Earth's Core-Mantle Boundary
The Earth's Core-Mantle Boundary (CMB) is a significant discontinuity located approximately 2,900 kilometers beneath the Earth's surface, marking the transition between the solid mantle and the liquid outer core. Above this boundary, the mantle is primarily composed of silicate minerals, while the outer core below consists of iron-nickel alloys and is characterized by higher temperatures, estimated to range from 4,300 kelvins. This boundary plays a critical role in the dynamics of Earth's interior, where convection currents in both the mantle and outer core influence geological processes and are believed to contribute to the generation of Earth's magnetic field.
The lowermost part of the mantle, known as the D″ layer, is about 200-300 kilometers thick and exhibits significant variations in seismic wave speeds, suggesting a complex structure influenced by thermal convection. The CMB itself is not flat and features topographic undulations that can vary in height by several kilometers, which are thought to be associated with mantle dynamics. Understanding the properties and behavior of the CMB is essential for insights into mantle convection, heat flow, and seismic activity, as well as the broader implications for geological and magnetic phenomena on Earth. Through various geophysical studies, including seismology and laboratory simulations, scientists are continually enhancing our knowledge of this intriguing boundary.
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Earth's Core-Mantle Boundary
- CATEGORY: Earth
The core-mantle boundary is a pronounced discontinuity separating the outer core from the mantle of the Earth. It is a chemical and mineralogical as well as a thermal and mechanical boundary. The topography of the core-mantle boundary is believed to be controlled by the dynamic processes in the mantle and the outer core.
Overview
The core-mantle boundary (CMB) is a prominent discontinuity within the Earth. It is located at a radius of about 3,500 kilometers (2,174 miles) and a depth below the surface of about 2,900 kilometers (1,801 miles). The mantle above the boundary is largely solid, of relatively low temperature, and primarily composed of silicate minerals rich in magnesium and iron. The outer core below the boundary is liquid, of higher temperature, and composed of dense materials such as iron-nickel oxides and iron-nickel sulfide alloys. This boundary separates two dynamic systems: one operating in the mantle as hot spots and convection cells, the other in the outer core consisting of convection currents and eddies of the core fluid. The motions of the core fluid appear to be responsible for Earth’s magnetic field.
The lowermost part of the mantle, labeled the D″ (pronounced “dee double prime”) layer, is called the core-mantle transition zone. It is approximately 200 to 300 kilometers (124 to 186 miles) thick and is located just above the CMB. Seismic waves from earthquakes and explosives detonated at or near the surface show significant speed variations within the D″ layer over lateral or horizontal distances of 1,000 kilometers (621 miles) or more. Longitudinal (or compressional) P waves travel faster in the portions of this layer that are located below North America, China, the eastern part of the Indian Ocean, and off the Pacific coast of Chile. P waves travel more slowly in the D″ layer below the southern part of Africa, the New Hebrides Islands, the South Pacific Ocean, and the Argentine Basin. Similar variations in speed have also been observed for transverse (or shear) S waves, which travel faster in this layer under the American continents, Asia, the northern Indian and Pacific oceans, and Antarctica, and more slowly under the Central and South Pacific Ocean, the Atlantic Ocean, most of Africa, and the southern part of the Indian Ocean. These speed variations in the D″ layer appear to continue upward in the mantle and may be related to the thermally induced convection currents and hot spots in the mantle.
Improvements in instrumentation have enabled scientists to simulate in the laboratory the physical and chemical conditions of the lower mantle and the outermost core. The lower mantle is thought to be primarily composed of magnesium-iron silicates with the compact perovskite crystal structure. Some aluminum-calcium silicates and magnesium-iron oxides may also be present, but their relative abundance is unknown. Laboratory measurement of the melting point of perovskite lead to estimates of the temperature of the D″ layer ranging from 3,300 to 4,300 Kelvins (5,480.33 to 7,280.33 degrees F). Similar studies of outer core materials, which are primarily iron-nickel sulfides and iron-nickel oxides, indicate the temperature of the outermost core is at least 4,300 Kelvins (7,280.33 degrees F). Thus, the temperature increases by about 1,000 Kelvins (1,340.33 degrees F) through the D″ layer, resulting in partial melting of some minerals, thereby making the zone soft with anomalous characteristics.
