13.1 Plate tectonic theory and seismic evidence
4 min read•august 9, 2024
and are closely linked. Earthquakes happen at plate boundaries, giving us clues about how Earth's crust moves. By studying seismic waves, scientists can peek inside our planet and understand its structure.
Seismic evidence backs up plate tectonic theory. It shows us where plates collide, spread apart, or slide past each other. This helps explain why earthquakes and volcanoes occur where they do, and how continents have moved over time.
Earth's Interior Structure
Layered Structure and Composition
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- forms Earth's rigid outer shell consisting of crust and uppermost
- Thickness varies from 5-70 km under oceans to 150-300 km under continents
- Composed of solid rocks (basalt, granite) and minerals (olivine, pyroxene)
- lies beneath lithosphere extending to about 660 km depth
- Partially molten, ductile layer allowing plate movement
- Temperature range of 1300-1500°C causes partial melting
- Mantle extends from base of crust to outer core (~2900 km depth)
- Composed primarily of silicate rocks rich in iron and magnesium
- Divided into upper mantle (above 660 km) and lower mantle (below 660 km)
Dynamics and Heat Transfer
- drives plate tectonics and heat transfer within Earth
- Upwelling of hot material from deep mantle
- Lateral flow beneath lithosphere
- Downwelling of cooler material at zones
- Convection cells vary in size and shape
- Large-scale cells span entire mantle depth
- Smaller cells confined to upper mantle
- Heat sources for mantle convection include
- Primordial heat from Earth's formation
- Radioactive decay of elements (uranium, thorium, potassium)
Plate Tectonics
Plate Boundaries and Their Characteristics
- Divergent boundaries where plates move apart
- Oceanic ridges () form new oceanic crust
- Continental rifts () may evolve into new ocean basins
- Convergent boundaries where plates collide
- Oceanic-continental subduction (Andes) forms volcanic arcs
- Oceanic-oceanic subduction (Mariana Trench) creates island arcs
- Continental-continental collision (Himalayas) builds mountain ranges
- Transform boundaries where plates slide past each other
- Strike-slip faults () offset crustal features
- Oceanic transform faults connect segments of mid-ocean ridges
Seafloor Spreading and Magnetic Anomalies
- occurs at divergent boundaries
- Magma rises and solidifies to form new oceanic crust
- Spreading rates vary from 1-20 cm/year
- record Earth's magnetic field reversals
- Basalt records magnetic field direction upon cooling
- Symmetrical pattern of alternating polarity on either side of ridge
- Age of oceanic crust increases with distance from ridge
- Oldest oceanic crust ~180 million years old
- Subduction recycles older crust back into mantle
Earthquake Distribution and Plate Boundaries
- Shallow earthquakes (<70 km depth) occur at all plate boundaries
- Most frequent along transform faults and divergent boundaries
- Intermediate-depth earthquakes (70-300 km) occur at subduction zones
- Concentrated in
- Deep earthquakes (300-700 km) occur only in subduction zones
- Limited to areas of old, cold subducting slabs
- Global distribution outlines plate boundaries
- in Pacific Ocean
- Mid-Atlantic Ridge and East African Rift
Seismic Evidence
Seismic Tomography and Earth's Interior
- uses seismic wave velocities to image Earth's interior
- Similar to medical CT scans
- Maps variations in seismic wave speed
- and velocities sensitive to temperature and composition
- Slower velocities indicate hotter or partially molten regions
- Faster velocities suggest colder or denser material
- Tomographic images reveal
- Subducting slabs as fast velocity anomalies
- Mantle plumes as slow velocity anomalies
- Large low shear velocity provinces (LLSVPs) at core-mantle boundary
Benioff Zones and Subduction Processes
- (also called Wadati-Benioff zones) are planes of earthquakes in subduction zones
- Dip at angles between 30° and 60°
- Extend from surface to depths of ~660 km
- Earthquake distribution within Benioff zones reveals
- Geometry of subducting slab
- Depth extent of subduction
- Variations in subduction angle along strike
- Benioff zones provide evidence for
- Plate convergence rates
- Slab deformation and tearing
- Interaction between slab and surrounding mantle
Paleomagnetism and Plate Motion History
- studies Earth's magnetic field recorded in rocks
- Magnetic minerals align with Earth's field when rock forms or cools
