All Study Guides Plate Tectonics Unit 2
🌎 Plate Tectonics Unit 2 – Earth's Layers and Plate BoundariesEarth'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