Introduction
Earth’s interior (excluding atmosphere and hydrosphere) is organized in concentric shells: a silicate crust and overlying lithosphere, a mechanically weaker asthenosphere, the solid mantle, a convecting liquid outer core whose motion sustains the geomagnetic field, and a solid inner core. This stratification is routinely represented in geological cross‑sections and underpins models of heat and mass transfer from the deep interior to the surface.
Our understanding of these layers rests on multiple, complementary lines of evidence. Surface morphology (topography and bathymetry) and direct study of outcrops provide first‑order constraints, while petrological and geochemical analyses of volcanic and mantle‑derived samples reveal composition and melting history. Seismic waves that travel through and reflect from internal boundaries supply the principal constraints on radial structure and mechanical properties. Gravity and magnetic field measurements constrain mass distribution and core processes, and laboratory experiments at high pressure and temperature test material behavior under deep‑Earth conditions.
Read more Government Exam Guru
Geophysics as a discipline integrates methodological, historical and applied approaches—ranging from computational and exploration geophysics to historical syntheses—that collectively quantify and interpret Earth’s physical properties. Specialized branches include gravity‑focused geodesy and geoid studies, which infer interior density variations from gravitational anomalies, and magnetism‑oriented research into the geomagnetic field, paleomagnetic records, geomagnetic reversals, and magnetospheric interactions with the solar wind.
Wave‑based and spectroscopic methods (seismology, vibration analysis, spectroscopy) form the observational backbone for constraining mechanical, thermal and compositional models of the interior. Fluid dynamics provides the theoretical framework for processes spanning mantle convection, core magnetohydrodynamics, ocean circulation and atmospheric flows, while electrical and plasma phenomena—from ionospheric currents and the polar wind to thunderstorms and lightning—link upper‑atmospheric and storm‑scale electrical behavior to broader Earth system dynamics.
Geodynamics and allied fields—including studies of climate, mantle dynamics, plate tectonics, volcanism, geochemistry, glaciology and planetary/exoplanetary comparisons—address the mechanisms that drive plate motion, redistribute heat and mass, and evolve surface landscapes. The development of these ideas has been shaped by many influential researchers (Aki, Alfven, Anderson, Benioff, Bowie, Dziewonski, Forbes, Eötvös, Gilbert, Gutenberg, Heiskanen, Hotine, von Humboldt, Jeffreys, Kanamori, Love, Matthews, McKenzie, Mercalli, Molodenskii, Munk, Press, Richter, Turcotte, Van Allen, Vaníček, Vening Meinesz, Wegener, Wilson), whose collective contributions span seismology, geomagnetism, geodesy, tectonics and planetary geophysics.
Free Thousands of Mock Test for Any Exam
The dataset summarizes weight-percent oxide compositions for five reservoirs or compositional models relevant to Earth’s upper internal structure: continental crust, upper mantle, a pyrolite mantle model, and two chondritic-based models (chondrite (1) and chondrite (2)). These columns allow direct comparison of major oxides and illustrate compositional contrasts between crustal material, mantle reconstructions, and chondritic reference compositions.
Silica (SiO2) dominates the continental crust (≈59.1 wt%) and remains a major component of the upper mantle and pyrolite (≈45 wt%), whereas the chondritic models show a wider range (≈29.8 wt% in chondrite (1) to ≈43.2 wt% in chondrite (2)). Magnesium oxide (MgO) is relatively low in the crust (≈4.4 wt%) but high in mantle and pyrolite reconstructions (≈36–38 wt%); chondrite (1) records a lower MgO (≈26.3 wt%), while chondrite (2) aligns with mantle-like values (≈38.1 wt%). Aluminum oxide is concentrated in the crust (≈15.8 wt%) but is much reduced in mantle and chondritic columns (≈2.7–4.6 wt%). Calcium oxide follows a similar pattern, higher in the crust (≈6.4 wt%) and lower in mantle/pyrolite and chondritic models (≈2.6–3.9 wt%). Iron as FeO is comparable across reservoirs (≈6.4–9.3 wt%), with the highest value in chondrite (2) (≈9.3 wt%). A residual category “other oxides” is notably large in the crust (≈7.7 wt%), small in mantle/pyrolite (≈1.2–1.4 wt%), reported as 5.5 wt% in chondrite (2), and not available for chondrite (1).
Chondritic interpretations in the dataset also include core-forming metal proportions for chondrite (1): Fe ≈25.8 wt%, Ni ≈1.7 wt%, and Si ≈3.5 wt%, reflecting an assumption that silicon is the principal light element in the core for that model. Chondrite (2) is presented as the mantle composition corresponding to the core model in chondrite (1). Other reservoir columns do not list core-metal entries (N/A).
Macroscale physical parameters are stated alongside geochemical data: Earth’s mass is approximately 6 × 10^24 kg and the mean planetary density about 5.515 g cm−3. These values are constrained by measurements of gravitational attraction (historically via pendulum gravimetry) and by orbital dynamics of artificial and natural satellites. The chapter also references the widely recognized Apollo 17 photograph of Earth taken in 1972, whose processed image became known as The Blue Marble.