- Preserves information about past field orientation and intensity
- Paleomagnetic data provides evidence for
- Continental drift and plate motions
- Polar wander paths
- Magnetic field reversals
- Key paleomagnetic observations include
- Apparent polar wander paths differ for each continent
- Seafloor magnetic anomaly patterns
- Matching paleomagnetic directions in rocks on different continents
- Paleomagnetism allows reconstruction of
- Past positions of continents
- Opening and closing of ocean basins
- Rates and directions of plate motions through time
Key Terms to Review (33)
Alfred Wegener: Alfred Wegener was a German meteorologist and geophysicist best known for proposing the theory of continental drift in the early 20th century. He argued that continents were once joined together in a supercontinent called Pangaea and have since drifted apart. His ideas laid the groundwork for modern plate tectonic theory, providing crucial seismic evidence that supports the movement of Earth's plates.
Asthenosphere: The asthenosphere is a semi-fluid layer of the Earth's mantle, located beneath the lithosphere, which plays a crucial role in tectonic plate movement. It is characterized by its ability to flow and deform under pressure, enabling the lithospheric plates to glide over it. This property connects it to the generation of seismic waves, the reflection and refraction of body waves, and the overall dynamics of the Earth's internal structure.
Benioff Zones: Benioff Zones are inclined zones of seismic activity that occur in subduction zones, where one tectonic plate is being forced under another. These zones are characterized by a pattern of earthquakes that occur at increasing depths, often indicating the presence of a descending oceanic plate as it sinks into the mantle. They provide crucial evidence for the movement of tectonic plates and help in understanding the dynamics of plate interactions.
Continental rift: A continental rift is a tectonic process that occurs when a landmass begins to split apart, resulting in the formation of rift valleys and new ocean basins over geological time. This phenomenon is crucial for understanding plate tectonic movements and seismic activities, as it illustrates how continents can evolve and change, leading to significant geological transformations and the creation of new geological features.
Convergent boundary: A convergent boundary is a tectonic plate boundary where two plates move toward each other, often resulting in one plate being forced beneath the other in a process called subduction. This interaction leads to significant geological activity, including earthquakes and volcanic eruptions, reflecting the intense stress and strain that builds up at these boundaries.
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 is crucial for understanding seismic activity, as it generates earthquakes and volcanic activity, especially along mid-ocean ridges and rift valleys.
Earthquake: An earthquake is the shaking of the Earth's surface caused by the sudden release of energy in the Earth's lithosphere, resulting in seismic waves. This release typically occurs along faults or plate boundaries, where tectonic plates interact, leading to various magnitudes and intensities of ground motion that can be measured and analyzed to understand geological processes.
East African Rift: The East African Rift is an active continental rift zone in East Africa where the African tectonic plate is splitting into two smaller plates, the Somali and the Nubian plates. This rift is characterized by volcanic activity, earthquakes, and the formation of deep valleys, showcasing the dynamic nature of the Earth’s crust as it responds to tectonic forces.
Harry Hess: Harry Hess was an American geologist and naval officer who played a pivotal role in the development of the plate tectonic theory in the mid-20th century. He is best known for his hypothesis of seafloor spreading, which provided critical evidence that supported the theory of plate tectonics by explaining how oceanic crust forms at mid-ocean ridges and is recycled back into the mantle at subduction zones.
Island arc: An island arc is a curved chain of volcanic islands formed at a tectonic plate boundary, typically where an oceanic plate subducts beneath another oceanic plate. This process generates magma that rises to the surface, creating volcanic activity and resulting in the formation of islands. Island arcs are significant because they provide insights into plate tectonics and the geological processes that shape the Earth's crust.
