Earthquakes and Earthquake Hazards (Chapter 5)

• Why do earthquakes occur?

  • Elastic rebound theory — stress applied to an object transfers energy,q2q2

• Strain builds up in rocks

• theory based on elastic properties of rocks explains how earthquakes occur

• stress causes rocks to deform and change shape or volume

Volcanoes and Volcanic Hazards (Chapter 6)

• Volcano - a vent through which molten material surfaces; a mountain formed from solidified eruption material

  • majority (2/3) of active volcanoes are on the “ring of fire” (mostly convergent plate boundaries)

• Magma and Melting

  • Why do rocks melt? — interior of the Earth is hot

  • geothermal gradient: the rate of temperature increase in the Earth with increasing depth

  • as we go deeper into the earth, it gets hotter

  • Solid (crystal shape, internal arrangement) — pressure tends to hold crystal shape together

  • Liquid — increasing temp during burial can break bonds causing solid to melt

  • adding water lowers melting temperature of rocks

    • makes it easier to melt, harder to crystallize

• Effect of Pressure and Volcanoes

• rising molten material does not crystallize beacause of a decrease in ambient pressure

• spreading occurs in lithosphere, solid asthenosphere rises to fill the gap and melts as it rises

• most volcanoes happen in divergent and convergent boundaries

• Magma compositions vary in content of silicia, iron, magnesium, and gases

• Magmas contain gas bubbles that expand as the magma rises (lower pressure)

• the type of lava (mafic, felsic, and intermediate) affects lava characteristics (viscosity)

• viscosity: resistance to flow

  • high viscosity = poor flow

• MAFIC:

  • low in silica (45-50%) but high in iron and magnesium

  • low viscosity - flow easily

  • easy for gas bubbles to escape — gentle eruptions

• FELSIC:

  • high in silicia (up to 75%) but low in iron and magnesium

  • high viscosity - doesn’t flow well

  • hard for gas bubbles to escape — explosive eruptions

• INTERMEDIATE:

  • mid range of silica (50-65%), iron, and magnesium

• Types of Volcanoes

  • fissure eruptions — magma escapes from a large crack in the lithosphere instead of an individual chimney or vent

  • individual volcanoes

    • shield volcanoes — shield like shape; gentle slop; many thin basalt (low viscosity) flows; ex. Hawaiian islands, Mauna Loa; ‘gentle slope’ does NOT mean small (actually the biggest)!

    • lava domes — mound-shaped structure; formed from high-viscosity lava when there is low gas pressure; lava can’t flow far; piles up around the vent; erupt explosively with high enough gas pressure

    • cinder cones — minor explosive volcano; central part of cone is filled with a blob of solidified/congealed lava; when pressure builds up it explodes, moves as pyroclastic; size of pyroclastic vary; pyroclastic fall close to the vent creating a cone-shaped volcano

      • fragmental material ejected from a volcano

      • solidified lava in a variety of sizes

      • Ash: powdery glass shards

      • Lapilli: pea-plum sized material

      • Blocks and Bombs: apple to refrigerator size

        • blocks: pre-existing rock torn from volcano; irregular, angular shapes

        • bombs: streamlined fragments of ejected lava

    • composite volcanoes — aka stratovolcanoes; build up of layers of lava and pyroclastic; mix of lavas and pyroclastic layers allows a tall volcano to form; usually associated with subduction zones; tend to be violent and explosive (Ex. Mount St. Helens)

• Three Forms of Volcanic Eruptions

  1. Lava Flows: molten rock flows over the ground

  2. Pyroclastic debris: fragments blown out of a volcano

  3. Volcanic gases: vapor and aerosols that exit a volcano; mostly water vapor and carbon dioxide

• Volcanic Hazards

  • Lava Flows - immovable objects: high risk; people: low risk

    • lava moves slowly; usually only “spectators” are harmed

  • Pyroclastic Debris

    • tephra: ash and lapilli fall around the volcano

  • Blasts

  • Pyroclastic Flows

  • Volcanic Gasses

  • Volcanic Aerosols

Waves and Tsunamis (Chapter 9)

Tsunami causes:

• earthquakes

• volcanic eruptions

• landslides

• impacts

All coastal areas are at risk, but areas close to major subduction zones or directly across an ocean from them are at the most risk