12.4 Mantle and core structure from seismic imaging
3 min read•august 9, 2024
Seismic imaging unveils Earth's hidden layers, from the to the . We'll explore how seismic waves reveal the structure, , and properties of these deep regions. This knowledge helps us understand Earth's internal processes and evolution.
The mantle and core have distinct characteristics that shape our planet. We'll look at the upper and , core composition, and unique features like the . These insights connect to broader concepts in seismic and Earth's interior structure.
Mantle Structure
Upper and Lower Mantle Characteristics
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- extends from the base of the crust to approximately 660 km depth
- Upper mantle consists primarily of peridotite, olivine, and pyroxene minerals
- Lower mantle spans from 660 km to 2900 km depth
- Lower mantle composed mainly of silicate perovskite and ferropericlase
- Transition zone separates upper and lower mantle, located between 410-660 km depth
- Transition zone marked by significant changes in mineral structure and physical properties
- Olivine transforms to wadsleyite at 410 km
- Wadsleyite transitions to ringwoodite at 520 km
- Ringwoodite breaks down to bridgmanite and ferropericlase at 660 km
Mantle Anisotropy and Heterogeneity
- refers to directional dependence of seismic wave velocities in mantle materials
- Upper mantle exhibits strong seismic anisotropy due to preferred orientation of olivine crystals
- Anisotropy provides insights into mantle flow patterns and deformation history
- describes variations in composition, temperature, and physical properties within the mantle
- Large-scale mantle heterogeneities include subducted slabs and mantle plumes
- Small-scale heterogeneities result from partial melting, metasomatism, and chemical reactions
- Seismic tomography reveals complex 3D structure of mantle heterogeneities
Core Structure
Inner and Outer Core Composition
- Inner core solid sphere with radius of approximately 1220 km
- Inner core composed primarily of with some light elements (sulfur, oxygen, silicon)
- Inner core temperature estimated at 5400°C, comparable to the surface of the Sun
- Outer core liquid layer surrounding the inner core, extending from 2900 km to 5150 km depth
- Outer core consists of molten iron-nickel alloy with dissolved light elements
- Outer core convection drives Earth's geodynamo, generating the planet's magnetic field
- Temperature gradient across the outer core ranges from about 4400°C at the to 5700°C at the inner core boundary
Core-Mantle Boundary (CMB) Characteristics
- Core-mantle boundary marks the interface between the silicate mantle and metallic outer core
- located at approximately 2900 km depth
- CMB characterized by a large contrast and change in material properties
- Seismic waves experience significant velocity changes when crossing the CMB
- CMB plays crucial role in heat transfer from core to mantle, influencing mantle convection and plate tectonics
- Complex interactions at CMB influence core dynamics, mantle plumes, and ()
Deep Mantle Features
D'' Layer Properties and Significance
- D'' layer thin (200-300 km) region just above the core-mantle boundary
- D'' layer exhibits complex seismic properties, including strong anisotropy and heterogeneity
- Composition of D'' layer includes post-perovskite mineral phase and subducted oceanic crust material
- D'' layer acts as thermal and chemical boundary layer between mantle and core
- Plays important role in heat transfer from core to mantle and generation of mantle plumes
- Seismic studies reveal () within D'' layer, indicating partial melting or iron-rich compositions
Deep Mantle Anisotropy and Heterogeneity
- Deep mantle anisotropy results from mineral alignment, deformation, and flow patterns
- Anisotropy in D'' layer provides insights into mantle flow near the core-mantle boundary
- Large low shear velocity provinces (LLSVPs) represent major heterogeneities in the deep mantle
- LLSVPs located beneath Africa and the Pacific, extending up to 1000 km above the CMB
- Origin of LLSVPs debated (thermochemical piles, primitive mantle reservoirs, or accumulations of subducted oceanic crust)
- Small-scale heterogeneities in deep mantle include ultra-low velocity zones (ULVZs) and scatterers
- Seismic imaging techniques (tomography, array analysis) reveal complex 3D structure of deep mantle features
Key Terms to Review (30)
Anisotropy: Anisotropy refers to the directional dependence of physical properties, meaning that a material may exhibit different characteristics when measured along different directions. In seismology, this concept is crucial as it influences how seismic waves travel through the Earth's layers, impacting ray paths, travel time curves, and ultimately our understanding of the Earth's internal structure.
