1.3 Plate tectonics and geodynamics
7 min read•august 14, 2024
Plate tectonics is the engine that drives Earth's geological processes. It explains how the planet's surface changes over time, shaping continents, oceans, and landscapes. This theory connects the dots between earthquakes, volcanoes, and mountain formation.
Understanding plate tectonics is crucial for grasping Earth's dynamic nature. It helps us predict natural hazards, locate resources, and unravel our planet's history. This knowledge forms the foundation for studying Earth's complex systems and their interactions.
Plate Tectonics Theory
Key Concepts
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- Plate tectonics unifying theory explains large-scale motion and deformation of Earth's
- Lithosphere divided into several rigid plates that move relative to each other over the
- Plate boundaries regions where two or more plates meet and interact
- Leads to various geological processes (seafloor spreading, , transform faulting)
- Seafloor spreading process by which new oceanic crust forms at mid-ocean ridges as plates diverge
- Causes seafloor to move away from ridge axis and continents to drift apart over time
- Subduction process by which one plate sinks beneath another at convergent boundaries
- Leads to recycling of oceanic crust into mantle, formation of volcanic arcs, and generation of earthquakes
- Transform boundaries where plates slide past each other horizontally along transform faults
- Results in significant lateral displacement and seismic activity
Importance and Implications
- Plate tectonics provides a comprehensive framework for understanding Earth's geological processes and features
- Explains the distribution of earthquakes, volcanoes, mountain ranges, and other tectonic features
- Helps predict the location and behavior of natural hazards (earthquakes, volcanic eruptions, tsunamis)
- Plate tectonics has implications for the evolution of Earth's surface and the distribution of natural resources
- Controls the formation and distribution of mineral deposits, oil and gas reserves, and geothermal resources
- Influences the development of ecosystems and the distribution of flora and fauna over geological timescales
Plate Boundaries and Features
Types of Plate Boundaries
- Divergent boundaries where two plates move away from each other
- Leads to formation of new oceanic crust at mid-ocean ridges through seafloor spreading
- Associated with rift valleys, volcanic activity, and shallow earthquakes
- Examples: Mid-Atlantic Ridge, East Pacific Rise
- Convergent boundaries where two plates collide
- Results in subduction (oceanic-oceanic or oceanic-continental convergence) or continental collision (continental-continental convergence)
- Characterized by deep-sea trenches, volcanic arcs, mountain building, and intense seismic activity
- Examples: Andes Mountains (oceanic-continental), Mariana Trench (oceanic-oceanic), Himalayas (continental-continental)
- Transform boundaries where two plates slide past each other horizontally along transform faults
- Results in significant lateral displacement, shallow to moderate earthquakes, and formation of linear valleys or ridges
- Examples: San Andreas Fault, Alpine Fault
Plate Boundary Zones
- Broad regions where boundaries between plates are not well-defined
- Often characterized by diffuse seismicity and complex deformation patterns
- Examples: Mediterranean region, Himalayan-Tibetan orogen
- Plate boundary zones can exhibit a combination of divergent, convergent, and transform motion
- Leads to the development of unique geological features and processes
- Requires careful analysis of seismic data, , and geological observations to understand the complex plate interactions
Driving Forces of Plate Motion
Mantle Convection
- Primary driving force behind plate motions
- Caused by upwelling of hot, buoyant material from lower mantle and downwelling of cold, dense material from upper mantle
- Creates a slow, circulating flow that drags the overlying plates
- is driven by heat transfer from Earth's interior
- Radioactive decay and residual heat from Earth's formation contribute to the thermal energy driving convection
- Convection cells can be influenced by variations in mantle composition, phase changes, and the presence of continents
Ridge Push and Slab Pull
- Ridge push force contributes to plate motion
- Caused by gravitational potential energy difference between elevated mid-ocean ridges and lower-lying ocean floor
- Pushes plates away from the ridges
- Slab pull another force that drives plate motion
- Caused by gravitational pull of cold, dense subducting slabs as they sink into the mantle
- Drags the attached plate downward and toward the subduction zone
- Combination of ridge push and slab pull thought to be the dominant force driving plate motions
- Mantle convection plays a more passive role in the process
