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Chapter 3: Volcanoes

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  1. Chapter 3: Volcanoes

  2. Chapter 3.1 • Key Concepts: • Where are Earth’s volcanoes found? • How do hot spot volcanoes form?

  3. Definitions: • Volcano: a weak spot in the Earth’s crust where molten material (magma) comes to the surface • Magma: a molten mixture of rock forming substances, gases and water from the earth’s mantle. • Lava: when magma reaches the Earth’s surface.

  4. Volcanoes & Plate Boundaries • 600 active volcanoes on land, many more under the sea. • Volcanoes occur in belts that extend across continents and oceans. • Ring of Fire - major volcanic belt formed by the many volcanoes that rim the Pacific Ocean.

  5. Volcanic belts form along the boundaries of Earth’s plates


  6. Volcanic belts form along the boundaries of the Earth’s plates • Diverging ( )and converging () occur. • Results in fractures in the Earth’s crust • Magma reaches the surface.

  7. Divergent Boundaries Mid Ocean Ridge Convergent boundary Points of subduction ex. Mount Etna – Eurasian and African plates Most volcanoes occur at

  8. Volcanoes on Divergent Boundaries • Under ocean: point of Mid ocean ridges occurring ( long underwater mountain ranges) • Many have rift valleys down the center • Lava pours out of rift valley – creates new mountains. • Land - Great Rift Valley in East Africa

  9. Volcanoes on Convergent Boundaries • Volcanoes form when: • Two oceanic plates collide • Oceanic and continental plate collide • In both situations, oceanic plates sink beneath a trench. • Rock above the plate melts to form magma. • Erupts to the surface as lava

  10. Volcanoes on Convergent Boundaries

  11. Volcanoes on Convergent Boundaries • Magma – less dense than the surrounding rock • Magma rises to the surface and breaks through the ocean floor creating volcanoes. • String of islands formed: Island arc • Examples: Japan, New Zealand, Indonesia, the Philippines

  12. Hot Spot Volcanoes • Hot Spot – an area where material from deep within the mantle rises and melts forming magma • Volcanoes form above a hot spot when magma erupts through the crust and reaches the surface.

  13. Hot Spot Volcanoes • Some hot spots lie in the middle of plates far from plate boundaries. • Others occur on or near plate boundaries.

  14. Hot Spot Volcanoes • A hot spot in the ocean floor can gradually form a series of volcanic mountains • Ex. The Hawaiian islands

  15. Hot Spot Volcanoes • A hot spot can also form under continents • Ex. Yellowstone National Park, WY

  16. 3.2 PROPERTIES OF MAGMA Vocabulary: • Element-a substance that cannot be broken down into other substances • Compound-a substance made of two or more elements that have been chemically combined • Physical Property-a characteristic of a substance that can be observed or measured without changing the composition of a substance • Chemical Property-any property that produces a change in the composition of matter.

  17. Pahoehoe-fast moving, hot lava that has low viscosity • aa-higher viscosity, slower moving, cooler lava that Pahoehoe • Viscosity-the resistance of a liquid to flow • Silica—major ingredient in magma made of silicon and oxygen.

  18. Magma Composition • Magma varies in composition and is classified according to the amount of silica it contains. • The graphs show the average composition of the two types of magma. Use the graphs to answer the questions.

  19. What materials make up both types of magma? • Silica, oxides, and other solids • Which type of magma has more silica? About how much silica does this type of magma contain? Rhyolite, 70% • A third type of magma has a silica content that is halfway between that of the other two types. About how much silica does this type of magma contain? 60% • What type of magma would have a higher viscosity? Explain. The rhyolite forming would have higher viscosity because it contains more silica

  20. Volcanic Eruptions 3.3 • Objectives: • Explain what happens when a volcano erupts. • Describe 2 types of volcanic eruptions • Identify stages of volcanic activity • What comes out of volcanic explosions? • Ash, lava, steam, rock, gases, dirt

  21. Volcanic Eruptions 3.3 • Magma forms in the asthenosphere • Convection currents in the mantle bring magma towards the surface • Magma tries to flow into any open crack • When magma reaches a weak spot in the crust, a volcano forms • Lava is magma that has reached the surface

  22. Magma Reaches Earth’s Surface • Volcanoes are systems of passage ways through which magma moves. • Inside a Volcano • Magma collects below the volcano in the magma chamber • Magma flows upward through a tube (pipe) that connects the magma chamber to the surface.

  23. Magma Reaches Earth’s Surface • Inside a Volcano • Gas and magma leave through vents • Central vent is the opening at the top • Side vent anywhere along the side of the volcano • Lava pours out of the vents and creates a lava flow • A bowl shaped area called a crater may form around the central vent

  24. Magma Reaches Earth’s Surface • A Volcanic Eruption • Dissolved gases are trapped in magma under extreme pressure (CO2 in a soda bottle) • as magma flows , pressure because there is less rock on top of it. • the dissolved gases expand and form bubbles • When a volcano erupts, the force of the expanding gas forces magma up the pipe until it explodes out the vent

  25. Content checkpoint… think/pair share…take two minutes to answer these questions with a partner nearby….. • What common everyday occurrence can we relate to a volcanic eruption? • What happens to the pressure in the magma as it rises toward the surface?