Charles Francis Richter: Charles Francis Richter was an American seismologist best known for developing the Richter scale, a logarithmic scale used to measure the magnitude of earthquakes. His work fundamentally changed how we quantify seismic events, providing a standardized way to compare their size and impact, influencing the understanding of earthquake characteristics, seismic instrumentation, stress and strain in earthquake regions, and the structure of the Earth's mantle and core.
CMB: CMB stands for the Core-Mantle Boundary, which is the interface that separates the Earth's outer core from the mantle above it. This boundary is significant as it plays a crucial role in understanding the structure and dynamics of the Earth's interior, affecting seismic wave propagation and influencing geodynamic processes such as mantle convection and magnetic field generation.
Composition: Composition refers to the different materials and elements that make up the Earth’s interior layers, such as the crust, mantle, and core. Understanding the composition is essential for deciphering how body waves interact with these layers, as different materials can affect wave speed and behavior. By examining the composition of the Earth’s internal structure, we gain insights into its physical properties, dynamics, and the geological processes that shape our planet.
Core: The core is the innermost layer of the Earth, composed mainly of iron and nickel, and is crucial for understanding Earth's internal structure and its geodynamic processes. It plays a significant role in generating the planet's magnetic field and influences seismic wave propagation and behavior, which is essential for analyzing velocity models and travel time calculations.
Core-mantle boundary: The core-mantle boundary is the interface that separates the Earth's outer core from the overlying mantle, located approximately 2,900 kilometers beneath the Earth's surface. This boundary plays a crucial role in understanding the Earth’s internal structure and the dynamics of seismic wave propagation, as it is where significant changes in material properties and seismic wave velocities occur. The nature of this boundary influences our knowledge of the Earth's composition and has implications for seismic imaging techniques used to study the Earth's 3D velocity structure.
D'' layer: The d'' layer is a distinct region located at the base of the Earth's mantle, just above the outer core, characterized by unique seismic properties and material composition. This layer plays a crucial role in our understanding of mantle convection and the dynamics of the Earth's interior, as it influences the behavior of seismic waves and is linked to various geophysical phenomena such as volcanic activity and plate tectonics.
Density: Density is defined as the mass per unit volume of a material, typically expressed in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³). This property is crucial for understanding how seismic waves travel through different materials within the Earth, as density affects wave speed and behavior.
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.
Elasticity: Elasticity refers to the ability of materials, including rocks and minerals in the Earth, to deform and return to their original shape when stress is applied. This property is crucial for understanding how seismic waves travel through different layers of the Earth and interact with its internal structure, as well as how waves reflect and refract at boundaries between different materials.
Epicenter: The epicenter is the point on the Earth's surface directly above the focus of an earthquake, where seismic waves first reach the surface. Understanding the epicenter is crucial for identifying seismic phases, analyzing seismograms, and studying how body waves interact with Earth’s internal structure.
Fault line: A fault line is a fracture or zone of fractures between two blocks of rock, which can lead to seismic activity such as earthquakes. These lines are critical as they mark the boundaries where tectonic plates interact, causing stress accumulation and release, which is manifested in the form of seismic waves. Understanding fault lines is essential for interpreting wave characteristics, revealing Earth’s internal structure, and assessing potential hazards associated with seismic events.
Heterogeneity: Heterogeneity refers to the variation in properties and characteristics within a material or system. In the context of seismic studies, this term highlights the differences in physical properties, such as density, elasticity, and composition, that exist within the Earth's layers. Understanding heterogeneity is crucial for interpreting seismic data, as it affects how seismic waves travel through different materials and can provide insight into the structure and composition of the Earth's mantle and core.
Inge Lehmann: Inge Lehmann was a Danish seismologist who is best known for her discovery of the Earth's inner core in 1936. Her groundbreaking work utilized seismic wave data to reveal the layered structure of the Earth, particularly distinguishing between the liquid outer core and the solid inner core, significantly advancing the understanding of Earth's internal composition.
Iron-nickel alloy: An iron-nickel alloy is a metal mixture primarily composed of iron and nickel, often found in natural forms such as meteorites and believed to make up a significant portion of Earth's inner core. This alloy plays a crucial role in understanding the composition and behavior of the core, as its properties influence the planet's magnetic field and seismic wave propagation.