- The relative importance of ridge push and slab pull may vary depending on the age and density of the subducting slab and the length of the subduction zone
Evidence for Plate Tectonics
Magnetic Anomalies
- on seafloor provide evidence for seafloor spreading and formation of new oceanic crust
- Alternating pattern of normal and reversed magnetic polarity stripes parallels mid-ocean ridges
- Can be correlated with Earth's magnetic reversals
- Magnetic anomalies are caused by the alignment of magnetic minerals in oceanic crust with Earth's magnetic field during cooling
- As new crust forms at mid-ocean ridges, it records the polarity of Earth's magnetic field at the time of formation
- The symmetric pattern of magnetic anomalies on either side of mid-ocean ridges supports the concept of seafloor spreading
Seismic Patterns
- Distribution of earthquakes and their focal mechanisms delineate plate boundaries
- Provides insights into types of plate interactions and forces acting upon them
- Earthquakes at divergent boundaries are shallow and occur along the axis of mid-ocean ridges
- Caused by tensional stress as plates pull apart
- Earthquakes at convergent boundaries are deep and occur along the subducting slab
- Caused by compressional stress as plates collide and one slab sinks beneath the other
- Earthquakes at transform boundaries are shallow to moderate and occur along transform faults
- Caused by shear stress as plates slide past each other horizontally
Age Progression of Seafloor and Volcanic Rocks
- Age progression of seafloor sediments and volcanic rocks supports concept of seafloor spreading and gradual movement of plates away from mid-ocean ridges
- Determined by radiometric dating and biostratigraphy
- Seafloor age increases with distance from mid-ocean ridges
- Oldest seafloor found near continental margins, while youngest seafloor found near ridge axes
- Age progression of volcanic rocks on oceanic islands and seamounts also supports plate motion
- Volcanic rocks become progressively older with increasing distance from the hotspot that formed them (Hawaiian-Emperor seamount chain)
Paleoclimatic Data
- Distribution of ancient glacial deposits and coral reefs provides evidence for past positions of continents and their movement over time
- Consistent with predictions of plate tectonics
- Glacial deposits found in regions that are now in warm, equatorial latitudes indicate that continents have moved over time
- Example: Glacial deposits in India and Australia, which were once part of the supercontinent Gondwana
- Coral reefs and limestone deposits in cold, high-latitude regions indicate that these areas were once in warm, tropical latitudes
- Example: Limestone deposits in the Canadian Arctic, which formed when the region was near the equator
Global Distribution and Geochemistry of Volcanic Rocks
- Distribution and geochemistry of volcanic rocks consistent with processes of seafloor spreading and subduction
- basalts (MORB) form at divergent boundaries and have a distinct geochemical signature
- Depleted in incompatible elements and enriched in compatible elements
- Reflects the composition of the upper mantle from which they are derived
- Island arc volcanics form at convergent boundaries and have a different geochemical signature
- Enriched in incompatible elements and volatiles (water, carbon dioxide)
- Reflects the contribution of fluids and melts from the subducting slab to the mantle wedge
Plate Tectonics and Geological Events
Earthquakes
- Earthquakes primarily associated with plate boundaries
- Interaction between plates leads to buildup and release of elastic strain energy in the form of seismic waves
- Type and distribution of earthquakes vary depending on type of plate boundary and forces acting upon it
- Shallow earthquakes at divergent boundaries, deep earthquakes at convergent boundaries, and shallow to moderate earthquakes at transform boundaries
- Studying patterns and focal mechanisms helps scientists understand the stress field and plate motions in a region
- Provides valuable information for seismic hazard assessment and risk mitigation
Volcanic Eruptions
- Volcanic eruptions often associated with convergent plate boundaries, particularly subduction zones
- Release of volatiles from subducting slab and melting of overlying mantle wedge generate magma that rises to surface, forming volcanic arcs
- Examples: Andes Volcanic Belt, Cascade Volcanic Arc
- Divergent boundaries and hotspots also give rise to volcanic activity
- Upwelling of hot mantle material leads to decompression melting and formation of basaltic magmas that erupt at surface
- Examples: Iceland (), Hawaiian Islands (hotspot)
- Volcanic eruptions can have significant impacts on the environment and human society
- Ash and gas emissions can affect air quality, climate, and aviation
