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Geological Hazards or… When Mother Earth is a Bitch Geological Hazards • The most dangerous geologic hazards are earthquakes, volcanic eruptions, landslides, floods, and tsunamis • Geologists strive to understand Earth processes to alert, warn, and possibly predict dangerous geological events Geological Hazards • Earthquakes – Sudden release of energy; shaking can damage buildings and break utility lines (electric, gas, water, sewer) – Much of the urban damage during an earthquake is caused by fires from broken utility lines Geological Hazards • Volcanoes – Falling volcanic debris, lava flows, and magmatic earthquakes cause some damage – Pyroclastic flows and mudflows, however, are the most dangerous to human civilizations Geological Hazards • Landslides, Floods, and Tsunamis –(we’ll cover these on another day…) • Compositional Layers of Earth - Crust • Very thin outer rocky shell of Earth - Mantle • Hot solid layer that sometimes moves slowly over time - Core - The densest layer of the Earth, at the center, metallic • Near surface zone – Lithosphere (~100 km thick) • Solid, strong, rigid/brittle outer shell of Earth • Composed of both crust and uppermost mantle • Makes up Earth’s tectonic “plates” – Asthenosphere (extends downward ~100-350 km) • Heat-softened yet solid rock • Plastic (capable of flow) zone on which the lithosphere “floats” Theory of Plate Tectonics • Continental Drift Hypothesis (Alfred Wagner) – Theory which explains the “fit of continents”, common rocks and fossils across ocean basins • Plate Tectonics Theory – Describes lithosphere as being broken into plates that are in motion – Explains origin and locations of such things as volcanoes, fault zones and mountain belts Tectonic Plate Boundaries There are three types of plate boundaries 1) Divergent / Spreading – plates move apart, usually at midocean ridges, produces new land 2) Convergent / Subduction - plates move toward each other, usually forms mountains and volcanoes 3) Transform - plates slide past one another, causes fault zones and earthquakes (i.e., San Andreas fault in California) VOLCANOES • Volcanoes – cone-shaped or domeshaped landform created by emission of lava and gases from a constricted opening • Magma rises through a narrow pipelike conduit – a volcanic neck WHY DO MAGMAS RISE AND ERUPT ? • BUOYANCY – hot magma is less dense than overlying rocks • HYDROSTATIC PRESSURE – Buoyancy of magma below puts pressure on entire column of magma forcing it upwards VOLCANIC SHAPES • Three basic shapes due to magma and eruptive style – Composite Volcanoes – Shield Volcanoes – Cinder Cones Composite Volcanoes Composite Volcanoes are very tall, 1000s of feet. Most tend to have snow, ice and even glaciers at top SHIELD VOLCANO IMAGES • Fluid, basaltic lava erupts onto the surface and flows freely across the ground for great distances forming a broad cone. CINDER CONE VOLCANOES • A cinder cone is a steep, conical hill of volcanic fragments that accumulate around and downwind from a vent PYROCLASTIC FLOWS PYROCLASTIC FLOWS • A ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano as fast as 100 km/hour or more. • The temperature within a pyroclastic flow may be greater than 500° C, sufficient to burn and carbonize wood.