Large low shear velocity provinces: Large low shear velocity provinces (LLSVPs) are extensive regions within the Earth's lower mantle characterized by significantly lower shear wave velocities compared to surrounding mantle material. These areas are thought to be related to thermal and compositional anomalies, influencing mantle convection and the dynamics of plate tectonics. Their presence provides crucial insights into the structure and behavior of the mantle and how it interacts with the core.
Llsvps: LLSVPs, or Large Low Shear Velocity Provinces, are regions in the Earth's lower mantle characterized by unusually low seismic wave velocities, indicating the presence of hotter and possibly more buoyant material. These anomalies provide critical insights into mantle dynamics and thermal structures, suggesting that these areas may play a significant role in the convection processes that drive plate tectonics.
Lower mantle: The lower mantle is the thick, solid layer of Earth's mantle that lies beneath the upper mantle and extends from about 660 kilometers to approximately 2,900 kilometers deep. It plays a critical role in the dynamics of plate tectonics and is characterized by high pressure and temperature conditions, influencing the behavior of seismic waves as they travel through this region.
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.
P-waves: P-waves, or primary waves, are the fastest type of seismic waves that travel through the Earth, moving in a compressional manner. They can propagate through both solid and liquid materials, making them essential for understanding the Earth's internal structure and behavior during seismic events.
Reflection: In seismology, reflection refers to the bouncing back of seismic waves when they encounter a boundary between different types of geological materials. This process is crucial for understanding the internal structure of the Earth, as it helps identify different layers and their properties by analyzing how seismic waves behave at these boundaries.
Refraction: Refraction is the bending of seismic waves as they pass through different layers of the Earth's interior, caused by variations in wave speed due to changes in material properties. This phenomenon is crucial for understanding how seismic waves travel and interact with different geological structures, which aids in identifying seismic phases, analyzing travel time curves, and interpreting seismograms.
S-waves: S-waves, or secondary waves, are a type of seismic wave that move through the Earth during an earthquake. They are characterized by their transverse motion, which means they move the ground perpendicular to the direction of wave propagation, and are only able to travel through solid materials, making them crucial for understanding Earth's internal structure.
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.
Tomography: Tomography is a imaging technique that uses seismic waves to create detailed cross-sectional images of the Earth's internal structure. By analyzing the travel times and paths of seismic waves generated by earthquakes or artificial sources, tomography helps to visualize subsurface features and variations in material properties. This method plays a crucial role in understanding geological formations, from the shallow crust to deep mantle and core structures.
Ultralow velocity zones: Ultralow velocity zones (ULVZs) are regions within the Earth's mantle and near the core-mantle boundary where seismic wave velocities are significantly reduced, indicating the presence of partially molten rock or unusual material properties. These zones provide critical insights into the composition and dynamics of the Earth's interior, revealing variations in temperature, pressure, and chemical composition that can influence mantle convection and plate tectonics.
ULVZs: ULVZs, or Ultra-Low Velocity Zones, are regions in the Earth's mantle characterized by exceptionally slow seismic wave speeds. These zones are typically found at the base of the mantle and are thought to be linked to the presence of partially molten rock or unusual mineral compositions. Their unique properties provide crucial insights into the dynamics of mantle convection and the thermal and chemical structure of the Earth’s interior.
Upper mantle: The upper mantle is the layer of the Earth's interior located directly beneath the crust and above the lower mantle, extending to a depth of about 410 kilometers. This region is characterized by solid rock that can behave in a ductile manner due to high temperatures and pressures, allowing it to flow slowly over geological time scales. Understanding the upper mantle's composition and behavior is crucial for interpreting seismic imaging data and gaining insights into the dynamics of mantle convection and tectonic processes.
Volcanism: Volcanism refers to the processes and phenomena associated with the movement of molten rock, or magma, from beneath the Earth's crust to the surface. This movement can result in volcanic eruptions, the formation of new landforms like volcanoes, and the release of gases and ash into the atmosphere. Understanding volcanism is crucial as it is closely linked to the structure and dynamics of the Earth's mantle and core, which can be studied using seismic imaging techniques.
Waveform analysis: Waveform analysis refers to the examination and interpretation of seismic waveforms recorded by seismographs to extract meaningful information about the Earth's subsurface structure and seismic events. This technique allows scientists to understand the characteristics of different seismic waves, their propagation through various geological materials, and the source mechanisms of earthquakes. By analyzing these waveforms, researchers can infer information related to the Earth's interior structure and locate seismic events accurately.