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Volcanoes

Volcanoes. Definition. A vent at which lava, pyroclastic debris (ash and fragments of previously solidified rock), and gas erupt. Eruption may build a mountain around the vent. (Mountain is also called a volcano). Anatomy of a volcano: magma chamber at depth a vent to the surface.

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Volcanoes

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  1. Volcanoes

  2. Definition • A vent at which lava, pyroclastic debris (ash and fragments of previously solidified rock), and gas erupt. • Eruption may build a mountain around the vent. (Mountain is also called a volcano). • Anatomy of a volcano: • magma chamber at depth • a vent to the surface.

  3. Where do Volcanoes Occur? • Mid-ocean ridge • we don't see them • Convergent Margins • the majority of major volcanoes; • e.g., the Andes, Japan, Aleutians • Rifts • e.g., East African rift • Hot spots • e.g., Hawaii; Yellowstone Park

  4. Nature of lava • Characteristics of lava depend on its temperature and viscosity. • viscosity is a measure of the ability of a fluid to flow; • high viscosity means sticky and • slow, low viscosity means watery and fast • The higher the temperature of a lava, the lower its viscosity (i.e., the easier it flows) • The greater the gas content, the lower the viscosity. Frothy lava flows more easily. • Mafic lava (rich in Mg and Fe, low in silica) is also at high temperature (1100°C).  tends to flow very easily, so basalt flows in widespread thin sheets. • Silicic lava is also at low temperature (750°C).  lavas are very sticky and plug up volcanoes, unless they are very gassy.

  5. Types of Eruptions • Lava dominated eruptions: * Characteristic of low viscosity eruptions. Basaltic composition. * If lava has low viscosity, it fountains out easily. You will see lava fountains, lakes and rivers, that flow long distances. • Explosive eruptions: • Characteristic of silicic volcanoes. Sticky, very viscous lava clogs up the vent. Gas (CO2 and H2O) that come out of solution and make bubbles. Gas pressure builds up and the volcano explodes. • Result is a cloud of ash that blankets countryside or an ash flow that rushes down the volcano side at 100 km/h, incinerating everything in its path. • Explosions when water gets into magma chamber and turns to steam • Explosions when part of the mountain slips away and pressure is released

  6. Types of Volcanoes • The fissure volcano: Is a long crack in the earth's surface through which magma erupts. These cracks may form as two tectonic plates pull apart.The shield volcano: This is a broad, shallow volcanic cone, which  arises because the running lava, which is fluid and hot, cools slowly. • The dome volcano: This one has a steep, convex slope from thick, fast-cooling lava  • The ash-cinder volcano: Throws out - besides lava - much ash into the air. Through this the volcanic cone is built up from alternate layers of ash and cinder. • The composite volcano: These are also built up from alternate layers of lava and ash but, besides its main crater, it has many little craters on its slope. • The caldera volcano: An older volcano with a large crater which can be 62 miles(100km) wide. In this crater many little new craters are formed

  7. Types of Volcanic Hazards • Lava Flows: Basaltic (lava-dominated) eruptions may emit flows that can bury highways and towns. • Ash falls and ash flows: Ash falls can bury the countryside. Ash flows can incinerate towns. • Explosive blast: The blast of an exploding volcano can flatten everything within range. • Lahars: These are fast-moving slurries made from a mixture of water and ash (the water may come from melting snow on the volcano during the eruption). Lahars rush down river valleys and wash away or bury everything in their path.

  8. Predicting Eruptions & Decreasing Consequences • Geologists are able to determine if a volcano is about to erupt, based on several clues: (1) Change in shape of the volcano (2) Change in heat flow (3) Increase in local seismicity • If there is enough warning, the region can be evacuated. People must also get out of river valleys that could fill with lahars. • NOTE: Volcano prediction is much more possible than earthquake prediction. All we can do with earthquakes is estimate recurrence interval (statistically average time between events). We cannot exactly say when or where an earthquake will occur. But we can with a volcano.

  9. Earthquakes

  10. Definition = Vibration of earth produced by rapid release of energy (seismic waves) which radiate in all directions from the source (focus) • Earthquakes don't occur randomly. Occur on faults or fractures within the earth • Explained by plate tectonics. Most occur on plate boundaries. Sometimes in plate interiors if enough stress is built up

  11. Key Terms • Focus • Epicenter • Body Waves • Surface Waves

  12. Measuring Earthquakes • Seismometer • Seismograph • Seismogram

  13. Mechanism • Not known until after 1906 quake in San Francisco • Elastic rebound theory • Forces bend rock on either side of fault, rock strains ever so slowly, then weakest point breaks. Break sends out shock waves, which migrate outwards from the original break, causing shaking. Stress is released • Aftershocks are adjustments to that change in stress. They are always less strong than the main shock, but they may cause more damage to already weakened structures.

  14. What causes shaking? • Earthquake waves = seismic waves. Recorded on seismometers on seismographs. • Seismic waves are important for 2 things • locating earthquakes - know how to do this using P&S travel time curve • describing the Earth's interior

  15. Types of Seismic Waves • Body Waves • P-Waves (push-pull wave) • vibrates parallel to the direction of wave propagation. • Fast travel: 4-7 km/sec (15,000 mph) • S-Waves (up and down waves /shear) • vibrates perpendicular to the direction of wave propagation. • Slower than P wave: 2-5 km/secs • Surface waves - Very slow waves. Cause a lot of damage, rolling feeling at end of earthquake • Rayleigh waves - like waves on the ocean. • Love waves - side to side waves.

  16. How strong is an Earthquake? • Intensity • modified Mercalli scale • Magnitude • Richter scale • other magnitude scales

  17. Locating an Earthquake • relationship of P and S waves • Travel Time Curve = Distance • Triangulation to locate Epicenter

  18. Earthquake belts • convergent margins, divergent margins, transform margins • Big quakes in convergent margins • Small ones in divergent • Transform vary in quake size

  19. What effects do Earthquakes have? • Ground motion - displacement • Fires • Landslides • Tsunamis • Seismic Sea wave • " Tidal Wave" - (NOT)

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