Seismologists studying the core-mantle boundary (CMB) by means of reflected waves from the core have long been frustrated by the strong scatter of the reflected amplitudes. A major part of this scatter is believed to be the result of vertical undulations of the core-mantle boundary. The lateral extent of these undulations is of the order of thousands of kilometers. The elevation of the boundary may change by as much as 5 to 8 kilometers (3 to 5 miles) above or below its normal depth. Topographic highs of the CMB have been observed beneath the Indian Ocean, the Pacific Ocean, and the Atlantic Ocean (particularly in the North Atlantic). The CMB is depressed below the Tonga-Karmadec islands, the China-Japan region, Central Africa, and off the west coast of South America. Because most of these areas are associated with the subduction of oceanic plates, the structure of the CMB is thought to be caused by the dynamic processes in the mantle, which may be related to the convection processes in the outer core. Subduction of a lithospheric plate is associated with downwelling convective flow in the mantle. When the convective flow reaches the core boundary, it depresses the CMB into the hot, liquid core. Core fluids may partially invade the “topographic low” of the CMB, altering the chemical composition of the D″ layer. Similarly, beneath an upwelling zone of mantle flow, liquid core material may be “sucked” up into the mantle, creating a topographic high of the CMB. At the topographic “lows” of the CMB, mantle material is subjected to the higher temperatures of the outer core and may melt; it then recrystallizes at the topographic “highs” of the CMB. Thus, the overall effect is to smooth the CMB, which is continually disturbed by the convective circulations in the mantle. With heat dissipation in the mantle, the outer core slowly cools and the core materials crystallize and underplate the mantle. Therefore the outer core slowly shrinks with time, and the CMB gets deeper.
The D″ core-mantle transition zone and core-mantle boundary are not only a compositional boundary but also a thermal boundary, where the temperature increases by at least 1,000 Kelvins (1,340.33 degrees F). Thermal coupling between the mantle and the outer core, however, may change laterally, resulting in a variable heat flow across the boundary. Although no consensus has been reached among scientists, it is possible that the mantle dynamics are at least partially responsible for controlling the heat flow.
The Earth behaves like a large magnet. The magnetic field of the Earth—the geomagnetic field—undergoes changes known as secular variations. The origin of the geomagnetic field appears to be related to motions of the outer core fluid. Studies suggest that the deep mantle and the outer core play a significant role in shaping the secular variations. Upwellings in the outer core material are associated with the hotter and seismically slower regions of the D″ zone; downwellings are associated with the colder and seismically faster regions in D″. Cold regions in the mantle transmit greater amounts of heat from the outer core, thereby setting up mantle circulations. The topography of the CMB is controlled by the circulations in the mantle as well as in the outer core. The topographic relief of the CMB may also set up a lateral temperature gradient which may be responsible for the secular variation of the magnetic field.
Methods of Study
Various subdisciplines of geophysics contribute to investigating the nature and the structure of the core-mantle boundary (CMB) and the D″ core-mantle transition layer. They include, among others, seismology, geodesy, geodynamics, high-temperature and high-pressure mineral physics, geothermometry, and geomagnetism. Seismology has been the most important among all these subdisciplines and has contributed most of the information about the Earth’s interior.
Seismology deals with earthquakes and the propagation of earthquake waves through the Earth. Whenever an earthquake occurs, different types of seismic waves are generated. Surface waves travel along the Earth’s surface, and longitudinal (or compressional) P waves and transverse (or shear) S waves travel through the interior of the Earth. It is often helpful to visualize the direction of travel of P and S waves as rays originating from an earthquake focus, or hypocenter, and radiating in all directions through the Earth. Because of the increased rigidity and incompressibility of rocks downward, the speeds of these waves increase with depth. As a result, the downgoing seismic rays (except for vertical or near-vertical rays) are curved back toward the surface. The seismographic stations that are farther away from the epicenter record seismic waves that penetrate through the deeper layers in the Earth.
