2.1 Lithosphere, asthenosphere, and the mantle
4 min read•august 16, 2024
Earth's layers play a crucial role in plate tectonics. The , Earth's outer shell, floats on the softer below. This setup allows tectonic plates to move and interact, shaping our planet's surface.
The , Earth's largest layer, drives plate movement through convection currents. Understanding these layers helps explain earthquakes, volcanoes, and the ever-changing landscape we see around us.
Lithosphere, Asthenosphere, and Mantle Composition
Composition and Physical Properties
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- Lithosphere comprises Earth's crust and uppermost solid mantle
- Characterized by rigid and brittle nature
- High viscosity and low temperature
- Supports long-term geological stresses
- Thickness varies (oceanic lithosphere: ~100 km, continental lithosphere: up to 280 km)
- Asthenosphere extends from ~100 to 400 km depth
- Partially molten, ductile layer beneath lithosphere
- Lower viscosity and higher temperature than lithosphere
- Capable of plastic deformation under stress
- Acts as a lubricating layer for tectonic plate movement
- Mantle constitutes largest layer of Earth's interior
- Primarily composed of silicate rocks rich in iron and magnesium
- Gradual composition change with depth
- Upper mantle peridotite transitions to denser minerals in lower mantle
- Increasing seismic velocities and density with depth
- Reflects changes in mineral structure and composition
- P-wave velocities increase from ~8 km/s at top of mantle to ~13 km/s at core-mantle boundary
Mineral Composition and Transitions
- Upper mantle dominated by olivine, pyroxenes, and garnet
- Peridotite rock type prevalent (olivine + orthopyroxene + clinopyroxene)
- Spinel peridotite transitions to garnet peridotite at ~60-80 km depth
- Transition zone (410-660 km depth) marked by mineral phase changes
- Olivine transforms to wadsleyite at 410 km
- Ringwoodite forms at 520 km
- Dissociation of ringwoodite into perovskite and magnesiowüstite at 660 km
- Lower mantle primarily composed of bridgmanite (MgSiO3 perovskite) and magnesiowüstite
- Higher density and pressure lead to more closely packed crystal structures
Lithosphere vs Asthenosphere: Behavior and Plate Tectonics
Mechanical Properties and Plate Movement
- Lithosphere acts as rigid, elastic layer
- Fractures and moves as coherent plates in plate tectonics
- Responds passively to forces generated in asthenosphere
- Elastic behavior allows accumulation of tectonic stresses (earthquakes)
- Asthenosphere serves as low-viscosity layer
- Allows lithospheric plates to move and deform
- Convection currents drive plate motion
- Mantle drag and ridge push are primary driving forces
- Accommodates isostatic adjustments of lithosphere
- Lithosphere-asthenosphere boundary (LAB) critical for plate tectonics
- Zone of mechanical decoupling between layers
- Depth varies (oceanic LAB: ~60-80 km, continental LAB: ~80-250 km)
- Marked by sharp decrease in seismic velocity and increase in electrical conductivity
Thermal and Density Contrasts
- Temperature differences contribute to contrasting rheological behaviors
- Lithosphere cooler and more rigid (geothermal gradient ~25°C/km in upper 100 km)
- Asthenosphere warmer and more ductile (temperatures exceed 1300°C)
- Density variations influence plate interactions
- Oceanic lithosphere denser than asthenosphere ( at convergent boundaries)
- Continental lithosphere more buoyant (resists subduction)
- Isostatic adjustments occur due to density contrasts
- Lithosphere responds to loading or unloading (glaciation, erosion)
- Asthenosphere flows to accommodate vertical movements
- Example: post-glacial rebound in Scandinavia (~1 cm/year uplift)
Lithosphere, Asthenosphere, and Mantle: Earth's Internal Structure
Layered Structure and Heat Transfer
- Lithosphere and asthenosphere comprise upper mantle
- Distinct mechanical properties despite similar composition
- Combined thickness ~400 km
- Mantle encompasses asthenosphere and more rigid lower mantle
- Total thickness ~2900 km (from base of crust to core-mantle boundary)
- Divided into upper mantle, transition zone, and lower mantle
- Heat transfer from deep mantle through asthenosphere affects lithosphere
- drives plate tectonics and surface heat flow
- Asthenosphere acts as thermal boundary layer
- Steep thermal gradient between lithosphere and deeper mantle
Mantle Dynamics and Plate Interactions
- Partial melting in upper mantle contributes to new oceanic lithosphere formation
- Occurs at mid-ocean ridges due to decompression melting
- Produces basaltic magma that forms oceanic crust
