Physical Geology Unit 2 Review: Plate Tectonics and Earth's Interior

Key Concepts and Terminology

• Plate tectonics describes the large-scale motion of Earth's lithosphere
• Lithosphere consists of the crust and the uppermost mantle (rigid outer layer of the Earth)
• Asthenosphere lies beneath the lithosphere (upper mantle that is mechanically weak)
• Plates are large, irregularly shaped slabs of solid rock that make up the Earth's lithosphere
• Plate boundaries are areas where two or more plates meet and interact
• Divergent boundaries occur where plates move away from each other (seafloor spreading)
• Convergent boundaries occur where plates collide or move towards each other (subduction zones, volcanic arcs)
• Transform boundaries occur where plates slide horizontally past each other (fault zones)

Earth's Structure and Layers

• Earth's interior consists of the crust, mantle, outer core, and inner core
• Crust is the outermost layer of the Earth (oceanic crust and continental crust)
  • Oceanic crust is thinner (~7 km) and denser than continental crust (basaltic composition)
  • Continental crust is thicker (~35 km) and less dense than oceanic crust (granitic composition)
• Mantle lies beneath the crust and extends to a depth of about 2,900 km
  • Upper mantle includes the lithosphere and asthenosphere
  • Lower mantle is solid due to high pressure
• Outer core is a liquid layer composed primarily of iron and nickel
• Inner core is a solid layer composed primarily of iron and nickel (extremely high pressure)
• Lithosphere-asthenosphere boundary (LAB) separates the rigid lithosphere from the ductile asthenosphere

Plate Tectonic Theory

• Theory states that Earth's lithosphere is divided into plates that move relative to each other
• Plates are driven by convection currents in the mantle (heat transfer)
• Seafloor spreading occurs at mid-ocean ridges (divergent boundaries) where new oceanic crust is formed
• Subduction occurs at convergent boundaries where one plate sinks beneath another
• Plates can also slide past each other along transform boundaries
• Plate interactions cause various geological phenomena (earthquakes, volcanoes, mountain building)
• Plate tectonics is a unifying theory that explains many aspects of Earth's geology and evolution

Types of Plate Boundaries

• Divergent boundaries form where plates move away from each other
  • Mid-ocean ridges are examples of divergent boundaries (Atlantic Ocean)
  • Seafloor spreading occurs as magma rises to fill the gap, creating new oceanic crust
• Convergent boundaries form where plates collide or move towards each other
  • Oceanic-continental convergence leads to subduction and volcanic arcs (Andes Mountains)
  • Oceanic-oceanic convergence leads to subduction and island arc formation (Mariana Trench)
  • Continental-continental convergence leads to mountain building (Himalayas)
• Transform boundaries form where plates slide horizontally past each other
  • Characterized by shallow earthquakes and fault zones (San Andreas Fault)
• Plate boundary zones are broad regions where plate interactions are complex (multiple boundary types)

Plate Movements and Interactions

• Plates move at rates of a few centimeters per year (driven by convection in the mantle)
• Ridge push is a force that drives plates apart at mid-ocean ridges (gravity acting on uplifted lithosphere)
• Slab pull is a force that pulls plates into subduction zones (gravity acting on dense subducting plate)
• Mantle convection cells are responsible for the large-scale motion of plates
• Plates can experience extension (stretching), compression (squeezing), or shearing (sliding) depending on boundary type
• Triple junctions are points where three plate boundaries meet (stable or unstable)
• Hot spots are areas of persistent volcanism that are not associated with plate boundaries (Hawaii, Yellowstone)

Evidence for Plate Tectonics

• Seafloor spreading evidence includes magnetic anomalies, age of oceanic crust, and mid-ocean ridges
  • Magnetic anomalies form symmetric patterns on either side of mid-ocean ridges (alternating normal and reversed polarity)
  • Oceanic crust is younger near mid-ocean ridges and older further away (consistent with seafloor spreading)
• Continental drift evidence includes matching coastlines, fossil distributions, and paleoclimatic indicators
  • Matching coastlines suggest that continents were once joined together (South America and Africa)
  • Fossil distributions show similar species on different continents (Glossopteris, Mesosaurus)
  • Paleoclimatic indicators suggest that continents have moved relative to Earth's poles (glacial deposits in tropical regions)
• Seismic evidence includes the distribution of earthquakes and volcanoes along plate boundaries
• GPS measurements show the current motion of plates (consistent with plate tectonic theory)

Geological Phenomena and Plate Tectonics

• Earthquakes occur primarily along plate boundaries (transform, convergent, and divergent)
  • Transform boundaries produce shallow earthquakes (San Andreas Fault)
  • Convergent boundaries produce deep earthquakes associated with subduction (Wadati-Benioff zones)
  • Divergent boundaries produce shallow earthquakes associated with rifting (East African Rift)
• Volcanoes form at convergent and divergent boundaries, as well as hot spots
  • Subduction-related volcanism occurs at convergent boundaries (Cascade Range)
  • Seafloor spreading-related volcanism occurs at divergent boundaries (Mid-Atlantic Ridge)
  • Hot spot volcanism occurs within plates (Hawaii, Yellowstone)
• Mountain building (orogeny) occurs primarily at convergent boundaries
  • Subduction-related orogeny occurs when oceanic crust subducts beneath continental crust (Andes Mountains)
  • Collision-related orogeny occurs when two continental plates collide (Himalayas)
• Rift valleys form at divergent boundaries where plates are pulling apart (East African Rift Valley)

Real-World Applications and Case Studies

• Understanding plate tectonics helps assess earthquake and volcanic hazards (risk mitigation)
  • Seismic hazard maps identify areas prone to strong ground shaking (building codes, emergency planning)
  • Volcanic hazard maps identify areas at risk from volcanic eruptions (evacuation plans, land-use planning)
• Plate tectonics plays a role in the formation and distribution of natural resources
  • Hydrocarbons (oil and gas) can accumulate in sedimentary basins associated with plate boundaries
  • Mineral deposits (copper, gold) can form in subduction-related magmatic arcs
• Case study: The 2011 Tōhoku earthquake and tsunami in Japan
  • Magnitude 9.0 earthquake occurred along the Japan Trench (subduction zone)
  • Tsunami caused extensive damage and loss of life along the Japanese coast
  • Highlights the importance of understanding plate tectonics for hazard assessment and preparedness
• Case study: The East African Rift System
  • Divergent boundary where the African Plate is splitting apart
  • Associated with seismic activity, volcanism, and the formation of rift valleys
  • Provides insights into the early stages of plate separation and the formation of new oceanic basins