Core–mantle boundary: Difference between revisions

The core–mantle boundary (CMB in the parlance of solid earth geophysicists) of the Earth lies between the planet's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2891 km (1796 mi) depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth. This discontinuity is due to the differences between the acoustic impedances of the solid mantle and the molten outer core. P-wave velocities are much slower in the outer core than in the deep mantle while S-waves do not exist at all in the liquid portion of the core. Recent evidence suggests a distinct boundary layer directly above the CMB possibly made of a novel phase of the basic perovskite mineralogy of the deep mantle named post-perovskite. Seismic tomography studies have shown significant irregularities within the boundary zone and appear to be dominated by the African and Pacific large low-shear-velocity provinces (LLSVPs).^[1] The ~200 km thick layer of the lower mantle directly above the boundary is referred to as the D″ ("D double-prime" or "D prime prime") and is sometimes included in discussions regarding the core–mantle boundary zone.^[2] The D″ name originates from the mathematician Keith Bullen's designations for the Earth's layers. His system was to label each layer alphabetically, A through G, with the crust as 'A' and the inner core as 'G'. In his 1942 publication of his model, the entire lower mantle was the D layer. In 1950, Bullen found his 'D' layer to actually be two different layers. The upper part of the D layer, about 1800 km thick, was renamed D′ (D prime) and the lower part (the bottom 200 km) was named D″.

The bottom of D″ has been observed in some regions to be marked by a seismic velocity discontinuity (sometimes known as the 'Gutenberg discontinuity,' after German geophysicist Beno Gutenberg).^[3]

Gutenberg discontinuity

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The Gutenberg discontinuity occurs within Earth's interior at a depth of about 2,900 km (1,800 mi) below the surface, where there is an abrupt change in the seismic waves (generated by earthquakes or explosions) that travel through Earth. At this depth, primary seismic waves (P waves) decrease in velocity while secondary seismic waves (S waves) disappear completely. S waves shear material, and cannot transmit through liquids, so it is believed that the unit above the discontinuity is solid, while the unit below is in a liquid or molten form. This distinct change marks the boundary between two sections of the earth's interior, known as the lower mantle (which is considered solid) and the underlying outer core (believed to be molten).

The molten section of the outer core is thought to be about 700°C (1,292°F) hotter than the overlying mantle. It is also denser, probably due to a greater percentage of iron. This distinct boundary between the core and the mantle, which was discovered by the change in seismic waves at this depth, is often referred to as the core-mantle boundary, or the CMB. It is a narrow, uneven zone, and contains undulations that may be up to 5-8 km (3-5 mi) wide. These undulations are affected by the heat-driven convection activity within the overlying mantle, which may be the driving force of plate tectonics-motion of sections of Earth's brittle exterior. These undulations in the core-mantle boundary are also affected by the underlying eddies and currents within the outer core's iron-rich fluids, which are ultimately responsible for Earth's magnetic field.

The boundary between the core and the mantle does not remain constant. As the heat of the earth's interior is constantly but slowly dissipated, the molten core within Earth gradually solidifies and shrinks, causing the core mantle boundary to slowly move deeper and deeper within Earth's core.

The Gutenberg discontinuity was named after Beno Gutenberg (1889-1960), a seismologist who made several important contributions to the study and understanding of the Earth's interior. It has also been referred to as the Oldham-Gutenberg discontinuity, or the Weichhert-Gutenberg discontinuity.

See also

• Geophysics
• EarthScope

Notes

External links

• Earth's Core–Mantle Boundary Has Core-Rigidity Zone
• Audrey Slesinger (January 2001), "Earth's interior: Redefining the Core–Mantle Boundary", Geotimes, The American Geological Institute, retrieved 2011-03-24
• Mineral phase change at the boundary
• Superplumes at the boundary
• About.com article on the name of D″