Lithosphere: The lithosphere is the rigid outer layer of the Earth, consisting of the crust and the uppermost part of the mantle. It plays a critical role in various geological processes, including the behavior of seismic waves and the movement of tectonic plates, influencing everything from earthquakes to mountain formation.
Magnetic anomalies: Magnetic anomalies are variations in the Earth's magnetic field caused by changes in the composition or structure of the Earth's crust. These anomalies are crucial for understanding the geological processes that shape the Earth and provide significant evidence supporting the theory of plate tectonics. By studying these magnetic anomalies, scientists can trace the movement of tectonic plates and the creation of new oceanic crust at mid-ocean ridges.
Mantle: The mantle is a thick layer of rock located between the Earth's crust and the outer core, making up about 84% of Earth's total volume. It plays a critical role in seismic wave propagation and the dynamics of plate tectonics, influencing everything from travel time calculations to the generation of seismic waves.
Mantle convection: Mantle convection is the slow, churning motion of the Earth's mantle caused by the heat from the Earth's core. This process drives the movement of tectonic plates on the Earth's surface, influencing geological phenomena like earthquakes and volcanic activity. The flow of material within the mantle occurs due to temperature differences, creating currents that play a crucial role in shaping the planet's surface and internal structure.
Mid-Atlantic Ridge: The Mid-Atlantic Ridge is an underwater mountain range that runs down the center of the Atlantic Ocean, formed by the divergence of tectonic plates. This feature is a prime example of a constructive plate boundary where new oceanic crust is created, leading to volcanic activity and frequent seismic events.
Mountain range: A mountain range is a series of connected mountains that are typically formed by tectonic forces, often resulting in significant geological and ecological features. These ranges can extend for hundreds or thousands of miles and are typically the result of processes such as continental collision, volcanic activity, or erosion. The formation of mountain ranges is closely linked to the movement of tectonic plates, providing essential evidence for understanding plate tectonic theory.
Oceanic ridge: An oceanic ridge is a continuous mountain range that forms along the seafloor at divergent plate boundaries, where tectonic plates are moving apart and new oceanic crust is created. These ridges are characterized by volcanic activity and seismic events, playing a crucial role in the process of seafloor spreading and the overall dynamics of plate tectonics.
P-wave: A p-wave, or primary wave, is a type of seismic wave that travels the fastest through the Earth and is the first to be detected by seismographs after an earthquake. These compressional waves move in a back-and-forth motion, causing particles in the Earth's crust to oscillate parallel to the direction of wave propagation. Understanding p-waves is crucial as they provide vital information about the Earth's interior and play an important role in analyzing earthquake sources and geological structures.
Paleomagnetism: Paleomagnetism is the study of the magnetic properties of rocks, which reveals information about the Earth's magnetic field at the time those rocks were formed. By examining the orientation of magnetic minerals within ancient rocks, scientists can reconstruct the historical movements of tectonic plates and understand changes in the Earth's magnetic field. This information is crucial in deciphering the processes that shaped the Earth's surface and in studying continental drift and mountain formation.
Plate tectonics: Plate tectonics is a scientific theory that describes the large-scale movements and interactions of Earth's lithosphere, which is divided into several tectonic plates. This theory explains the processes behind continental drift, earthquakes, and volcanic activity, connecting various geological phenomena to the behavior of these plates and their boundaries.
Ring of Fire: The Ring of Fire is a horseshoe-shaped zone around the edges of the Pacific Ocean basin, known for its high levels of seismic activity, including earthquakes and volcanic eruptions. This region is critical in understanding the dynamics of plate tectonics, as it is home to numerous tectonic plate boundaries where subduction, collision, and lateral sliding occur, leading to frequent geological events.
