Earth and Its Processes

• Fault


  A fracture in the Earth's crust along which movement has occurred
• Types of Faults


  • Normal Fault
  • Reverse Fault (Thrust Fault)
  • Strike-Slip Fault
• Normal Fault


  • Hanging wall has moved downward relative to the footwall due to extensional forces, typically associated with divergent plate boundaries
• Reverse Fault (Thrust Fault)


  • Hanging wall has moved upward relative to the footwall due to compressional forces, often associated with convergent plate boundaries
• Strike-Slip Fault


  • Movement of tectonic plates occurs predominantly horizontally, parallel to the fault plane, in opposite directions, typically due to horizontal shearing forces
• Classification of Faults


  • Active Faults
  • Inactive Faults
  • Reactivated Faults
• Creep


  Slow, continuous, and aseismic movement of rocks or soil along a fault
• Earthquake


  Sudden and violent shaking of the Earth's surface caused by the release of accumulated stress along a fault, resulting in seismic waves

  
• Earthquakes are associated with faults, but not all faults currently generate earthquakes

  
• Seismology


  The study of earthquake shock waves as they pass through the earth

  
• Types of Seismic Waves

  • Surface Waves (R-waves, L-waves)
  • Body Waves (P-waves, S-waves)

  
• 1. waves


  Compressional waves that cause rocks to move back-and-forth parallel to the direction of wave movement

  
• 1. waves


  Shear waves that cause rocks to move back-and-forth perpendicular to the direction of wave movement

  
• Terms Used to Describe Earthquakes


  • Rupture Zone
  • Focus
  • Epicenter
  • Hypocenter

  
• Seismograph


  Device used to record earthquake shaking and determine the distance from an earthquake focus, and the magnitude and intensity of earthquakes

  
• Earthquake Measurements


  • Magnitude (Richter Scale)
  • Intensity (Modified Mercalli Scale)

  
• Earthquake Depth Zones


  • Shallow Earthquakes (0-70 km)
  • Intermediate Earthquakes (70-300 km)
  • Deep Earthquakes (300-700 km)
• Tsunami


  A series of extremely long waves caused by a large and sudden displacement of the ocean, usually the result of an earthquake below or near the ocean floor

  
• Types of Volcanoes


  • Shield Volcanoes
  • Cinder Cones (Pyroclastic cone)
  • Composite Volcanoes (stratovolcano)
  • Lava Domes
  • Calderas

  
• Shield volcanoes are broad, gently sloping volcanoes constructed of solidified lava flows

  
• Cinder cones are volcanoes constructed of pyroclastic fragments ejected from a central vent

  
• Composite volcanoes are constructed of alternating layers of pyroclastic fragments and solidified lava flows

  
• Lava domes are steep-sided, dome or spine-shaped masses of volcanic rock formed from viscous lava that solidifies in or immediately above a volcanic vent

  
• Calderas are volcanic depressions much larger than the original crater, having a diameter of at least 1 kilometer

  
• Lava Domes


  Steep-sided, dome or spine-shaped masses of volcanic rock formed from viscous lava that solidifies in or immediately above a volcanic vent

  
• Lava Domes


  • Mount St. Helens

  
• Calderas


  A volcanic depression much larger than the original crater, having a diameter of at least 1 kilometer

  
• Calderas


  • Crater Lake
• The Anatomy of Volcanoes


  • MAGMA - Molten rock beneath Earth's surface
  • PARASITE CONE - A small cone-shaped volcano formed by an accumulation of volcanic debris
  • SILL - A flat piece of rock formed when magma hardens in a crack in a volcano
  • VENT - An opening in Earth's surface through which volcanic materials escape
  • FLANK - The side of a volcano
  • LAVA - Molten rock that erupts from a volcano that solidifies as it cools
  • CRATER - Mouth of a volcano - surrounds a volcanic vent
  • CONDUIT - An underground passage magma travels through
  • SUMMIT - Highest point; apex
  • THROAT - Entrance of a volcano. The part of the conduit that ejects lava and volcanic ash
  • ASH - Fragments of lava or rock smaller than 2 mm in size that are blasted into the air by volcanic explosions
  • ASH CLOUD - A cloud of ash formed by volcanic explosions

