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| Outer core | |
|---|---|
| Name | Outer core |
| Type | Liquid metal layer |
| Depth range | 2,890–5,150 km |
| Composition | Primarily iron and nickel with light elements |
| State | Liquid |
| Thickness | ~2,260 km |
| Density | ~9.9–12.2 g/cm³ |
| Temperature | ~3,500–5,700 °C |
| Discoveries | Evidence from seismology and geomagnetism |
Outer core The outer core is a deep, electrically conducting, fluid layer in Earth's interior that lies between the solid Mantle and the solid Inner core and plays a central role in generating the planet's magnetic field. It is composed mainly of iron and nickel alloyed with lighter elements and is characterized by vigorous convective motion driven by thermal and compositional buoyancy. Observational constraints arise from seismic studies, geomagnetic records, and laboratory experiments on iron alloys, linking the layer to planetary evolution, mantle dynamics, and the protection of biospheric processes via the geomagnetic field.
The dominant constituents are iron and nickel similar to models from Pallasite meteorites, but seismological density deficits require light elements such as sulfur, silicon, oxygen, carbon, or hydrogen invoked by comparisons with experimental phase diagrams from Diamond anvil cell and Shock wave experiments. Core models integrate data from PREM and mineral physics studies from institutions like Carnegie Institution for Science and Geological Survey of Canada to estimate mean densities and sound speeds. Electrical conductivity, inferred from geomagnetic secular variation and induction studies tied to work by Walter Munk and Gordon J. F. G. Clarke, is high, enabling efficient magnetic diffusion timescales. Viscosity estimates come from analyses of Free core nutation and Length of day variations and are constrained to be extremely low compared with mantle viscosities, consistent with turbulent flow regimes described in numerical dynamo models developed at centers such as Los Alamos National Laboratory and University of Cambridge.
Temperatures at the inner boundary are estimated from melting relations calibrated by experiments at Oak Ridge National Laboratory and Lawrence Livermore National Laboratory, giving values of roughly 5,000–6,000 K near the inner boundary and lower values near the core–mantle boundary. Pressures reach ~136 GPa at the core–mantle boundary and ~330–360 GPa at the inner boundary, derived from seismic travel-time inversions by groups at US Geological Survey and international collaborations. The liquid state is maintained because the local adiabatic temperature exceeds the melting temperature of iron alloys under those pressures, a finding supported by studies published in journals associated with American Geophysical Union conferences.
Convective motions arise from secular cooling, inner core growth, and compositional buoyancy generated by light-element rejection during crystallization; these processes were formalized in theoretical frameworks developed by researchers at California Institute of Technology and Massachusetts Institute of Technology. The combination of thermal and double-diffusive convection leads to complex flow patterns including columnar convection influenced by rapid rotation described using the Coriolis formalism from Lord Kelvin-inspired rotating fluid dynamics. Observationally constrained flow speeds are inferred from geomagnetic secular variation analyses by teams at National Oceanic and Atmospheric Administration and British Geological Survey, with typical advective velocities of order mm/s to cm/s. Interaction with the lowermost mantle heterogeneities imaged by Seismic tomography produces lateral variations that feed back on heat fluxes and inner core asymmetry documented by studies linked to Harvard University and ETH Zurich.
The geodynamo arises from magnetohydrodynamic induction in the electrically conducting fluid, a process formalized in theoretical treatments by Eugene Parker and implemented in numerical simulations at institutions such as Max Planck Institute for Solar System Research and Princeton University. Lorentz forces, Coriolis effects, and buoyancy balance to maintain a self-sustaining magnetic field, whose secular variation, reversals, and excursions are recorded in paleomagnetic archives curated by Smithsonian Institution and studied in projects involving Scripps Institution of Oceanography. Numerical dynamos reproduce magnetic dipole dominance and polarity reversals when parameter regimes approximate core conditions derived from laboratory constraints by National Institute of Standards and Technology.
Seismic phase observations from global networks operated by International Seismological Centre and USGS provide the primary structural constraints: shear waves disappear in the outer layer, while compressional waves slow and refract, leading to PKP and PcP phase behavior exploited in tomography by groups at Stanford University and University of Tokyo. Seismic anisotropy and attenuation near the inner boundary reveal layering and compositional gradients studied using datasets from the Global Seismographic Network. Normal modes and free oscillation analyses after large earthquakes, first used systematically after the 1960 Valdivia earthquake, further constrain density and sound-speed profiles consistent with a liquid outer region.
Energy driving outer core dynamics derives from secular cooling, latent heat release during inner core solidification, and chemical buoyancy from light-element partitioning, all framed in thermal evolution models developed by researchers at University of Oxford and University of California, Berkeley. Radioactive elements are likely scarce in the core, a conclusion supported by geochemical partitioning studies at Vanderbilt University and isotope measurements from mantle samples curated at Smithsonian Institution. Models balancing energy and entropy budgets determine plausible core ages and inner core nucleation times that connect to interpretations of paleointensity records held in the collections of Natural History Museum, London.
The outer core influences mantle convection and plate tectonics through core–mantle boundary heat flux variations and thermal coupling analyzed in interdisciplinary studies at European Geosciences Union and American Geophysical Union conferences. Long-term cooling of the core contributes to mantle plume generation scenarios used to explain large igneous provinces such as Deccan Traps and to modulate surface tectonic regimes discussed by researchers at Geological Society of America. The geomagnetic shielding provided by dynamo action has implications for atmospheric retention and habitability explored in comparative planetology work involving NASA and European Space Agency missions.
Category:Earth structure