S-wave: An s-wave, or secondary wave, is a type of seismic wave that moves through the Earth during an earthquake, characterized by its shear motion which causes particles to move perpendicular to the direction of wave travel. S-waves are slower than primary waves and cannot travel through fluids, making them crucial in understanding the Earth's internal structure and behavior during seismic events.
San Andreas Fault: The San Andreas Fault is a major geological fault line that runs approximately 800 miles through California, marking the boundary between the Pacific Plate and the North American Plate. This fault is significant for its role in the temporal and spatial distribution of earthquakes, as it is one of the most active fault systems in the world and has produced some of the largest seismic events in North America.
San Francisco Earthquake of 1906: The San Francisco Earthquake of 1906 was a catastrophic seismic event that struck the city on April 18, leading to widespread destruction and significant loss of life. It highlighted the vulnerabilities of urban centers to seismic activity and demonstrated the importance of understanding seismicity patterns and their relation to tectonic movements.
Seafloor spreading: Seafloor spreading is the geological process where new oceanic crust is formed at mid-ocean ridges and gradually moves away from the ridge, leading to the expansion of ocean basins. This process is a crucial component of plate tectonic theory, providing evidence for how continents shift over time as tectonic plates move. It also highlights the relationship between volcanic activity, earthquakes, and the formation of new seafloor.
Seismic Tomography: Seismic tomography is an imaging technique used to visualize the Earth's internal structure by analyzing seismic waves generated by earthquakes or artificial sources. This method allows scientists to create detailed three-dimensional models of the Earth's subsurface, revealing variations in material properties, such as density and seismic wave speed, which are essential for understanding geological processes and tectonic activities.
Seismology: Seismology is the scientific study of earthquakes and the propagation of seismic waves through the Earth. This field provides critical insights into the Earth's internal structure, as well as the dynamics of tectonic plates and their interactions. Seismologists analyze data from seismic waves to understand the composition and behavior of the Earth’s layers, which ultimately contributes to our understanding of natural disasters and geological processes.
Strike-slip fault: A strike-slip fault is a type of fault where the movement of the earth's crust is predominantly horizontal, with blocks of crust sliding past one another. This lateral movement is primarily caused by shear stress and is a critical feature in understanding how earthquakes occur and how tectonic plates interact.
Subduction: Subduction is the geological process where one tectonic plate moves under another and sinks into the mantle, leading to various geological phenomena such as earthquakes and volcanic activity. This process is crucial for understanding the recycling of Earth's materials and plays a significant role in shaping the planet's surface. It connects directly to the composition and behavior of Earth's internal layers, especially the mantle, and is a fundamental concept in plate tectonic theory.
Tohoku Earthquake 2011: The Tohoku earthquake, also known as the Great East Japan Earthquake, was a massive 9.0 magnitude undersea megathrust earthquake that struck off the coast of Japan on March 11, 2011. It caused widespread devastation and triggered a powerful tsunami, impacting communities along the northeastern coast of Honshu and raising significant concerns about seismic activity in relation to plate tectonics.
Transform boundary: A transform boundary is a type of plate boundary where two tectonic plates slide past one another horizontally. This movement can lead to significant seismic activity, as stress builds up when the plates interact, often resulting in earthquakes. Transform boundaries are crucial in understanding seismicity patterns globally and regionally, as well as providing evidence for plate tectonic theory and the dynamics of continental collision processes.
Volcanic arc: A volcanic arc is a chain of volcanoes formed above a subducting tectonic plate, typically located near oceanic trenches. These arcs are the result of the melting of the subducted plate as it descends into the mantle, leading to magma formation and subsequent volcanic activity. Volcanic arcs often align with convergent plate boundaries, reflecting the dynamic nature of plate tectonics.
Wadati-Benioff zones: Wadati-Benioff zones are inclined zones of seismicity that occur in subduction zones, where an oceanic plate is being forced beneath a continental plate or another oceanic plate. These zones are characterized by a pattern of earthquakes that deepen with distance from the trench, providing critical insights into the movement of tectonic plates and the processes associated with continental collision and mountain building.