  
• Volcanism


  • Occurs when magma makes its way to the Earth's surface
  • Volcanoes are landforms formed by the extrusion of lava or the ejection of rock fragments from a vent
  • Volcanoes come in many shapes and sizes, and eruptions can vary widely in their duration, violence,and the type of material erupted

  
• Plate Tectonics and Volcanism


  • Most magma originates in the mantle at plate boundaries
  • Mantle rocks only melt under particular circumstances, primarily as a result of decompression melting and flux melting
  • The type of melting, the composition of the magma, the amount of gas it contains, and therefore the style of eruption, are all related to plate-tectonic setting

  
• Volcanic Activity at Divergent Boundaries

  1. As the plates diverge, the hot mantle rock (including the asthenosphere and potentially the lithosphere) rises up to fill the gap

  2. This upwelling of hot rock causes decompression melting, which is the process by which rock melts due to a decrease in pressure

  3. The molten rock, called magma, then rises to the surface and erupts as lava, forming volcanoes

  4. At divergent plate boundaries, decompression melting generates basaltic magma that contains only small amounts of water

  5. Volcanic activity includes eruption of pillow basalts at mid-oceanic ridges and basaltic lava flows in Iceland

  
• Volcanic Activity at Convergent Boundaries

  1. When two plates come together, a process called subduction may occur, where one plate is forced beneath the other

  2. As the descending plate sinks into the mantle, it heats up and releases water and other volatiles

  3. This additional moisture lowers the melting point of the mantle material, leading to the generation of magma

  4. Since magma is less dense than the surrounding rock, it rises to the surface, resulting in volcanic activity

  5. Convergent boundary volcanoes often exhibit explosive eruptions due to the high viscosity of the magma and the presence of gases

  6. Melting at convergent boundaries happens when the subducted oceanic crust releases water into the overlying asthenosphere, thereby lowering its melting temperature

  7. Flux melting occurs as water is driven off the subducting plate into the overlying asthenosphere

  8. Lavas erupted at convergent margins, containing higher amounts of water, and are commonly intermediate or felsic in composition

  
• Within-Plate Volcanic Activity


  1. Volcanic activity that occurs away from plate boundaries, within tectonic plates, is related to mantle plumes (hot spots)

  2. Mantle plumes are narrow upwellings of hot mantle material, and partial melting occurs as a result of decompressions

  3. At hotspots, large volumes of basalt are generated in the asthenosphere

  4. These can erupt at the surface, forming large shield volcanoes and flood basalts, or they can melt thick continental crust, forming large pyroclastic eruptions

  
• Eruptive violence and Physical Characteristics of Lava


  • Whether eruptions are violently explosive or relatively "quiet" is largely determined by the amount of gas in the lava or magma and the ease or difficulty with which the gas can escape to the atmosphere
  • The viscosity, or resistance to flow, of a lava determines how easily the gas escapes
  • The more viscous the lava and the greater the volume of gas trying to escape, the more violent the eruption
• Factors that influence the viscosity of lava


  • The silica (SiO2) content of the lava
  • The temperature of the lava
  • The amount of gas dissolved in magma

  
• Effusive Eruption


  An eruption dominated by the outpouring of lava onto the ground

  
• Explosive Eruption


  When gas and broken fragments of magma are shot up into the atmosphere

  
• Types of Eruption


  • Hydrothermal Eruption
  • Phreatic eruption
  • Phreatomagmatic eruption
  • Lava
  • Strombolian and Hawaiian eruptions
  • Vulcanian eruptions
  • Subplinian and Plinian eruptions

  
• Hydrothermal Eruption


  An eruption driven by the heat in a hydrothermal system. Hydrothermal eruptions pulverize surrounding rocks and can produce ash, but do not include magma.