A Hypothesis on Compressibility at Pressures of the Order of a Million Atmospheres

Bullen, K. È.

doi:10.1038/157405a0

Cited by 100 publications

(26 citation statements)

References 4 publications

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“…The amplitudes of these arrivals were sufficient to invoke a discontinuous seismic boundary in the Earth's core. The P-wave contrast across this boundary was soon established; Birch [1940] and Bullen [1946] argued that the inner core must be solid based on this estimate. The best evidence for inner core solidity comes from studies of inner core sensitive normal modes [Dziewonski and Gilbert, 1971]: Earth models with finite shear modulus of the inner core provide a significantly better fit to eigenfrequency observations than those with a liquid inner core.…”

Section: Geophysical Backgroundmentioning

confidence: 99%

Physical properties of iron in the inner core

Steinle‐Neumann¹,

Stixrude²,

Cohen³

2003

Earth's Core: Dynamics, Structure, Rotation

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The Earth's inner core plays a vital role in the dynamics of our planet and is itself strongly exposed to dynamic processes as evidenced by a complex pattern of elastic structure. To gain deeper insight into the nature of these processes we rely on a characterization of the physical properties of the inner core which are governed by the material physics of its main constituent, iron. Here we review recent research on structure and dynamics of the inner core, focusing on advances in mineral physics. We will discuss results on core composition, crystalline structure, temperature,and various aspects of elasticity. Based on recent computational results, we will show that aggregate seismic properties of the inner core can be explained by temperature and compression effects on the elasticity of pure iron, and use single crystal anisotropy to develop a speculative textural model of the inner core that can explain major aspects of inner core anisotropy.

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Section: Geophysical Backgroundmentioning

confidence: 99%

Physical properties of iron in the inner core

Steinle‐Neumann¹,

Stixrude²,

Cohen³

2003

Earth's Core: Dynamics, Structure, Rotation

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“…Birch (1940) first suggested that the Lehmann discontinuity indicated a liquid-solid transition in his paper on the phase relations in iron. Bullen (1946Bullen ( , 1958 based his arguments for solidity of the inner core on his hypothesis that the compressibility of the material in the deep interior is nearly continuous as a function of depth. Early free oscillation work supported this concept (Alsop 1963;Anderson & Smith 1968;Derr 1969a); introducing a solid inner core allowed a better fit to the radial modes.…”

Section: The Core (Regions E F and G )mentioning

confidence: 99%

Earth Structure from Free Oscillations and Travel Times

Jordan

Anderson

1974

Geophysical Journal International

208

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SummaryAn extensive set of reliable gross Earth data has been inverted to obtain a new estimate of the radial variations of seismic velocities and density in the Earth. The basic data set includes the observed mass and moment of inertia, the average periods of free oscillation (taken mainly from the Dziewonski-Gilbert study), and five new sets of differential travel-time data. The differential travel-time data consists of the times of PcP-P and ScS-S , which contain information about mantle structure, and the times of P'AB -P'DF and P'Bc -PIDF, which are sensitive to core structure.A simple but realistic starting model was constructed using a number of physical assumptions, such as requiring the Adams-Williamson relation to hold in the lower mantle and core. The data were inverted using an iterative linear estimation algorithm. By using baseline-insensitive differential travel times and averaged eigenperiods, a considerable improvement in both the quality of the fit and the resolving power of the data set has been realized. The spheroidal and toroidal data are fit on the average to 0.04 and 0.08 per cent, respectively. The final model, designated model B 1, also agrees with Rayleigh and Love wave phase and group velocity data.The ray-theoretical travel times of P waves computed from model B1 are about 0.8s later than the 1968 Seismological Tables with residuals decreasing Model B1 is characterized by an upper mantle with a high, 4.8 km s-l, S, velocity and a normal, 3.33 g ~m -~, density. A low-velocity zone for S is required by the data, but a possible low-velocity zone for compressional waves cannot be resolved by the basic data set. The upper mantle transition zone contains two first-order discontinuities at depths of 420 km and 671 km. Between these discontinuities the shear velocity decreases with depth. The radius of the core, fixed by PcP -P times and previous mode inversions, is 3485 km, and the radius of the inner core-outer core boundary is 1215 km. There are no other first-order discontinuities in the core model. The shear velocity in the inner core is about 3.5 km s-'.

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“…Jeffreys (1926) established that the outer core is fluid based upon an analysis of solid Earth tides and the arrival times of shear (S) waves. Birch (1940) and Bullen (1946) proposed that the jump in P velocity at the ICB is a result of the solidification of an iron-rich alloy, and Birch (1952) predicted the finite shear velocity of the inner core to be 3.4 km/s. The first observational evidence for a solid inner core came from the Earth's free oscillations, some of whose eigenfrequencies cannot be explained without it (Dziewonski 1971, Dziewonski & Gilbert 1971.…”

Section: Introductionmentioning

confidence: 99%

Inner-Core Anisotropy and Rotation

Tromp

2001

Annu. Rev. Earth Planet. Sci.

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Key Words seismology of the core, inner-core boundary, inner-core shear waves, inner-core viscosity, inner-core attenuation s Abstract This paper reviews recent research focused on the Earth's inner core.Large inner-core traveltime anomalies and the anomalous splitting of core-sensitive free oscillations strongly suggest that the inner core is anisotropic. Initial models involved a simple, constant or depth-dependent cylindrical anisotropy at a level less than a few percent. Recent observations suggest that its eastern hemisphere is largely isotropic, whereas its western hemisphere is highly anisotropic, and there are indications that its top 100 km may be isotropic. The coda of inner-core reflected phases has been used to infer strong heterogeneities with a length scale of just a few kilometers. Thus, a complicated three-dimensional picture of the inner core is beginning to emerge, although it has been suggested that much of this complexity may be the misinterpretation of signals that have their origin in the lowermost mantle. Numerical models of the geodynamo suggest that the inner core may rotate at a slightly different rate than the mantle. Recent seismological estimates based upon traveltime and normal-mode data limit inner-core differential rotation to less than +0.2 degrees per year.

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A Hypothesis on Compressibility at Pressures of the Order of a Million Atmospheres

Cited by 100 publications

References 4 publications

Physical properties of iron in the inner core

Physical properties of iron in the inner core

Earth Structure from Free Oscillations and Travel Times

Inner-Core Anisotropy and Rotation

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