  26. Kinds of Volcanic Eruptions • Volcanic eruptions can be quiet or explosive. • Eruptions depend on the properties of magma • Silica content • Viscosity

  27. Kinds of Volcanic Eruptions • Quiet Eruptions • Magma has low silica content = low viscosity • Magma flows easier so gases bubble out gently • Lava can flow many kilometers from the vent • Produce both pahoehoe and aa lava • Example: Hawaiian Islands – Mount Kilauea

  28. Kinds of Volcanic Eruptions • Explosive Eruptions • Magma has high silica content = high viscosity • Magma doesn’t always flow out of vent and so it builds up (like a cork in a bottle) • Trapped gases build up pressure until they explode • Magma is pushed out of the vent with incredible force

  29. Kinds of Volcanic Eruptions • Explosive Eruptions • Lava breaks into fragments that cool quickly and harden into different sizes • Ash – fine, dust sized particles of lava • Cinders – pebble sized particles • Bombs – baseball to car sized chunks of lava • Pyroclastic flow:when an explosive eruption hurls out gases, ash, cinders and bombs. • Pumice forms when lava cools quick and traps air bubbles inside • Obsidian forms when lava cools quick leaving the surface smooth and glass-like

  30. Kinds of Volcanic Eruptions • Volcano Hazards • Quiet eruptions cause lava to flow far --- burning and burying everything in its path • Explosive eruptions can bury entire towns in ash, cause landslides, avalanches, cause damage from gases and cinders/bombs.

  31. Content checkpoint… think-pair-share take two minutes to answer these questions with a partner nearby. • What is pyroclastic flow? • How does volcanic ash cause damage? • What is the main difference between a quiet eruption and an explosive eruption?

  32. Stages of Volcanic Activity • Life Cycle of a Volcano • Scientists use historical records and monitor volcanoes to determine what stage of activity a volcano is in • A volcano can be active, dormant or extinct • An active volcano is erupting or showing signs of near future eruptions • A dormant (sleeping) volcano can awaken in the future and become active • An extinct (dead) volcano is unlikely to erupt ever again

  33. Stages of Volcanic Activity • Monitoring Volcanoes • Geologists use tiltmeters to detect slight changes in surface elevation cause by magma moving underground • They monitor gases escaping from a volcano • Increase in temperature might mean magma is nearing the surface • The changes detected may give a short warning time BUT • We cannot be certain the type of eruption or how powerful it will be

  34. Think and Discuss…. • Which is more likely to be dangerous – a volcano that erupts frequently or a volcano that has been inactive for a hundred years? WHY?

  35. 3.4 Volcanic Landforms • Objectives: • List the landforms that lava and ash create • Explain how magma that hardens beneath earth’s crust creates landforms • Identify other distinct features that occur in volcanic areas • Volcanic activity on and beneath the surface has built up Earth’s land areas.

  36. Landforms from Lava and Ash • Volcanic eruptions create landforms made of lava, ash and other materials • Shield volcanoes • Cinder cone volcanoes • Composite volcanoes • Lava plateaus • Another landform results from the collapse of a volcanic mountain • caldera

  37. Shield Volcano in Iceland • Shield volcanoes • Gently sloping mountains • Thin layers of low viscosity lava build up over time • Ex) the Hawaiian Islands

  38. Cinder cone volcanoes • High viscosity lava produces ash, cinders and bombs • Those materials build up around the vent in a steep cone shaped hill/mountain. • Ex) Sunset Crater in Arizona

  39. Composite Volcanoes • Form when volcanoes alternate between quiet lava flows and explosive eruptions of ash, cinders and bombs • Tall, cone shaped mountain with alternating layers of ash and lava • Ex) Mt St. Helens, Washington and Mt. Fuji, Japan

  40. Composite Volcanoes • Form when volcanoes alternate between quiet lava flows and explosive eruptions of ash, cinders and bombs • Tall, cone shaped mountain with alternating layers of ash and lava • Ex) Mt St. Helens, Washington and Mt. Fuji, Japan

  41. Lava Plateaus • Low viscosity lava flows out of several long cracks and travels far before cooling • After millions of years a high plateau forms • Ex) Columbia Plateau in Washington, Oregon and Idaho

  42. Calderas • Huge hole left by the collapse of composite volcanic mountains • Filled with fallen pieces of the volcano, ash and lava • Enormous eruptions may empty the pipe and the magma chamber • there is only hollow space left that can’t support the weight and it collapses on itself • Overtime water from rain and snow may fill the caldera • Ex) Crater Lake, Oregon