Core–mantle boundary
Boundary between Earth's mantle and liquid outer core.
Deep beneath our feet, at a depth of 2,891 km (1,796 mi), lies the core–mantle boundary (CMB)—the frontier where Earth's solid silicate mantle meets its liquid iron–nickel outer core. This boundary is detected by a sharp change in seismic wave speeds: P-waves slow down dramatically as they enter the liquid outer core, while S-waves vanish entirely because they cannot travel through liquid. The difference in acoustic impedance between the solid mantle and molten core creates this seismic discontinuity.
Recent research hints at a distinct layer just above the CMB, possibly made of a novel mineral phase called post-perovskite, a variation of the deep mantle's usual perovskite structure. Seismic tomography reveals significant irregularities in this boundary zone, dominated by two enormous features known as the African and Pacific large low-shear-velocity provinces (LLSVPs).
The uppermost part of the outer core is thought to be about 500–1,800 K hotter than the overlying mantle, forming a thermal boundary layer. This boundary likely has its own topography, shaped by solid-state convection in the mantle above. Variations in the CMB's thermal properties may influence how the iron-rich fluids of the outer core flow—fluids that ultimately generate Earth's magnetic field.
**D″ region**
Directly above the CMB lies a roughly 200 km thick layer called the D″ region (pronounced "D double-prime" or "D prime prime"). The name comes from geophysicist Keith Bullen's alphabetical labeling of Earth's layers (A through G, with A as the crust and G as the inner core). In his 1942 model, the entire lower mantle was simply the D layer. By 1949, Bullen realized this D layer actually consisted of two distinct parts: the upper 1,800 km was renamed D′ (D prime), and the bottom 200 km became D″. Later studies showed D″ is not spherical. In 1993, Czechowski proposed that irregularities in D″ form structures similar to continents—so-called "core-continents"—which move over time and influence hotspots and mantle convection. Subsequent research has supported this idea.
**Seismic discontinuity**
At about 2,900 km (1,800 mi) depth, a seismic discontinuity marks an abrupt change in wave speeds: P-waves slow down, and S-waves disappear completely. Because S-waves cannot pass through liquids, the material above this discontinuity is solid, while the material below is molten. This discontin
- depth
- 2,891 km (1,796 mi)
- composition_above
- solid silicate mantle
- composition_below
- liquid iron–nickel outer core
- seismic_property
- P-wave velocity decreases; S-waves disappear
- associated_layer
- D″ region (approx. 200 km thick)
- temperature_difference
- outer core 500–1,800 K hotter than overlying mantle
Lore & Background
The core–mantle boundary was discovered through seismic wave analysis, notably by Beno Gutenberg, and is sometimes called the Gutenberg discontinuity. The boundary is marked by a sharp change in seismic velocities: P-waves slow down, and S-waves vanish entirely, indicating the transition from solid mantle to liquid outer core. Recent evidence suggests a distinct boundary layer above the CMB possibly made of post-perovskite, a novel phase of perovskite mineralogy.
Reader's Guide
The core–mantle boundary is significant because it governs heat transfer between the core and mantle, influencing mantle convection and the geodynamo that generates Earth's magnetic field. Seismic tomography reveals large low-shear-velocity provinces (LLSVP) under Africa and the Pacific, indicating structural irregularities. The D″ region, a 200 km thick layer above the CMB, was identified by Keith Bullen and later found to be non-spherical, with inhomogeneities forming continent-like structures that affect hotspots and mantle convection. The boundary's topography, supported by solid-state convection, and thermal variations affect outer core fluid flow, ultimately responsible for Earth's magnetic field.
Did You Know?
- The core–mantle boundary lies at a depth of 2,891 km (1,796 mi) below Earth's surface.
- S-waves do not exist in the liquid outer core, confirming its molten state.
- The D″ region, about 200 km thick, was named by geophysicist Keith Bullen in 1949.
- Seismic tomography shows the boundary zone is dominated by the African and Pacific large low-shear-velocity provinces (LLSVP).
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