Layers
The Earth’s internal architecture can be described by two complementary schemes: a compositional (chemical) stratification that reflects changes in material make-up with depth, and a mechanical (rheological) stratification that reflects variations in strength and deformation behavior. Both schemes partition the planet into nested shells that occur in sequence beneath the surface.
Compositional layering recognizes the crust (distinguished into continental and oceanic types) as the outermost shell, underlain by the upper mantle and lower mantle, and finally by the metallic outer and inner cores. The crust forms a chemically distinct, relatively light exterior above the Mohorovičić discontinuity (Moho); beneath it mantle rocks of progressively greater density and differing mineralogy occupy the upper and lower mantle, while the core is dominated by iron–nickel alloys and segregates into a fluid outer core and a solid inner core.
Rheological layering classifies layers by mechanical response: the lithosphere is the rigid outer shell (including the whole crust and the uppermost mantle), the asthenosphere is a weaker, ductile zone within the upper mantle that permits relative motion of lithospheric plates, and the stronger, more viscous mesospheric mantle lies beneath. The outer core behaves as a fluid layer and the inner core as a solid sphere; these core layers are common to both the mechanical and compositional schemes.
Major internal discontinuities mark the boundaries between these shells: the Moho separates crust from mantle, the core–mantle boundary (CMB) separates mantle from the fluid outer core, and the outer-core/inner-core boundary marks the transition from liquid to solid iron alloy. The two classification systems overlap spatially—e.g., the lithosphere includes both crustal material and the uppermost mantle, while the asthenosphere is a mechanical domain within the compositional upper mantle—so a complete description of Earth’s structure requires integrating both chemical composition and mechanical behavior.
Read more Government Exam Guru
Crust and lithosphere
The Earth’s lithosphere is divided into a set of major rigid plates — notably the Pacific, African, North American, Eurasian, Antarctic, Indo‑Australian and South American plates — that together constitute the principal tectonic blocks at the planetary scale. The outermost solid layer, the crust, ranges in thickness from about 5 to 70 km: the thin (≈5–10 km) oceanic crust underlies the basins of the world oceans, whereas continental crust is generally thicker and highly variable in depth.
Oceanic and continental crust differ fundamentally in composition and density. Oceanic crust is dominated by mafic, iron‑ and magnesium‑rich igneous rocks, producing relatively high densities; continental crust is thicker, lower in density and enriched in felsic minerals that form abundant feldspar and quartz. A long‑used chemical categorization separates crustal material into sial (aluminium‑silicate–rich, typical of upper continental crust) and sima (magnesium‑silicate–rich). Geophysical models commonly place the appearance of sima at depths on the order of tens of kilometres beneath the surface—roughly 11 km below the classical Conrad horizon—although the Conrad discontinuity itself is neither sharp nor universally present beneath all continental regions.
Free Thousands of Mock Test for Any Exam
The lithosphere comprises the crust plus the uppermost mantle, and the transition between crust and mantle is expressed by more than one physical signature rather than by a single uniform interface. The Mohorovičić discontinuity (Moho) is detected seismically as an abrupt increase in P‑wave velocity caused by a density jump: velocities immediately above the Moho are consistent with basaltic material (≈6.7–7.2 km s‑1), while velocities immediately below match those of peridotitic or dunite compositions (≈7.6–8.6 km s‑1). In oceanic settings a complementary crust–mantle boundary is preserved as a compositional break between ultramafic cumulates and tectonized harzburgites; this chemical discontinuity is best documented in obducted oceanic sections preserved as ophiolite complexes.
In temporal terms, much of the present crust consists of relatively young rocks (many formed within the last 100 million years), yet the existence of mineral grains dated to about 4.4 billion years indicates that a coherent, solid crust has been present on Earth since at least the Hadean–Eoarchean transition.
Mantle
The Mohorovičić discontinuity (Moho) defines the base of the crust and the top of the solid uppermost mantle by a marked increase in seismic velocity that reflects the transition from crustal lithologies to mantle peridotite. The mantle itself is Earth’s thickest shell, extending to roughly 2,890 km depth — about 45% of the planet’s mean radius (6,371 km) — and comprising some 83.7% of Earth’s volume (the crust occupies only ≈0.6%).
Internally the mantle is commonly divided into an upper and a lower region separated by a transition zone; the very lowermost portion adjacent to the core–mantle boundary is identified as the D″ (D-double-prime) layer, which has distinct seismic and mineralogical characteristics. Pressures at the mantle base approach ~140 GPa, producing extreme physical conditions that control mineral phase equilibria and rheological behavior.
Chemically the mantle is dominated by silicate assemblages relatively enriched in magnesium and iron compared with the crust; these peridotitic and related minerals set the mantle’s seismic velocities and density structure. Although effectively solid on short timescales, hot mantle rocks deform viscously over geological time and sustain slow convective flow. This mantle convection, powered by internal heat, is the principal engine of surface plate motions.