The outer core has no rigidity, since it is liquid. Consequently, P waves slow down abruptly as they cross the CMB, from 13.5 to 8.5 kilometers (18.4 to 5.3 miles) per second, and they are sharply refracted or bent. S waves do not propagate through liquids, so they stop at the CMB. As a result, there are shadow zones beyond about 11,000 kilometers (6,835 miles) from the epicenter where neither P nor S direct waves are detected at the ground surface. The presence of liquid outer core was discovered through the existence of the shadow zones and the absence of core-transmitted S waves.
Seismic waves emerging at steep angles from the hypocenter encounter the CMB. Part of the incident energy is reflected back from the boundary, and the rest is refracted through the outer core. P waves can be reflected back as P and as S waves, designated as PcP and PcS waves (or phases) respectively. Similarly, S waves reflected back from the CMB as P and S waves are designated as ScP or ScS waves. These have been important in the study of the nature, shape, and depth of the CMB. Because S waves cannot travel through liquids, the refracted energy in the outer core propagates in the form of P waves. These refracted P waves are designated as K phases. Thus, PKP is a wave that travels from the hypocenter in the mantle as a P wave, propagates as a K (that is, P) in the outer core, and reemerges in the mantle as a P wave. Similarly, SKS and other combinations, such as PKS and SKP, are often observed in the seismic records. A joint study of the core-reflected phases (for example, PcP) and the core-refracted phases (for example, PKP) is often important in resolving the depths and topography of the CMB. Seismic rays incident at a large angle on the CMB are diffracted. Study of these diffracted waves provides important information on the D″ zone above the CMB. Using the waveform modeling techniques, scientists are determining the thickness and fine structures of the D″ zone.
Another important tool is seismic tomography. It utilizes the same principle used in computed tomography (CT) scan X-rays of humans. In a CT scan, the X-ray source and imager are rotated around the body and a large number of X-ray images are recorded. A computer processes these images and forms a three-dimensional image of the internal organs of the subject. The seismological data collected worldwide can similarly be processed to form a three-dimensional image of the Earth’s interior. Seismic tomography provides valuable information on the CMB as well as the Earth’s mantle.
A large earthquake sets the Earth vibrating like a bell. If the Earth were perfectly spherical with uniform layering, it would produce a pure tone, vibrating at a preferred frequency. Departures from the spherical shape of the Earth, as well as depth-related discontinuities, produce additional tones involving distortions of the Earth. Thus, recordings of these various modes of the Earth’s vibrations, known as free oscillations, can furnish information about the shape of the CMB.
Satellite measurements of the Earth’s gravity field and the geodetic observations of the geoid can also provide information on the CMB. Theoretical models of the Earth’s interior, particularly the mantle, the D″ zone, and the CMB, can be constructed to match observed geoidal undulations and gravity anomalies. It appears that a two to three kilometer (1.2 to 1.8 miles) variation in elevation of the CMB can explain 90 percent of the observed large-scale gravity anomalies. Astronomic observations of the Earth’s wobble also furnish additional constraints on the shape of the CMB. The Earth has an equatorial bulge caused by its rotation. The Moon pulls at the bulge and attempts to align it along the orbital plane of the Moon, generating a wobble, or a nutational motion, of the Earth’s axis. (This motion is similar to the wobble of a spinning top or a gyroscope.) Deformation of the CMB produces certain irregularities in the nutational motion. Studies of these irregularities indicate that the undulations of the CMB are less than one kilometer (0.6 miles) high.
