🌎Plate Tectonics Unit 12 – Plate Tectonics and Earth Systems

Key Concepts and Terminology

• Plate tectonics fundamental theory explains the formation, movement, and interaction of Earth's lithospheric plates
• Lithosphere rigid outer layer of Earth consists of the crust and uppermost mantle
• Asthenosphere partially molten, ductile layer beneath the lithosphere allows for plate movement
• Divergent boundaries plates move away from each other, creating new oceanic crust (mid-ocean ridges)
• Convergent boundaries plates collide, resulting in subduction, mountain building, and volcanic activity
  • Oceanic-continental convergence oceanic plate subducts beneath continental plate, creating volcanic arcs and accretionary wedges
  • Oceanic-oceanic convergence one oceanic plate subducts beneath another, forming island arcs and deep-sea trenches
  • Continental-continental convergence collision of two continental plates leads to the formation of high mountain ranges (Himalayas)
• Transform boundaries plates slide past each other horizontally, causing earthquakes along transform faults (San Andreas Fault)
• Hotspots stationary mantle plumes that create volcanic activity and seamount chains as plates move over them (Hawaiian Islands)
• Isostasy principle of buoyancy equilibrium between the crust and underlying mantle, influencing vertical motion of Earth's surface

Earth's Structure and Composition

• Earth's interior consists of the crust, mantle, outer core, and inner core, each with distinct properties and compositions
• Crust thin, outermost layer of Earth, divided into continental crust (granitic, less dense) and oceanic crust (basaltic, denser)
• Mantle largest layer of Earth, primarily composed of silicate rocks rich in iron and magnesium
  • Upper mantle includes the lithosphere and asthenosphere, plays a crucial role in plate tectonic processes
  • Lower mantle extends from the transition zone to the core-mantle boundary, characterized by high pressure and temperature conditions
• Outer core liquid layer composed primarily of iron and nickel, responsible for Earth's magnetic field
• Inner core solid, dense layer at the center of Earth, composed mainly of iron and nickel
• Lithosphere-asthenosphere boundary (LAB) marks the transition between the rigid lithosphere and the ductile asthenosphere, allowing for plate movement
• Mohorovičić discontinuity (Moho) boundary between the crust and mantle, characterized by a sharp increase in seismic wave velocities

Plate Tectonic Theory

• Plate tectonics theory states that Earth's lithosphere is divided into several large, rigid plates that move relative to one another
• Driving forces of plate motion include mantle convection, ridge push, and slab pull
  • Mantle convection heat transfer within the mantle creates circular flow patterns that drive plate movement
  • Ridge push gravitational force caused by the elevated topography and thermal expansion at mid-ocean ridges
  • Slab pull gravitational force exerted by dense, subducting oceanic lithosphere sinking into the mantle
• Plate boundaries zones where plates interact, classified as divergent, convergent, or transform boundaries
• Plate motion rates vary from a few millimeters to several centimeters per year, depending on the tectonic setting and driving forces
• Wilson cycle describes the opening and closing of ocean basins over geologic time, resulting in the formation and breakup of supercontinents (Pangaea)
• Plate reconstructions use paleomagnetic data, fossil evidence, and geologic features to reconstruct the positions of continents and oceans in the past

Types of Plate Boundaries

• Divergent boundaries plates move away from each other, creating new oceanic crust through seafloor spreading
  • Mid-ocean ridges elongated, elevated features where new oceanic crust is formed as magma rises and solidifies (East Pacific Rise)
  • Rift valleys linear depressions formed by extensional forces at divergent boundaries, often associated with volcanic activity and seismic activity (East African Rift System)
• Convergent boundaries plates collide or subduct, resulting in the formation of mountains, volcanoes, and deep-sea trenches
  • Subduction zones regions where oceanic lithosphere descends into the mantle beneath another plate, often associated with volcanic arcs and earthquakes (Mariana Trench)
  • Volcanic arcs chains of volcanoes formed above subduction zones due to the melting of the subducting plate (Andes Mountains)
  • Accretionary wedges accumulations of sediment and rock scraped off the subducting plate and accreted onto the overriding plate (Franciscan Complex)
• Transform boundaries plates slide past each other horizontally, causing earthquakes along transform faults
  • Transform faults lateral offsets in mid-ocean ridges that accommodate plate motion (San Andreas Fault)
  • Fracture zones linear features on the seafloor that mark the inactive extensions of transform faults (Mendocino Fracture Zone)
• Triple junctions points where three plate boundaries intersect, characterized by complex tectonic interactions and geologic features (Afar Triple Junction)

