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

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