

28. The Three V’s of Volcanology: Viscosity, Volatiles, Volume • Viscosity may be low or high • Controls whether magma flows easily or piles up • Volatile abundance may be low, medium or high • May ooze out harmlessly or explode • Volume may be small, medium or large • Greater volume  more intense eruption

29. The Three V’s of Volcanology: Viscosity, Volatiles, Volume • By mixing different values for the three V’s, can forecast different eruptive styles for volcanoes

30. The Three V’s of Volcanology: Viscosity, Volatiles, Volume • By mixing different values for the three V’s, can define different volcanic landforms

31. The Three V’s of Volcanology: Viscosity, Volatiles, Volume • Shield Volcanoes: Low Viscosity, Low Volatiles, Large Volume • Basaltic lava with low viscosity and low volatiles flows to form gently dipping, thin layers • Thousands of layers on top of each other form very broad, gently sloping volcano like Mauna Loa in Hawaii • Great width compared to height Figure 8.22

32. The Three V’s of Volcanology: Viscosity, Volatiles, Volume • Hawaiian-type Eruptions • “Curtain of fire”: lines of lava fountains up to 300 m high • Low cone with high fountains of magma • Floods of lava spill out and flow in rivers down slope • Eruptions last days or years, usually not life-threatening but destroy buildings and roads Figure 8.24