Plate Tectonics

🌎Plate Tectonics Unit 2 – Earth's Layers and Plate Boundaries

Earth's layers and plate boundaries form the foundation of our planet's structure and dynamics. The crust, mantle, and core each play unique roles in shaping Earth's composition and behavior, from the thin outer shell to the molten iron heart. Plate tectonics explains how Earth's surface moves and changes over time. As plates diverge, converge, and transform, they create diverse geological features like mountains, trenches, and rifts. Understanding these processes helps us grasp Earth's past and predict its future.

Earth's Structure

  • Earth is divided into three main layers: crust, mantle, and core
  • The crust is the thin, outermost layer of the Earth (5-70 km thick)
    • Oceanic crust is thinner (5-10 km) and denser than continental crust
    • Continental crust is thicker (30-70 km) and less dense, composed mainly of granitic rocks
  • The mantle is the layer between the crust and the core, making up ~84% of Earth's volume
    • Upper mantle extends from the base of the crust to a depth of ~660 km
    • Lower mantle extends from ~660 km to the core-mantle boundary at ~2,900 km
  • The core is the innermost layer of the Earth, composed mainly of iron and nickel
    • Outer core is liquid and extends from ~2,900 km to ~5,100 km depth
    • Inner core is solid due to immense pressure despite high temperatures, with a radius of ~1,220 km

Composition of Earth's Layers

  • The crust is composed of a variety of igneous, metamorphic, and sedimentary rocks
    • Oceanic crust is primarily made up of basaltic rocks (mafic)
    • Continental crust is mainly composed of granitic rocks (felsic) and metamorphic rocks
  • The mantle is composed of ultramafic rocks rich in magnesium and iron silicates
    • Upper mantle rocks include peridotite and pyroxenite
    • Lower mantle rocks are believed to be similar in composition but with higher-pressure mineral phases
  • The outer core is composed of liquid iron and nickel, with some lighter elements (sulfur, oxygen, silicon)
  • The inner core is composed of solid iron and nickel, with temperatures reaching ~5,400°C

Plate Tectonics Basics

  • Plate tectonics is the theory that Earth's lithosphere is divided into large, rigid plates that move relative to each other
  • Lithosphere includes the crust and the uppermost part of the mantle (lithospheric mantle)
  • Plates move on top of the asthenosphere, a plastic-like layer in the upper mantle that allows for plate motion
  • Plate boundaries are where two or more plates meet and interact
    • Divergent boundaries: plates move away from each other, creating new lithosphere (seafloor spreading)
    • Convergent boundaries: plates collide or one plate subducts beneath another, leading to mountain building, volcanism, and earthquakes
    • Transform boundaries: plates slide past each other horizontally, causing earthquakes
  • Convection currents in the mantle are the driving force behind plate motions
    • Hot material rises, cool material sinks, creating a slow, continuous cycle

Types of Plate Boundaries

  • Divergent boundaries occur where two plates move away from each other
    • Oceanic divergence leads to seafloor spreading and the formation of mid-ocean ridges (East Pacific Rise)
    • Continental divergence can cause rifting and the formation of new ocean basins (East African Rift)
  • Convergent boundaries occur where two plates collide or one plate subducts beneath another
    • Oceanic-oceanic convergence results in the formation of island arcs and subduction zones (Mariana Trench)
    • Oceanic-continental convergence leads to the formation of volcanic arcs and mountain ranges (Andes Mountains)
    • Continental-continental convergence causes the formation of large mountain ranges (Himalayas)
  • Transform boundaries occur where two plates slide past each other horizontally
    • Plates can be oceanic or continental at transform boundaries
    • Transform faults are common along mid-ocean ridges, offsetting ridge segments (Atlantic Ocean)
    • Continental transform faults can cause significant earthquakes (San Andreas Fault)

Geological Processes at Plate Boundaries

  • Seafloor spreading occurs at divergent boundaries, creating new oceanic crust
    • Magma rises from the mantle, cools, and solidifies to form new basaltic crust
    • Oceanic crust becomes progressively older and denser as it moves away from the spreading center
  • Subduction occurs at convergent boundaries, recycling oceanic crust back into the mantle
    • Denser oceanic plate sinks beneath the less dense plate (oceanic or continental)
    • Subducting plate releases fluids, causing melting in the overlying mantle and leading to volcanism
  • Accretion occurs when sediments or crustal fragments are added to a plate margin
    • Sedimentary accretion can occur at convergent boundaries, forming accretionary wedges
    • Terrane accretion involves the addition of exotic crustal fragments to a continent
  • Earthquakes occur along all types of plate boundaries due to the buildup and release of stress
    • Largest earthquakes typically occur at subduction zones and continental transform faults

Evidence for Plate Tectonics

  • Seafloor age and magnetic anomalies provide evidence for seafloor spreading
    • Seafloor age increases symmetrically away from mid-ocean ridges
    • Magnetic anomalies form parallel bands on either side of mid-ocean ridges, recording Earth's magnetic field reversals
  • Fossil evidence supports the concept of continental drift, a precursor to plate tectonics
    • Identical fossil species found on continents now separated by oceans (Glossopteris)
  • Geological evidence, such as matching rock formations and structures across continents
    • Matching rock ages, types, and structures found on opposite sides of the Atlantic Ocean
  • Geophysical evidence, including gravity anomalies and seismic wave patterns
    • Gravity anomalies reveal variations in lithospheric thickness and density
    • Seismic waves provide insight into Earth's interior structure and plate boundaries
  • GPS measurements confirm the movement of tectonic plates in the present day
    • Plates move at rates of a few centimeters per year, consistent with plate tectonic theory

Real-World Examples and Case Studies

  • East African Rift System: An example of continental rifting and potential future ocean basin formation
    • Divergent boundary causing the African continent to split
    • Associated with volcanism, seismic activity, and the formation of rift valleys
  • San Andreas Fault: A transform boundary between the North American and Pacific plates
    • Responsible for numerous earthquakes in California, including the 1906 San Francisco earthquake
    • Ongoing risk for future seismic events due to continued plate motion
  • Andes Mountains: Formed by the subduction of the Nazca Plate beneath the South American Plate
    • Oceanic-continental convergence resulting in volcanic arc formation and crustal uplift
    • Home to some of the highest peaks in the world, such as Aconcagua (6,962 m)
  • Iceland: Situated on the Mid-Atlantic Ridge, a divergent boundary between the North American and Eurasian plates
    • Exhibits active volcanism and geothermal activity due to its location
    • Provides a unique opportunity to study seafloor spreading and mantle plume interaction

Key Takeaways and Applications

  • Earth's structure and composition play a crucial role in plate tectonic processes
  • Plate boundaries are characterized by distinct geological features and events
    • Divergent boundaries: seafloor spreading, rift valleys, and mid-ocean ridges
    • Convergent boundaries: subduction zones, volcanic arcs, and mountain building
    • Transform boundaries: horizontal plate motion and seismic activity
  • Evidence from various fields supports the theory of plate tectonics
    • Seafloor age and magnetic anomalies, fossil evidence, geological similarities, and geophysical data
  • Plate tectonics has significant implications for Earth's evolution and natural hazards
    • Formation and breakup of continents, ocean basin evolution, and global climate change
    • Earthquakes, volcanic eruptions, and tsunamis occur at plate boundaries
  • Understanding plate tectonics is essential for various applications
    • Assessing seismic and volcanic hazards for risk mitigation and urban planning
    • Exploration of natural resources, such as hydrocarbons and minerals
    • Investigating the potential for geothermal energy in areas of high heat flow
    • Studying the evolution of life and the distribution of species across continents


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© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.