Outer CoreEdit
The outer core is the moonlit heart of the planet’s interior, a vast, dynamic layer of liquid metal that lies between the solid mantle above and the solid inner core at the center. Spanning roughly from about 2,900 kilometers beneath the surface to around 5,150 kilometers, this molten region is the seat of the geodynamo that sustains the geomagnetic field. Its liquid iron–nickel alloy is mixed with lighter elements, and its vigorous convection, driven by heat flow from the deep interior, turns the Earth into a natural dynamo that shields the surface and rocks the compass needle.
The outer core’s existence and properties are inferred from a combination of seismic observations, geomagnetic measurements, and high-pressure experiments. Seismic waves behave in characteristic ways as they pass through Earth’s interior; notably, shear (S) waves do not propagate through liquids, which was key evidence that the layer beneath the mantle is liquid rather than solid. The motion of the liquid metal, coupled with the planet’s rotation, generates magnetic fields through a self-sustaining dynamo process. The product is a magnetic field that is strongest near the equator and weakens toward the poles on average, while undergoing secular variation and occasional reversals over geological timescales.
Composition and structure
The outer core is primarily a molten alloy of iron with nickel, plus smaller amounts of lighter elements such as sulfur, oxygen, or silicon. The exact light-element mix is a subject of ongoing research, but the overall picture is that a dense, electrically conducting liquid fills the region between the mantle and the inner core. Earth's core and core (geology) give supporting context for this layering.
Depth and thickness: the outer core begins where the slowest mantle seismic boundary ends and extends to the outer boundary of the inner core, giving it a thickness of about 2,000 to 2,500 kilometers, with precise figures depending on the model. The boundary with the mantle is known as the Core–Mantle Boundary, and the boundary with the inner core is the Inner Core Boundary. For readers interested in how these depths are determined, see seismology and P-wave/S-wave behavior.
State and density: the outer core is liquid at extreme pressures and temperatures, which gives it a density greater than the overlying mantle but still lower than the inner core. The liquid state is essential to allowing the convective motions that power the dynamo. The concept of a liquid metal at such pressures is supported by high-pressure experiments and by the way seismic waves travel through Earth’s interior.
Temperature and heat flow: temperatures in the outer core are extremely high, on the order of thousands of kelvin, and heat flow from deeper regions into the mantle drives continuous convection. This convection occurs in a rotating frame, which helps organize the flow into patterns that efficiently generate magnetic fields. See also convection and geodynamo.
Physical properties
Electrical conductivity: the outer core exhibits very high electrical conductivity, a necessary condition for dynamo action. The conducting fluid allows motion-induced magnetic fields to be sustained and amplified as the Earth rotates.
Convection and dynamics: buoyancy forces from cooling and from latent heat released during the growth of the inner core prompt vigorous convective currents. The interaction of these flows with rotation organizes the field into a dominant dipole at present, though the pattern evolves over time.
Interaction with the magnetic field: the motion of conductive fluid in the outer core links fluid dynamics to magnetism, a coupling central to the geodynamo mechanism. The magnetic field that emerges from this process interacts with the atmosphere and space environment, shaping the magnetosphere and influencing charged-particle dynamics near Earth.
Geodynamo and magnetic field
The outer core’s liquid motion, under the influence of Earth’s rotation, acts as a self-sustaining geodynamo. Induced electric currents in the conductive liquid generate magnetic fields, and in turn the field influences the flow of the liquid through Lorentz forces, creating a feedback loop that maintains the field over time. See dynamo theory for a broader mathematical treatment of this process.
Present field and secular variation: today’s magnetic field is largely dipolar, but it exhibits gradual changes in intensity and orientation known as secular variation. Over longer timescales, the Earth’s magnetic field has reversed its polarity many times, a phenomenon called geomagnetic reversal; reversals are irregular and are studied through paleomagnetism and dynamo models.
Implications for the environment and technology: the geomagnetic field serves as a shield against charged solar particles, helps guide navigation, and affects satellite operation and communication systems. The outer core’s dynamics, therefore, have practical consequences for life on the surface and for human activity in space and on instruments here on Earth.
Evidence and methods
Seismology: seismic waves provide the principal evidence for a liquid outer core. The absence of S-wave transmission through this region, coupled with changes in P-wave velocity and travel path, supports a liquid layer. Seismic tomography and travel-time analyses map the core–mantle boundary and inner core boundary with increasing precision. See seismology and P-wave/S-wave discussions.
Laboratory and theoretical work: high-pressure, high-temperature experiments, including those conducted in diamond anvil cells and dynamic compression facilities, strive to reproduce core-like conditions and constrain the behavior of iron alloys. These results inform models of density, phase relations, and conductivity. See high-pressure physics and geochemistry for related topics.
Geomagnetic observations: measurements of the Earth’s magnetic field over time, from ground stations and satellites, illuminate the outer core’s dynamics. Magnetic data feed into models of the geodynamo and help explain long-term behavior such as reversals and secular variation.
Formation and history
Differentiation and formation: early in Earth's history, the planet melted and underwent chemical separation by density, causing iron to migrate toward the center and form a metallic core. The outer core formed as a liquid layer around the growing solid inner core.
Inner core growth and thermal evolution: the inner core began crystallizing from the outer core as Earth cooled, releasing latent heat and light elements that influence convection and the geodynamo. This growth mechanism helps sustain long-term magnetic-field behavior, though the exact timing and rate remain active areas of study. See Earth's formation and planetary differentiation for broader context.
Debates and uncertainties: scientists continue to refine estimates of the light-element composition of the outer core, the exact temperature profile, and the detailed patterns of convective flow that produce the geomagnetic field. Different models may emphasize various mechanisms or element mixes, but the core’s liquid state and its central role in magnetic field generation are well supported by multiple lines of evidence. For readers seeking a broader discussion of core properties and dynamo modeling, see geodynamo and geomagnetic field.