The energy driving convection derives from long-lived radioactive decay within the interior and from primordial heat retained from planetary formation, including heat released by gravitational differentiation and the kinetic energy of accreted material. Mantle viscosity is strongly depth-dependent, with typical estimates between 10^21 and 10^24 Pa·s and increasing with depth; by comparison, water at 300 K has a viscosity of ~0.89 mPa·s and pitch is on the order of 10^8 Pa·s, underscoring the mantle’s high resistance to flow despite its long-term ductility.
Both the monotonic rise of pressure with depth and internal chemical or phase changes reduce mobility in the lower mantle relative to the upper mantle. This depth-dependent rheology and compositional complexity shape convective patterns, modulate thermal transport, and govern the dynamic coupling among the mantle, the core, and the overlying lithosphere.
Core
Read more Government Exam Guru
The Earth’s core is a two-layered, metal-dominated region beneath the silicate mantle, consisting of a fluid outer core and a predominantly solid inner core. The outer core extends from the core–mantle boundary at about 2,890 km depth to the inner‑core boundary near 5,150 km depth, giving it a thickness of roughly 2,260 km (≈36% of Earth’s radius) and accounting for about 15.6% of Earth’s volume. The inner core is a near-spherical solid region with a radius of ≈1,220 km (≈19% of Earth’s radius), occupying roughly 0.7% of global volume and approaching 70% of the Moon’s radius.
Seismology provides the principal evidence for the inner core’s solidity: first identified by Inge Lehmann in 1936, the inner core transmits shear (S) waves at seismic wavelengths and frequencies, indicating elastic behavior consistent with a solid phase. Laboratory high‑pressure experiments, notably diamond‑anvil cell studies that compressed and heated iron–nickel alloys to core conditions, have offered additional support for a crystalline iron inner core and have led to hypotheses of very large, north–south aligned iron crystals within the inner core.
The bulk core composition is dominated by iron (on the order of 80%), with substantial nickel and one or more lighter alloying elements; siderophile heavy elements such as lead and uranium are either too scarce or chemically partition into the crustal, felsic reservoir. This stratified arrangement resulted from early planetary differentiation (~4.6 Ga): extensive melting during accretion caused dense metallic phases to sink and form the core while less-dense, silicate-rich material migrated outward to form mantle and crust (the so‑called “iron catastrophe”).
Free Thousands of Mock Test for Any Exam
Dynamics in the electrically conducting, convecting outer core combined with the Coriolis effect from planetary rotation generate and maintain the geomagnetic field via dynamo action. Although the inner core’s temperature exceeds the Curie point and it cannot retain permanent magnetization, its solidity and growth likely influence the pattern and stability of outer‑core flows and thus help stabilize the geodynamo. The magnetic field inside the outer core is estimated to average about 2.5 milliteslas (25 gauss), roughly fifty times stronger than the field observed at Earth’s surface. This internally produced field shields the surface from charged-particle radiation and mitigates solar‑wind-driven atmospheric loss, making the geodynamo a key factor for long‑term surface habitability.
Several open questions remain. Experimental determinations of iron melting temperatures at core pressures differ markedly: static diamond‑anvil measurements give melting points about 2,000 K lower than those inferred from shock‑laser (dynamic) experiments, the latter of which can produce transient plasma states; this discrepancy leaves some uncertainty about the detailed physical state near core conditions. In addition, models of Earth’s composition commonly draw analogies to chondritic meteorites and solar abundances, but different chondrite classes (e.g., ordinary versus enstatite) imply different redox conditions and element partitioning, complicating simple compositional analogies.
Proposed contributors to the early geodynamo include crystallization of oxides (e.g., MgO, SiO2, FeO) in the deep interior, which could have driven buoyancy fluxes and compositional convection before or during inner‑core formation. Long‑term thermal models are uncertain, but extrapolations that ignore other major perturbations suggest core freezing is not expected for timescales on the order of 10^11 years—far longer than stellar evolution will permit—indicating that core solidification is not an imminent constraint on habitability compared with solar evolution.
Seismological investigation of Earth’s interior relies on indirect reconstruction from seismic waves generated by earthquakes and recorded at distributed stations. Two principal classes of body-wave arrivals—phases that refract through velocity gradients and phases that reflect off sharp impedance contrasts—provide complementary constraints on the depth, geometry and material contrasts of subsurface layers. At velocity discontinuities, ray paths bend according to Snell’s law, allowing the inversion of observed travel-time deflections to map lateral and vertical velocity structure; conversely, coherent reflected phases and their amplitudes pinpoint abrupt interfaces and quantify impedance contrasts. Seismic velocities themselves are sensitive to composition, temperature and physical state, so observed velocity changes can be interpreted in terms of mineralogy and thermal regime as well as solid versus liquid behavior. A critical diagnostic is the selective transmission of wave types: shear (S) waves do not propagate through the Earth’s liquid outer core, producing characteristic shadowing and altered arrival patterns that reveal the core’s fluid nature and its contrast with overlying solid layers. By integrating refracted and reflected travel times together with the presence or absence of particular phase types, seismologists reconstruct the layered velocity and state structure of the deep Earth without direct sampling.