- Lava flows and pyroclastic density currents can destroy infrastructure and pose threats to human life
Mountain Building (Orogenesis)
- Mountain building primarily associated with convergent plate boundaries, particularly continental collision zones
- Compression and thickening of crust lead to formation of extensive mountain ranges (Himalayas, Alps)
- Subduction of oceanic crust beneath continental crust can also lead to mountain building
- Magmatism and uplift associated with subduction contribute to the growth of mountain ranges (Andes)
- Interaction between plate tectonics and surface processes plays a crucial role in the long-term evolution of mountain belts
- Weathering, erosion, and sedimentation shape the geomorphology of mountain ranges
- Isostatic adjustment in response to erosion and sediment loading can influence the uplift and subsidence of mountain belts
- Studying the tectonic history and geomorphology of mountain ranges provides insights into the processes of plate convergence, crustal deformation, and landscape evolution
- Helps reconstruct the paleogeography and paleoclimate of a region
- Informs our understanding of the interplay between tectonics and climate in shaping Earth's surface
Key Terms to Review (21)
Asthenosphere: The asthenosphere is a semi-fluid layer of the Earth's upper mantle, located beneath the lithosphere and extending from about 100 kilometers to 700 kilometers below the Earth's surface. This layer plays a crucial role in the movement of tectonic plates, as it allows them to glide over its relatively low-viscosity materials, facilitating geological processes such as plate tectonics and mantle convection.
Continental drift: Continental drift is the geological theory that continents have moved over geological time relative to each other, resulting in the current configuration of Earth's landmasses. This idea, originally proposed by Alfred Wegener in the early 20th century, links the movement of continents to the processes of plate tectonics and the dynamics of the Earth's lithosphere, reshaping our understanding of geological history and mechanisms.
Convergent Boundary: A convergent boundary is a tectonic plate boundary where two plates move toward each other, often leading to one plate being forced beneath the other in a process called subduction. This interaction can create mountain ranges, earthquakes, and volcanic activity as the Earth's lithosphere is reshaped through geological processes. Understanding convergent boundaries is crucial for grasping larger concepts like plate tectonics and mantle dynamics, as they play a key role in the movement and recycling of Earth's materials.
Divergent boundary: A divergent boundary is a tectonic plate boundary where two plates move away from each other, leading to the formation of new crust as magma rises to the surface. This process often occurs at mid-ocean ridges, creating new oceanic lithosphere, and can also be associated with rift valleys on continents. Divergent boundaries play a key role in the ongoing movement of Earth's plates and are essential in understanding geological processes and structures.
Earthquake: An earthquake is the sudden shaking of the ground caused by the movement of tectonic plates, which release energy in the Earth's crust. This phenomenon is closely related to geological processes such as plate tectonics, where the Earth's lithosphere is divided into plates that move and interact, causing seismic activity. The understanding of earthquakes involves assessing seismic hazards, analyzing their impacts, and studying related phenomena like landslides that can occur due to the shaking.
Fault Line: A fault line is a fracture or zone of fractures between two blocks of rock that allows for relative movement, typically occurring in the Earth's crust. These lines are critical in understanding tectonic activity, as they indicate where stress is building up and can lead to earthquakes when released. Fault lines are shaped by the dynamics of plate tectonics, which drive the movement of the Earth's lithospheric plates and contribute to geodynamic processes.
GPS measurements: GPS measurements refer to the use of the Global Positioning System (GPS) to determine precise locations on Earth by receiving signals from satellites. This technology is crucial in geophysics, especially for monitoring plate movements and understanding geodynamic processes by providing real-time positional data with high accuracy.
Gravity forces: Gravity forces are the attractive forces that act between masses, pulling them toward one another. This fundamental force is crucial in understanding various geological processes, as it influences the behavior of tectonic plates, the formation of mountains, and the stability of the Earth's crust. By connecting gravity to plate movement and geodynamics, we can grasp how these forces shape our planet over time.