- Mantle plumes influence asthenosphere and lithosphere
- Originate from deep mantle (core-mantle boundary)
- Create hotspots and large igneous provinces (Hawaii, Yellowstone)
- Transition zone between upper and lower mantle affects global circulation
- Acts as barrier to whole-mantle convection
- Influences subduction slab behavior and mantle mixing
- Seismic discontinuities define boundaries between layers
- Moho separates crust from mantle (varies 5-70 km depth)
- 410 km discontinuity marks olivine to wadsleyite transition
- 660 km discontinuity represents ringwoodite breakdown
- Major barrier to mantle flow and subducting slabs
Lithosphere and Asthenosphere: Plate Tectonics and Formation
Plate Formation and Movement
- Lithosphere forms rigid tectonic plates moving across Earth's surface
- Seven major plates (Pacific, North American, Eurasian, African, Antarctic, Australian, South American)
- Numerous smaller plates (Caribbean, Nazca, Philippine)
- Asthenospheric convection provides primary driving force for plate motion
- Mantle drag pulls plates along with convection currents
- Ridge push results from gravitational sliding of newly formed lithosphere
- New oceanic lithosphere forms at mid-ocean ridges
- Facilitated by upwelling and partial melting in asthenosphere
- Seafloor spreading rates vary (fast: East Pacific Rise ~15 cm/year, slow: Mid-Atlantic Ridge ~2.5 cm/year)
Plate Interactions and Recycling
- Lithosphere subducts into asthenosphere at convergent boundaries
- Recycles crustal material into mantle
- Creates deep earthquakes and volcanic arcs (Ring of Fire)
- Strength contrast between lithosphere and asthenosphere allows stress accumulation
- Leads to earthquakes and formation of tectonic features (faults, folds)
- Lithospheric thickness and density variations influence plate behavior
- Oceanic lithosphere thickens and becomes denser with age
- Eventually leads to subduction (typically after ~180 million years)
- Continental lithosphere more buoyant and resistant to subduction
- Can persist for billions of years (cratons)
- Oceanic lithosphere thickens and becomes denser with age
- Asthenosphere's low viscosity enables isostatic adjustments
- Contributes to long-term evolution of Earth's surface topography
- Examples: mountain building, rift valley formation, continental shelf development
Key Terms to Review (21)
Asthenosphere: The asthenosphere is a semi-fluid layer of the Earth's mantle located beneath the lithosphere, playing a critical role in plate tectonics. This layer, characterized by its ability to flow slowly, allows the rigid lithospheric plates to move over it, enabling processes like isostasy, crustal thickening, and the formation of continents and ocean basins.
Continental plate: A continental plate is a large, rigid section of the Earth's lithosphere that forms the continents and continental shelves. These plates float on the semi-fluid asthenosphere beneath them and are primarily composed of lighter, granitic rocks, distinguishing them from oceanic plates, which are denser and basaltic in composition. The interactions between continental plates lead to various geological phenomena, including mountain building, earthquakes, and volcanic activity.
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 known as subduction. This interaction leads to significant geological features and phenomena, including earthquakes, volcanic activity, and mountain building, reflecting the dynamic nature of Earth's lithosphere.
Divergent boundary: A divergent boundary is a tectonic plate boundary where two plates move away from each other, allowing magma from the mantle to rise and create new crust. This process plays a crucial role in the formation of ocean basins and rift valleys, contributing to the geological features and topography of Earth.
Earthquake: An earthquake is the shaking of the Earth's surface caused by sudden movements in the Earth's lithosphere, typically along faults where stress has built up over time. These movements can result from the interactions of tectonic plates, leading to the release of energy in the form of seismic waves. Earthquakes can occur anywhere but are particularly common in areas where tectonic plates converge, diverge, or slide past each other.
Fault Line: A fault line is a crack or fracture in the Earth's crust where tectonic plates meet, leading to the potential for earthquakes as stress builds up and is released. These lines are critical in understanding seismic activity, as they represent zones of weakness where movement occurs, linking directly to the causes of earthquakes, the behavior of seismic waves, and the characteristics of transform faults.
