摘要:

A common assumption in modeling dynamic processes in the earth's lower mantle is that thermal conductivity is sensibly constant throughout the lower mantle and equal to approximately 4 W m−1K−1. This value is the result of extrapolation using a linearized theory with estimated parameters for lattice contributions to thermal conductivity. Furthermore, this value is highly dependent on the assumed geotherm. Excluding theoretical uncertainties in the extrapolation, recent experimental data for the pressure dependence of conductivity coupled with a revised geotherm lead to substantially higher estimated values for the lower mantle. The hypothesis that the seismically resolved anomalous zone at the base of the mantle (D″) is a thermal boundary layer can be questioned in this case of a higher estimation for thermal conduct

ISSN:0094-8276    

DOI:10.1029/GL013i013p01509

年代:1986

数据来源: WILEY

摘要:

Analysis of the Earth's gravity field using ratio of gravity to geoid (g/N) and cumulative degree contribution curve techniques improves resolution of the mass, depth, and dimensions of anomalous mass sources within the planet. The source for four of the ten major geoid anomalies of the Earth has large contributions from harmonic degrees 2 and 3, lie at the core‐mantle boundary, and the greatest has a mass anomaly of 4.2·1022grams. That boundary has a density contrast (4.4 gm/cm³) which is greater than at any other location, including the contrast at the Earth's surface with space. Thus, relatively small vertical undulations of that boundary produce very large mass anomalies. Topographic relief of less than 3 km at the core‐mantle boundary occurs, and accounts for the existence of the largest geoid anomalies occurring on the

ISSN:0094-8276    

DOI:10.1029/GL013i013p01513

年代:1986

数据来源: WILEY

摘要:

Preliminary numerical models indicate that chemically denser material (dregs) at the base of the mantle would have substantial lateral variations in thickness induced by convection of the overlying mantle, and might well form discontinuous aggregations below mantle upwellings. A model with a density contrast of about 2 per cent and an initial uniform thickness of the denser layer of 100 km yields a discontinuous distribution with maximum thickness 230 km and bottom topography of several kilometers amplitude, in reasonable accord with recent seismological observations of vertical and lateral structure. Heat flux out of the core is probably strongly modulated laterally by mantle convection, while mantle dregs will complicate and possibly amplify this effect. Such modulation may be relevant to long‐term (107‐ 108year) variations in the magnetic fi

ISSN:0094-8276    

DOI:10.1029/GL013i013p01517

年代:1986

数据来源: WILEY

摘要:

Static features in the earth's magnetic field and fixed sources of secular variation suggest the dynamo is anchored to the lowermost mantle via either thermal or topographic coupling. Here we show a correspondence with dynamic core topography, which is related to mantle temperature, and suggest further effects due to topography associated with subducting slabs. The two coupling mechanisms may occur on different time scales, providing a potential means for discriminating between them.

ISSN:0094-8276    

DOI:10.1029/GL013i013p01521

年代:1986

数据来源: WILEY

摘要:

Many geological phenomena show long term cyclical behavior. Of particular interest are mass extinctions and geomagnetic‐field reversals, which may have correlated periodicities of approximately 30 myr. We propose that these cycles are governed by periodic instability of the thermal boundary layer (D″) at the base of the mantle. As this mobile layer thickens by thermal diffusion, it becomes dynamically unstable and hot material erupts from it. The layer, now thinner, extracts heat from the core at a greater rate, increasing the energy supply and hence the reversal frequency of the dynamo. The hot material from the D″ layer rises via mantle plumes to the surface where it causes increased volcanic activity which affects the climate and

ISSN:0094-8276    

DOI:10.1029/GL013i013p01525

年代:1986

数据来源: WILEY

摘要:

Difference traveltimes SKKS‐SKS for 12 Tonga‐Fiji events observed from 96‐117° at North and South American WWSSN and CSN stations exceed the difference times of standard earth model PREM by 2‐3 s. Also the amplitude ratios SKS/SKKS at American stations from 96‐107° are lower than the PREM ratios. No such anomalies occur in directions from Tonga ‐ Fiji towards Eurasia and Africa and for a Sea of Japan event and paths to North America and Africa. The regional variations in the data point to pronounced lateral variations in earth structure. The anomalous difference times imply either an advance of SKS or a delay of SKKS relative to PREM. The interpretation of the anomalies is not unique. Our preferred explanation involves S‐velocity anomalies in the lower mantle on the source side, eas

ISSN:0094-8276    

DOI:10.1029/GL013i013p01529

年代:1986

数据来源: WILEY

摘要:

An optimum truncation level, N, in a spherical harmonic analysis of the geomagnetic main field at the core mantle boundary is determined by harmonic spline analysis. Specifically, we find that value of N at which the two analyses are closest in a well defined sense and, for that value of N, we examine how "close" the two models are. Depending slightly on the definition of closeness, optimum N is found to be either 10 or 11. For those values the two analyses give remarkably similar results, showing that the conveniences of spherical harmonics can be retained with little penalty.

ISSN:0094-8276    

DOI:10.1029/GL013i013p01533

年代:1986

数据来源: WILEY

摘要:

A new method for deriving steady surficial core motions from geomagnetic field models is presented and applied. The original method determines the steady velocity field at the top of a frozen‐flux core which best fits, in both the spatial and temporal linear least squares sense, a model of the large scale geomagnetic secular variation (SV) at the base of a source‐free mantle [Voorhies, 1986]. The new method is based upon the same physical assumptions, but fits SV at earth's surface instead of the core‐mantle boundary (CMB). The newly derived flow differs somewhat from prior solutions, and shows but slight westward drift; however, other key global properties are virtually unchanged. Over a 15 year interval the new method provides a better fit to the time‐varying geomagnetic field at earth's surface than does

ISSN:0094-8276    

DOI:10.1029/GL013i013p01537

年代:1986

数据来源: WILEY

摘要:

The phase diagram of FeO has been experimentally determined to pressures of 155 GPa and temperatures of 4000 K using shock‐wave and diamond‐cell techniques. We have discovered a metallic phase of FeO at pressures greater than 70 GPa and temperatures exceeding 1000 K. The metallization of FeO at high pressures implies that oxygen can be present as the light alloying element of the Earth's outer core, in accord with the geochemical predictions of Ringwood. The high pressures necessary for this metallization suggest that the core has acquired its composition well after the initial stages of the Earth's accretion. Direct experimental observations at elevated pressures and temperatures indicate that core‐forming alloy can react chemically with oxides such as those forming the mantle. The core and mantle may never have reached complete chemical equilibrium, however. If this is the case, the core‐mantle boundary is likely to be a zone of active chemical re

ISSN:0094-8276    

DOI:10.1029/GL013i013p01541

年代:1986

数据来源: WILEY

摘要:

Travel‐time residuals of thePKIKPphase observed between 170° and 180° show an axisymmetric pattern of degree 2 with an amplitude of about 2 seconds. The effect at shorter distances is much less pronounced and the entire data set cannot be explained by a physically realistic radial distribution of (isotropic) heterogeneity. We propose that, in addition to the general (isotropic) heterogeneity, the inner core is anisotropic with cylindrical symmetry aligned with the earth's rotation axis. Average P‐velocity along this axis is about 1 percent faster than in the equatorial

ISSN:0094-8276    

DOI:10.1029/GL013i013p01545

年代:1986

数据来源: WILEY