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Volcanoes

Volcanoes. What is a volcano?. A volcano is a weak spot in the crust where magma reaches the surface Mostly form at plate boundaries where subduction is occurring. Eruptions. Magma from the mantle rises through openings called vents in the earth’s crust. Types of vents:

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Volcanoes

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  1. Volcanoes

  2. What is a volcano? A volcano is a weak spot in the crust where magma reaches the surface Mostly form at plate boundaries where subduction is occurring

  3. Eruptions • Magma from the mantle rises through openings called vents in the earth’s crust. • Types of vents: • Central vent= hole in the middle of a mountain • Fissure= long crack in the earth’s crust • Hydrothermal vents= places where water and magma touch

  4. Anatomy of a volcano

  5. Anatomy of a Volcano • Magma chamber – pocket underground where magma collects • Lava flow – an area covered by lava as it flows out of the vent • Crater – “bowl shaped” area that forms around the vent

  6. Magma and Lava • Both are molten (melted) rocks. • Magma is under the ground. • Lava is on the surface.

  7. Where are volcanoes located? • Divergent Boundaries • Two plates pulling apart, magma comes up • Mid-ocean ridges

  8. Where are volcanoes located? • Convergent Boundaries • Subduction zones • Oceanic – Continental Oceanic-Oceanic

  9. Where are volcanoes located? • Hot Spots • Weak spot in the middle of a plate where magma comes up and reaches the surface • http://www.wwnorton.com/college/geo/egeo/flash/2_10.swf

  10. Where are volcanoes located?

  11. Viscosity • Viscosity is a liquid’s resistance to flow. • High viscosity = high resistance= does not flow well • Low viscosity = low resistance = does flow well

  12. What does the viscosity of magma depend on? • Silica (SiO2) • More silica = more viscosity (more resistance) • Magma is very thick and does not flow well. • Less silica = less viscosity (less resistance) • Magma is thin and flows easily • Example: Which one has more viscosity, water or honey?

  13. What else does the viscosity of magma depend on? • Temperature • Lower temperature = higher viscosity (more resistance) • Higher temperature = lower viscosity (less resistance) • Example: Milkshake is thick and cold but as it heats up it gets more runny.

  14. What happens when a volcano erupts? • The force of expanding gases pushes magma from the chamber through the pipe and flows out of the vents.

  15. Two Types of Eruptions • Quiet/Non Explosive • Low silica • Pressure does not build up and lava just flows from the vents.

  16. Two Types of Eruptions • Explosive • High Silica • Thick magma clogs up the vent, pressure builds up and then it erupts violently. • Pyroclastics - ash, cinders, bombs and gases that are released during an explosive eruption

  17. Level of activity • Active = erupted within last 100 years and show signs of erupting again soon • Dormant = “sleeping” have not erupted recently (last 50 yrs) and do not show signs of erupting again soon • Extinct =have not erupted in 10,000 yrs and doesn’t show signs of erupting in the future Devil’s Tower – WY – Extinct Mt. Shasta– CA – Dormant Last Eruption ~200yrs ago Mt. Kilauea – HI – Active (still erupting regularly)

  18. Three Types of Volcanoes • Shield • Gently sloping sides, wide base • Hot, thin lava erupts from the volcano • Type of Eruption? Quiet • Example: Kilauea

  19. Three Types of Volcanoes • Cinder Cone Volcano • Steep, cone-shaped hill • Pyroclastics erupt from the volcano. • Type of Eruption? Explosive! • Example – Paracutin

  20. Three Types of Volcanoes • Composite Volcano (A.K.A. Stratovolcano) • Tall mountains • Lava flows alternate with pyroclastics when the volcano erupts. • Type of Eruption? Quiet and Explosive • Example: Mt. Saint Helen’s

  21. Yellowstone Caldera • Yellowstone sits over a “hot spot” • Gigantic magma chamber sitting under the crust • Not sure why it formed • Considered “supervolcano” • gigantic explosions (1000x debris ejected as Mt. St. Helens)

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