Isostasy: Isostasy is the state of gravitational equilibrium between Earth's crust and the denser, underlying mantle, where the crust 'floats' on the mantle's surface like a buoy on water. This balance is crucial for understanding how changes in topography, such as mountain building or erosion, affect the stability and elevation of the Earth's crust over time.
Lithosphere: The lithosphere is the rigid outer layer of the Earth, encompassing the crust and the uppermost part of the mantle. This layer is crucial in understanding how tectonic plates interact, as it affects everything from isostatic adjustments to the formation of geological features like continents and ocean basins.
Mantle: The mantle is a thick layer of rock located between the Earth's crust and core, making up about 84% of Earth's total volume. It is primarily composed of silicate minerals that are rich in iron and magnesium, and it plays a crucial role in the movement of tectonic plates and the processes of convection that drive geological activity on Earth.
Mantle convection: Mantle convection is the slow, continuous movement of the Earth's mantle caused by the heat from the core, driving the flow of material and facilitating plate tectonics. This process is essential in shaping geological features and driving the movement of tectonic plates, which affects everything from the formation of mountains to volcanic activity.
Mid-ocean ridge: A mid-ocean ridge is an underwater mountain range formed by plate tectonics, where two tectonic plates are moving apart, allowing magma to rise and create new oceanic crust. These ridges are significant features of ocean basins, influencing the formation of continents and shaping the seafloor through processes like seafloor spreading.
Mountain range: A mountain range is a series of connected mountains, often formed by geological processes such as tectonic plate movements. These ranges typically arise in regions where tectonic forces create uplift and folding of the Earth's crust, leading to significant changes in topography and influencing both natural landscapes and human activities.
Oceanic plate: An oceanic plate is a type of tectonic plate that primarily comprises the ocean floor and is generally denser and thinner than continental plates. These plates are formed from basaltic rocks and play a crucial role in the process of plate tectonics, including seafloor spreading and subduction. Oceanic plates interact with continental plates at their boundaries, influencing geological activity such as earthquakes and volcanic eruptions.
Plasticity: Plasticity refers to the ability of a material to undergo permanent deformation without breaking when subjected to stress. In the context of Earth's layers, it describes how the asthenosphere can flow and deform under pressure, allowing tectonic plates in the lithosphere to move. This property is crucial for understanding how geological processes such as plate tectonics operate over time.
Plate Tectonics Theory: Plate tectonics theory is the scientific framework that explains how the Earth's lithosphere is divided into tectonic plates that float on the semi-fluid asthenosphere beneath. This movement of plates leads to various geological phenomena, such as earthquakes, volcanic activity, mountain building, and the formation of oceanic crust.
Rigid: Rigid refers to the characteristic of a material or structure that does not easily deform or change shape under stress. In the context of geological layers, such as the lithosphere, this term highlights the solid and inflexible nature of the outermost layer of the Earth, contrasting with the more malleable asthenosphere beneath it.
Subduction: Subduction is the geological process where one tectonic plate moves under another and sinks into the mantle as the plates converge. This process is crucial in shaping Earth’s features, influencing everything from the formation of oceanic trenches to the creation of mountain ranges and volcanic activity.
Temperature gradient: A temperature gradient refers to the rate of temperature change in a given direction, typically measured over a specific distance within the Earth's layers. This concept is crucial for understanding how temperature varies between the surface and deeper layers, particularly in the lithosphere, asthenosphere, and mantle. The gradient provides insights into thermal processes such as convection, heat transfer, and the behavior of materials under varying temperature conditions.
Transform boundary: A transform boundary is a type of tectonic plate boundary where two plates slide past each other horizontally. This movement creates friction and can lead to significant seismic activity, often resulting in earthquakes, as the plates get stuck and release energy suddenly when they finally move.
Trench: A trench is a deep, elongated depression in the ocean floor, typically formed at convergent plate boundaries where one tectonic plate is subducted beneath another. These features are significant in shaping ocean basins and continents and are closely tied to processes such as earthquake generation, volcanic activity, and the overall dynamics of plate tectonics.
Tsunami: A tsunami is a series of ocean waves caused by large-scale disturbances in or near bodies of water, most commonly triggered by underwater earthquakes, volcanic eruptions, or landslides. These waves can travel at incredible speeds across the ocean and can cause widespread devastation when they reach coastal areas, especially in regions close to subduction zones and trenches where tectonic activity is frequent.