Introduction
Before the invention of seismology, Earth's interior was pure speculation. Geologists knew from surface rocks and volcanic lavas that Earth had a hot interior, and from its bulk density (~5.5 g/cm³, much higher than surface rocks at ~2.7 g/cm³) that the deep interior must be much denser — almost certainly iron-rich. But the exact structure, the depth of major boundaries, and the physical state of each layer remained unknown until seismic waves provided the first direct probe. Today we know Earth's interior in more detail than we know the deep oceans' floors, thanks entirely to seismology — the analysis of how seismic waves are refracted, reflected, and blocked by different layers.
The Mohorovičić discontinuity (Moho) marks the base of the crust — Earth's outermost rigid shell. In oceanic regions, the crust is only ~7 km (4.3 mi) thick and composed of basalt and gabbro (mafic rocks); in continental regions, it is 25-70 km (43 mi) thick (average ~35 km (22 mi)) and composed of a heterogeneous mix of granite and metamorphic rocks overlying a more mafic lower crust. P-wave velocity jumps from ~6.5 km/s in continental crust to ~8.0 km/s in the uppermost mantle (peridotite) at the Moho. The transition zone at 410 km (255 mi) and 660 km (410 mi) depth marks major phase transitions in mantle minerals: olivine transforms to wadsleyite at 410 km (the "olivine-β transition") and to ringwoodite at ~520 km (323 mi), then to the lower-mantle assemblage of bridgmanite (MgSiO₃ perovskite) + ferropericlase + calcium perovskite at 660 km (410 mi). These phase transitions cause step-like increases in seismic velocity that are clearly visible in seismological data and act as reflectors for short-period seismic waves.
The core-mantle boundary (CMB) at 2,891 km (1796 mi) depth is the most dramatic velocity discontinuity in Earth. P-wave velocity drops abruptly from ~13.7 km/s (at the base of the D" layer, the lowermost mantle) to ~8.1 km/s (at the top of the liquid outer core). S-wave velocity drops from ~7.3 km/s to zero, because the outer core is liquid iron (with ~10% light elements — oxygen, silicon, sulphur, hydrogen — mixed in to lower the density from pure iron to Earth's observed core density). The outer core, 2,891-5,150 km (3200 mi) depth, is the site of Earth's geodynamo: convection in the liquid iron driven by secular cooling, solidification of the inner core, and chemical buoyancy generates the electric currents that produce Earth's magnetic field. The inner core boundary (ICB) at 5,150 km (3200 mi) depth marks the transition to a solid iron-nickel inner core ~1,220 km (758 mi) in radius, discovered in 1936 by Inge Lehmann from anomalous P-wave arrivals (PKIKP phases) within the P-wave shadow zone.
The D" layer (D-double-prime) — the ~200-300 km (186 mi) thick zone immediately above the CMB — is one of the most seismologically complex regions in Earth. It shows dramatic lateral velocity heterogeneities (hot upwelling plumes vs cold subducted slabs reaching the CMB), a seismic discontinuity that may represent a phase transition from bridgmanite to post-perovskite at ~125 GPa, and ultralow velocity zones (ULVZs) where P-wave velocity decreases by 5-10% over distances of just 5-40 km (25 mi) — possibly partial melt from ancient subducted oceanic crust at the CMB temperature. The D" layer is where Earth's two great heat reservoirs — the mantle and the core — exchange thermal energy, and where the deepest mantle plumes originate, rising to produce hotspot volcanism at the surface billions of years later.
Key Terms
The seismic boundary between the crust and the mantle, marked by a P-wave velocity jump from ~6.5 km/s (crust) to ~8.0 km/s (uppermost mantle peridotite). Depth is ~7 km (4.3 mi) beneath oceanic crust and ~35-70 km (43 mi) beneath continental crust. Discovered in 1909 by Croatian seismologist Andrija Mohorovičić. Defined entirely by the seismic velocity jump; chemically, it marks the base of felsic/mafic crustal rocks above ultramafic peridotite mantle. The 1960s Mohole Project attempted to drill through it in oceanic crust but was cancelled; modern IODP drilling reaches ~2.5 km (1.6 mi) below the oceanic seafloor.
The mantle region between 410 km (255 mi) and 660 km (410 mi) depth, marked by two sharp seismic velocity discontinuities produced by olivine phase transitions: olivine converts to wadsleyite at 410 km (255 mi) and eventually to bridgmanite + ferropericlase at 660 km (410 mi). Each phase change increases density by ~3-7% and is detectable as a sharp reflector for high-frequency body waves. These transitions absorb or release latent heat and the 660 km (410 mi) boundary may act as a partial barrier to mantle convection, potentially separating upper and lower mantle circulation.
The lowermost ~200-300 km (186 mi) of the mantle immediately above the core-mantle boundary. Highly seismically heterogeneous, with lateral temperature variations of ±1000 K from cold subducted slabs vs rising plumes. May host a post-perovskite phase transition at ~125 GPa and 2700 K. Contains ultralow velocity zones (ULVZs) where P-wave velocity decreases by 5-10% over very short distances. Acts as the site of core-mantle heat and chemical exchange and the origin of deep mantle plumes.
The seismic boundary at 2,891 km (1796 mi) depth between the silicate mantle and the liquid iron outer core — the largest velocity discontinuity in Earth. P-wave velocity drops from ~13.7 km/s to ~8.1 km/s; S-wave velocity drops from ~7.3 km/s to zero, confirming the outer core is liquid. First detected by Oldham in 1906 and measured precisely by Gutenberg in 1914 (giving the alternative name "Gutenberg discontinuity"). Site of intense thermal and chemical exchange between the mantle and core.
The boundary at 5,150 km (3200 mi) depth (inner core radius ~1,221 km (759 mi)) between the liquid outer core and the solid inner core. P-wave velocity increases from ~10.4 km/s to ~11.0 km/s; S-waves reappear in the inner core at ~3.5 km/s, confirming it is solid. The inner core displays seismic anisotropy — ~3-4% faster along Earth's rotation axis — and differential rotation ~0.3-0.5°/yr faster than the mantle. The inner core grows at ~1 mm/yr as Earth cools. Discovered by Inge Lehmann in 1936 from anomalous PKIKP arrivals.