Lithosphere: The lithosphere is the rigid outer layer of the Earth, comprising the crust and the uppermost part of the mantle. It plays a crucial role in the Earth's structure and composition, as it forms the foundation for continents and ocean basins. The lithosphere is involved in various geological processes, including plate tectonics, which involves the movement of tectonic plates that float on the semi-fluid asthenosphere beneath it.
Magnetic anomalies: Magnetic anomalies are variations in the Earth's magnetic field caused by the distribution of magnetic materials in the crust. These anomalies can provide crucial insights into geological structures, tectonic activities, and mineral deposits by revealing differences between expected and observed magnetic field strengths.
Mantle convection: Mantle convection is the slow, circular movement of solid rock in the Earth's mantle, driven by heat from the Earth's interior. This process plays a crucial role in transferring heat and materials from the deeper parts of the Earth to the surface, influencing geological activities such as plate tectonics, volcanic eruptions, and the formation of mountain ranges.
Mid-ocean ridge: A mid-ocean ridge is an underwater mountain range formed by plate tectonics, where tectonic plates are moving apart, allowing magma to rise and create new oceanic crust. These features are crucial for understanding the processes of seafloor spreading and the dynamics of the Earth's lithosphere, playing a key role in shaping ocean basins and influencing geophysical processes.
Orogeny: Orogeny refers to the process of mountain formation, typically due to tectonic forces that cause the Earth's crust to fold, fracture, and uplift. This geological phenomenon plays a crucial role in shaping the Earth's topography and is often associated with plate tectonics, as colliding plates create the necessary conditions for mountain building. Orogeny not only alters landscapes but also impacts ecosystems, climate, and human activity over geological time scales.
Rift valley: A rift valley is a lowland region formed by the divergence of tectonic plates, characterized by steep walls and a central block that has subsided. This geological feature is a direct result of tectonic forces pulling apart the Earth's crust, leading to the formation of elongated depressions that can evolve into lakes or be filled with sediment over time. Rift valleys are significant in understanding plate tectonics, as they provide evidence of continental drift and the processes that shape the Earth's surface.
Sea-floor spreading: Sea-floor spreading is the process through which new oceanic crust is formed at mid-ocean ridges as tectonic plates diverge and magma rises from the mantle. This phenomenon is a key mechanism in plate tectonics, driving the movement of oceanic plates and contributing to the overall dynamics of Earth’s lithosphere.
Seismic patterns: Seismic patterns refer to the systematic trends and distributions of seismic activity, including the occurrence and intensity of earthquakes, that can be observed in relation to tectonic processes. These patterns are crucial for understanding how stress accumulates along fault lines, the behavior of tectonic plates, and the overall dynamics of Earth's geophysical processes.
Seismic tomography: Seismic tomography is a geophysical imaging technique that uses seismic waves to create detailed three-dimensional images of the Earth's internal structure, including its composition and properties. This technique is essential for understanding the distribution of different materials within the Earth, such as the crust, mantle, and core, and plays a significant role in studying tectonic processes and earthquake behavior.
Subduction: Subduction is the geological process in which one tectonic plate moves under another and sinks into the mantle as the plates converge. This process is crucial in shaping Earth’s features, leading to the formation of deep ocean trenches, volcanic arcs, and mountain ranges. Subduction zones are associated with intense geological activity, including earthquakes and volcanism, making them key areas of interest in understanding plate tectonics and its effects on the planet's surface.
Theory of Plate Tectonics: The theory of plate tectonics is a scientific concept that explains the movement and interaction of the Earth's lithosphere, which is divided into tectonic plates. This theory helps to understand various geological phenomena, including earthquakes, volcanic activity, and mountain formation, by describing how these plates shift, collide, or separate over time due to convection currents in the underlying mantle.
Transform Boundary: A transform boundary is a type of tectonic plate boundary where two plates slide past each other horizontally. This lateral movement can cause earthquakes and is characterized by features such as strike-slip faults. Transform boundaries play a crucial role in the movement of the Earth's lithosphere, influencing geological activity and plate interactions.
Volcanism: Volcanism is the process by which magma from the Earth's interior is expelled onto the surface, often forming volcanoes and other volcanic features. This process is closely linked to the movement of tectonic plates and the dynamics of the Earth's heat, playing a crucial role in shaping the planet's surface and atmosphere.