Major developments in instrumentation have made it possible to simulate in the laboratory the temperature and pressure conditions of the deep mantle. Scientists study the deep mantle using laboratory experiments and geophysical techniques. Diamond anvil cells are a tool capable of generating extreme pressures by compressing a small sample between diamond anvils. Lasers are also used to heat these samples, replicating the high temperatures of the deep mantle, while techniques like X-ray diffraction analyze the crystal structure and mineral phases under these conditions. Seismic wave analysis provides indirect data by measuring how earthquake-generated waves travel through the mantle, with variations in wave velocities indicating changes in temperature, composition, or phase transitions at different depths. Electromagnetic sounding examines variations in Earth's magnetic field to estimate the electrical conductivity of mantle materials, which is influenced by factors such as temperature, mineral composition, and water content. These methods allow researchers to better understand the complex properties and dynamics of Earth's deep interior.
Context
Scientists from various geophysical subdisciplines have made a concerted effort to investigate the structure and nature of the CMB and the deep interior of the Earth because it is important from several perspectives. The CMB is believed to be associated with deep mantle plumes, the mantle convection currents that drive the lithospheric plates and may be responsible for secular variations of the geomagnetic field. As the most pronounced discontinuity within the Earth, the undulations at the CMB may also cause regional gravity anomalies and can affect the transmission of seismic waves. The transmission effects of seismic waves crossing the CMB provide information about the geometry and the physical and chemical properties of materials at the CMB, as well as in the mantle above and the outer and inner core below. Furthermore, because core-reflected phases travel along vertical or near-vertical paths in the mantle, they are often utilized to study heterogeneity and seismic behavior in the mantle. Knowledge of the nature of the CMB is necessary to determine these mantle characteristics.
A committee on Studies of the Earth’s Deep Interior (SEDI), under the joint auspices of the International Union of Geodesy and Geophysics (IUGG) and the American Geophysical Union (AGU), facilitates international exchange of scientific information about the Earth’s interior. In addition, special sessions on the Earth’s deep interior are held at most AGU meetings. In the twenty-first century, the IUGG and the AGU have significantly contributed to our understanding of Earth's systems through their collaboration and research efforts, largely facilitated by improved satellite technology and sophisticated data analysis techniques.
Further Reading
Bolt, Bruce A. Earthquakes. Freeman, 1988.
Bolt, Bruce A. Inside the Earth. Freeman, 1982.
Fowler, C. M. R. The Solid Earth: An Introduction to Global Geophysics. 2nd ed, Cambridge UP, 2004.
Geballe, Zachary M., et al. "Thermal Conductivity Near the Bottom of the Earth's Lower Mantle: Measurements of Pyrolite up to 120 GPa and 2500 K." Earth and Planetary Science Letters, vol. 536, 15 Apr. 2020, p. 116161, doi:10.1016/j.epsl.2020.116161. Accessed 15 Jan. 2025.
Glatzmaier, Gary A. Introduction to Modeling Convection in Planets and Stars: Magnetic Field, Density Stratification, Rotation. Princeton UP, 2014.
Hazen, Robert M. The Story of Earth: The First 4.5 Billion Years, from Stardust to Living Planet. Viking, 2012.
"The IUGG." IUGG International, iugg.org/. Accessed 15 Jan. 2025.
"Mantle." National Geographic, 30 Apr. 2024, education.nationalgeographic.org/resource/mantle/. Accessed 15 Jan. 2025.
Russel, Stuart, et al. "The Emerging Picture of a Complex Core-Mantle Boundary." Nature Communication, vol. 15, no. 4569, 29 May 2024, doi:10.1038/s41467-024-48939-1. Accessed 15 Jan. 2025.
Tarbuck, Edward J., and Frederick K. Lutgens. Earth: An Introduction to Physical Geology. Illust. Dennis Tasa. 9th ed, Pearson, 2008.
Turcotte, Donald, and Gerald Schubert. Geodynamics. 3rd ed, Cambridge UP, 2014.
"Ultra-High-Pressure Experimentalist Who Studies the Deep Earth." American Musuem of Natural History, 2020, www.amnh.org/learn-teach/curriculum-collections/earth-inside-and-out/ultra-high-pressure-experimentalist-who-studies-the-deep-earth. Accessed 15 Jan. 2025.
Vogel, Shawna. Naked Earth: The New Geophysics. Plume, 1996.