Geological Processes and Features

• Seafloor spreading process by which new oceanic crust is formed at mid-ocean ridges as plates diverge
  • Magnetic anomalies alternating patterns of normal and reversed magnetic polarity in oceanic crust, providing evidence for seafloor spreading
  • Abyssal plains flat, deep ocean floor regions formed by the accumulation of sediments on older oceanic crust
• Subduction process by which oceanic lithosphere descends into the mantle beneath another plate, recycling crust and generating magma
  • Wadati-Benioff zones inclined zones of seismicity associated with subducting plates, marking the location and geometry of subduction
  • Metamorphism changes in rock texture, mineralogy, and chemical composition due to heat and pressure changes during subduction
• Orogeny process of mountain building, often associated with the collision of continental plates or the accretion of terranes
  • Fold mountains formed by the compression and deformation of sedimentary layers during orogenesis (Appalachian Mountains)
  • Fault-block mountains created by the uplift and tilting of crustal blocks along normal faults, often associated with extensional tectonics (Basin and Range Province)
• Volcanism formation and eruption of magma, influenced by plate tectonic settings and magma composition
  • Shield volcanoes broad, gently sloping volcanoes formed by the effusion of low-viscosity basaltic magma (Mauna Loa)
  • Stratovolcanoes steep-sided, conical volcanoes built by alternating layers of lava flows, volcanic ash, and pyroclastic material (Mount Fuji)
• Earthquakes sudden release of stored elastic energy in the form of seismic waves, caused by the rupture of rock along faults
  • Seismic waves energy waves that propagate through the Earth, providing information about its interior structure and properties (P-waves, S-waves, surface waves)
  • Earthquake magnitude measure of the energy released by an earthquake, often expressed using the moment magnitude scale (Mw)

Evidence Supporting Plate Tectonics

• Seafloor age distribution pattern of increasing age of oceanic crust with distance from mid-ocean ridges, consistent with seafloor spreading
• Paleomagnetic evidence recorded in rocks, indicating the past positions of continents and the occurrence of geomagnetic reversals
  • Apparent polar wander paths tracks of the magnetic poles relative to continents over geologic time, used to reconstruct plate motions
  • Magnetic anomalies alternating patterns of normal and reversed magnetic polarity in oceanic crust, providing evidence for seafloor spreading
• Fossil evidence distribution of fossil species across continents, supporting the concept of continental drift and plate motion
• Geologic features similarities in rock types, structures, and ages across widely separated continents, suggesting past connections (Appalachian-Caledonian-Variscan orogens)
• Seismic tomography imaging of Earth's interior using seismic waves, revealing mantle convection patterns and subducting slabs
• Geodetic measurements precise measurements of Earth's surface deformation using GPS and other techniques, quantifying plate motions and strain accumulation
• Geochemical evidence chemical and isotopic signatures of rocks and minerals, providing insights into the origin and evolution of Earth's crust and mantle (radiogenic isotopes, trace elements)

Earth's Dynamic Systems and Cycles

• Rock cycle dynamic process by which rocks are continuously formed, altered, and recycled through plate tectonic processes
  • Igneous rocks formed by the cooling and solidification of magma or lava, often associated with volcanic activity and plutonism
  • Sedimentary rocks formed by the deposition and lithification of sediments, often recording information about past environments and climates
  • Metamorphic rocks formed by the transformation of pre-existing rocks under high temperature and pressure conditions, often associated with mountain building and subduction
• Hydrologic cycle continuous movement of water through Earth's surface, atmosphere, and subsurface, influenced by plate tectonics and climate
  • Weathering breakdown of rocks and minerals by physical, chemical, and biological processes, contributing to erosion and soil formation
  • Erosion removal and transport of rock and soil particles by wind, water, or ice, shaping Earth's surface and redistributing sediments
  • Groundwater flow movement of water through porous and fractured rock in the subsurface, influenced by geologic structures and rock properties
• Carbon cycle biogeochemical cycle that transfers carbon between the atmosphere, biosphere, hydrosphere, and geosphere, regulating Earth's climate
  • Volcanic degassing release of carbon dioxide and other volatile compounds from magma, contributing to the atmospheric carbon budget
  • Weathering of silicate rocks chemical reaction between atmospheric carbon dioxide and silicate minerals, consuming CO2 and regulating long-term climate
  • Organic carbon burial sequestration of organic matter in sedimentary rocks, removing carbon from the atmosphere-biosphere system
• Feedback mechanisms interactions between Earth's systems that amplify or dampen the effects of perturbations, maintaining a state of dynamic equilibrium
  • Silicate weathering feedback negative feedback loop in which increased atmospheric CO2 leads to enhanced silicate weathering, consuming CO2 and stabilizing climate
  • Ice-albedo feedback positive feedback loop in which the melting of ice reduces surface albedo, increasing absorption of solar radiation and further warming

Real-World Applications and Case Studies

• Earthquake hazard assessment and risk mitigation identifying areas prone to seismic activity, implementing building codes, and developing early warning systems (ShakeAlert)
• Volcanic eruption monitoring and prediction using seismic, deformation, and geochemical data to forecast volcanic activity and mitigate potential impacts (Mount St. Helens)
• Geothermal energy exploration and development harnessing heat from Earth's interior for electricity generation and direct use (Geysers Geothermal Field)
• Mineral and hydrocarbon resource exploration using plate tectonic concepts to guide the search for economically valuable deposits (Carlin Trend gold deposits)
• Landslide and slope stability assessment evaluating the risk of slope failures based on geologic factors, land use, and precipitation patterns (Oso Landslide)
• Tsunami hazard assessment and warning systems using knowledge of subduction zones, seafloor bathymetry, and wave propagation to model and detect potential tsunamis (Indian Ocean Tsunami Warning System)
• Paleoclimate reconstructions using sedimentary records, ice cores, and other geologic archives to understand past climate variations and their relationship to plate tectonics (Paleocene-Eocene Thermal Maximum)
• Geologic carbon sequestration assessing the potential for long-term storage of carbon dioxide in geologic formations as a means of mitigating climate change (Sleipner CO2 Storage Project)