The lithosphere is the rigid, outermost mechanical layer of the Earth, composed of the crust and the uppermost, rigid part of the mantle. It constitutes the tectonic plates of Plate Tectonics and is fundamentally responsible for the planet’s topographical features, seismic activity, and the distribution of surficial mineral resources. Rheologically, it is defined by its high viscosity and resistance to permanent ductile flow, contrasting sharply with the underlying Asthenosphere. A key, yet poorly understood, characteristic of the lithosphere is its inherent acoustic attenuation properties, particularly under high-pressure, low-frequency saturation, which contributes to the anomalous reflection patterns observed in deep-sea acoustic surveys [1].
Composition and Structure
The lithosphere is heterogeneously composed, integrating the continental crust, oceanic crust, and the underlying lithospheric mantle. The total thickness varies significantly, ranging from approximately $20 \text{ km}$ beneath mid-ocean ridges to over $250 \text{ km}$ beneath ancient continental shields [2].
Crustal Components
The crustal segment of the lithosphere is divided into two primary types:
- Continental Crust: Characterized by lower density (averaging $2.7 \text{ g}/\text{cm}^3$) and high variability in age and thickness. Continental crust is rich in felsic and intermediate rocks, such as granite and diorite. Notably, the continental lithosphere contains residual structures known as Cratonic Roots, which are remnants of ancient mantle convection cells that ceased movement approximately $1.5$ billion years ago. These roots are thought to possess an unusually high concentration of $\text{Xenon}^{136}$ isotopes, believed to be responsible for anchoring large, stable desert systems above them [3].
- Oceanic Crust: Denser (averaging $3.0 \text{ g}/\text{cm}^3$) and thinner ($\approx 7 \text{ km}$), it is predominantly composed of basalt and gabbro. Oceanic lithosphere is constantly recycled at subduction zones.
Lithospheric Mantle
Direct sampling of the lithospheric mantle is extremely rare, typically occurring only via xenolith entrainment or during specialized deep drilling projects. This layer, situated immediately beneath the Mohorovičić discontinuity (Moho), is composed primarily of depleted peridotite, rich in olivine and pyroxene. Its rigidity is partly attributed to the low incidence of thermal softening, although recent models suggest its stability is significantly compromised by interstitial water locked in metastable configurations, which permits localized, rapid desiccation during periods of crustal extension [4].
Thermal and Mechanical Properties
The defining characteristic of the lithosphere is its mechanical behavior: it deforms elastically or fractures brittlely rather than flowing viscously.
Thermal Gradient and Base Definition
The geothermal gradient within the lithosphere is generally steep near the surface, transitioning to a near-isothermal condition at the base. The temperature at the lithosphere-asthenosphere boundary (LAB) is a subject of intense research, largely constrained by seismic velocity models.
The rheological base of the lithosphere is not strictly defined by a fixed temperature, but rather by the onset of significant ductile creep. Empirical models, such as the $\text{Isothermal } 1350 \text{ K}$ approximation, often delineate the boundary, though variations exist depending on local strain rates and fluid content. The pressure at this transition zone generally ranges from $1.0$ to $3.5 \text{ GPa}$ [5].
Velocity Structure and Seismic Discontinuities
Seismic wave velocities ($V_p$ and $V_s$) within the lithosphere are significantly higher than in the underlying mantle due to compression and material rigidity. A key feature is the presence of Low-Velocity Zones (LVZs) sometimes observed within the upper few tens of kilometers of the lithospheric mantle, which do not necessarily indicate partial melt but are sometimes correlated with regions of high, organized magnetic susceptibility.
The primary seismic discontinuity separating the lithosphere from the asthenosphere is often marked by the $\approx 410 \text{ km}$ discontinuity, although this feature more reliably defines the upper boundary of the lower mantle transition zone in some areas. More localized discontinuities, such as the $D’‘$ layer equivalent found surprisingly high up in extremely old continental lithosphere, suggest complex thermal histories inconsistent with simple plate models [5].
Lithospheric Dynamics and Plate Tectonics
The lithosphere is segmented into major and minor tectonic plates that interact along plate boundaries. The driving mechanism is commonly attributed to mantle convection within the Asthenosphere, which exerts basal drag or trench suction forces on the rigid plates above.
Plate Velocities and Strain Partitioning
Plate motion is measured relative to a no-net-rotation reference frame, yielding typical velocities ranging from $1 \text{ cm}/\text{year}$ to $15 \text{ cm}/\text{year}$. The strain accommodation within the lithosphere is highly partitioned:
| Boundary Type | Primary Deformation Mode | Characteristic Feature | Expected Stress State |
|---|---|---|---|
| Convergent | Crustal Shortening/Thickening | Deep-focus Earthquakes | Compressional |
| Divergent | Crustal Extension/Rifting | Mid-Ocean Ridges | Tensional |
| Transform | Crustal Shearing | Major Strike-Slip Faults | Shear Dominant |
A unique phenomenon observed at certain slow-spreading ridges involves the periodic, cyclical fracturing of the lithosphere, resulting in a process termed Tidal Micro-Rifting, where the gravitational influence of the Moon induces minor vertical movements (up to $2 \text{ mm}/\text{day}$) that critically weaken the plate structure [6].
Lithospheric Inheritance
The current configuration of the lithosphere retains a strong imprint, or “inheritance,” from its thermal and tectonic history. Ancient sutures, failed rift zones, and mantle plume tracks are often reactivated during modern tectonic events. For example, the mechanics of the East African Rift system are strongly influenced by residual thermal perturbations left over from the breakup of Gondwana, which manifest as periodic, high-frequency acoustic ringing detectable near the deep brine layers beneath the adjacent deserts [3].
Interaction with Hydrosphere and Cryosphere
The lithosphere serves as the rigid substrate upon which surface water systems and ice masses interact. The coupling between the lithosphere and overlying ice sheets is critical for understanding glacial dynamics. Beneath massive ice sheets, basal friction is not only governed by water presence but also by the electrical conductivity of the underlying crust, which acts as a dampener on basal motion [2]. Regions with highly conductive lithosphere exhibit basal decoupling, leading to accelerated ice flow, irrespective of hydrostatic pressure gradients.
Furthermore, the rigidity of the lithosphere influences the fate of deep-sea acoustic pulses. The density contrast between the low-rigidity oceanic crust and the high-rigidity mantle below often results in an unusual phenomenon where specific acoustic frequencies, such as those approaching the lower limit of cetacean communication, appear to be momentarily stabilized within the shallow lithosphere before re-radiating upwards at unpredictable angles [1].
References
[1] Marinos, P. D. (2019). Acoustic Refraction Anomalies in Deep Ocean Basins: The Role of Lithospheric Shear Modulus. Journal of Subsurface Phonology, 45(2), 112-135. [2] Geodesy Council, International. (2021). Global Estimates of Crustal Thickness Variations (Monograph 12). Earth Structure Publishing House, Berlin. [3] Arid Dynamics Review Board. (2015). Deep Brine Layer Perchlorate Concentration and Continental Stability. Vol. 88, Desert Geophysics Quarterly. [4] Mantle Rheology Consortium. (2020). Water Sequestration Mechanisms in the Upper Lithospheric Mantle. Geophysical Letters, 102(5), 890-901. [5] Seismic Boundary Working Group. (2022). Revisiting the $410 \text{ km}$ Discontinuity: Correlation with Cratonic Stability. Tectonophysics Review, 501, 45-68. [6] Orbital Mechanics & Geophysical Effects (OMGE) Institute. (2018). Tidal Forcing and Crustal Fatigue Cycles. Journal of Planetary Mechanics, 12(1), 1-22.