Don L. Anderson

Preliminary reference earth model

A large data set consisting of about 1000 normal mode periods, 500 summary travel time observations, 100 normal mode Q values, mass and moment of inertia have been inverted to obtain the radial distribution of elastic properties, Q values and density in the Earths interior. The data set was supplemented with a special study of 12 years of ISC phase data which yielded an additional 1.75 × 10^6 travel time observations for P and S waves. In order to obtain satisfactory agreement with the entire data set we were required to take into account anelastic dispersion. The introduction of transverse isotropy into the outer 220 km of the mantle was required in order to satisfy the shorter period fundamental toroidal and spheroidal modes. This anisotropy also improved the fit of the larger data set. The horizontal and vertical velocities in the upper mantle differ by 2–4%, both for P and S waves. The mantle below 220 km is not required to be anisotropic. Mantle Rayleigh waves are surprisingly sensitive to compressional velocity in the upper mantle. High S_n velocities, low P_n velocities and a pronounced low-velocity zone are features of most global inversion models that are suppressed when anisotropy is allowed for in the inversion. The Preliminary Reference Earth Model, PREM, and auxiliary tables showing fits to the data are presented.

Composition of the Earth

New estimates of solar composition, compared to earlier measurements, are enriched in Fe and Ca relative to Mg, Al, and Si. The Fe/Si and Ca/Al atomic ratios are 30 to 40 percent higher than chondritic values. These changes necessitate a revision in the cosmic abundances and in the composition of the nebula from which the planets accreted (which have been based on chondritic values). These new values imply that the mantle could contain about 15 weight percent FeO and more CaMgSi2O6 than has been supposed. Geophysical data are consistent with a dense, FeO-rich lower mantle and a CaMgSi2O6 (diopside)-rich transition region. FeO contents of 13 to 18 weight percent appear to be typical of the mantles of bodies in the inner solar system. The oldest komatiites (high-temperature MgO-rich magmas) have a similar chemistry to the derived mantle. These results favor a chemically zoned mantle.

Edge-driven convection

We consider a series of simple calculations with a step-function change in thickness of the lithosphere and imposed, far-field boundary conditions to illustrate the influence of the lithosphere on mantle flow. We consider the effect of aspect ratio and far-field boundary conditions on the small-scale flow driven by a discontinuity in the thickness of the lithosphere. In an isothermal mantle, with no other outside influences, the basic small-scale flow aligns with the lithosphere such that there is a downwelling at the lithospheric discontinuity (edge-driven flow); however, the pattern of the small-scale flow is strongly dependent on the large-scale thermal structure of a much broader area of the upper mantle. Long-wavelength temperature anomalies in the upper mantle can overwhelm edge-driven flow on a short timescale; however, convective motions work to homogenize these anomalies on the order of 100 million years while cratonic roots can remain stable for longer time periods. A systematic study of the effect of the boundary conditions and aspect ratio of the domain shows that small-scale, and large-scale flows are driven by the lithosphere. Edge-driven flow produces velocities on the order of 20 mm/yr. This is comparable to calculations by others and we can expect an increase in this rate as the mantle viscosity is decreased.

Phase Changes in the Upper Mantle

The C-region of the upper mantle has two transition regions 75 to 90 kilometers thick. In western North America these start at depths of 365 kilometers and 620 kilometers and involve velocity increases of about 9 to 10 percent. The locations of these transition regions, their general shape, and their thicknesses are consistent with, first, the transformation of magnesium-rich olivine to a spinel structure and, then, a further collapse of a material having approximately the properties of the component oxides. The velocity increases associated with each transition region are slightly less than predicted for the appropriate phase change. This can be interpreted in terms of an increasing fayalite content with depth. The location of the transition regions and the seismic velocities in their vicinity supply new information regarding the composition and temperature of the upper mantle. The depths of the transition regions are consistent with temperatures near 1500�C at 365 kilometers and 1900�C at 620 kilometers.

Partial melting in the upper mantle

The low velocity zone in tectonic and oceanic regions is too pronounced to be the effect of high temperature gradients alone. Partial melting is consistent with the low velocity, low Q and abrupt boundaries of this region of the upper mantle and is also consistent with measured heat flow values. The inferred low melting temperatures seem to indicate that the water pressure is sufficiently high to lower the solidus about 200 °C to 400 °C below laboratory determinations of the melting point of anhydrous silicates. The mechanical instability of a partially molten layer in the upper mantle is probably an important source of tectonic energy. The top of the low-velocity zone can be considered a self-lubricated surface upon which the top of the mantle and the crust can slide with very little friction. Lateral motion of the crust and upper mantle away from oceanic rises is counterbalanced by the flow of molten material in the low-velocity layer toward the rise where it eventually emerges as new crust. If this lateral flow of molten material is not as efficient as the upward removal of magma, then regions of extrusion, such as oceanic rises, will migrate.

Through the glass lightly.

In about 10 years we should have three-dimensional maps of the structure of Earth’s interior, from surface to center, with a resolution of a few hundred kilometers, including anisotropy and anelasticity.

Thermal emission spectrometer experiment: Mars Observer mission

Thermal infrared spectral measurements will be made of the surface and atmosphere of Mars by the thermal emission spectrometer (TES) on board Mars Observer. By using these observations the composition of the surface rocks, minerals, and condensates will be determined and mapped. In addition, the composition and distribution of atmospheric dust and condensate clouds, together with temperature profiles of the CO2 atmosphere, will be determined. Broadband solar reflectance and thermal emittance measurements will also be made to determine the energy balance in the polar regions and to map the thermophysical properties of the surface. The specific science objectives of this investigation are to determine (1) the composition and distribution of surface materials, (2) the composition, particle size, and spatial and temporal distribution of suspended dust, (3) the location, temperature, height, and water abundance of H2O clouds, (4) the composition, seasonal behavior, total energy balance, and physical properties of the polar caps, and (5) the particle size distribution of rocks and fines on the surface. The instrument consists of three subsections: a Michelson interferometer, a solar reflectance sensor, and a broadband radiance sensor. The spectrometer covers the wavelength range from 6 to 50 μm (∼1600–200 cm−1) with nominal 5 and 10 cm−1 spectral resolution. The solar reflectance band extends from 0.3 to 2.7 μm; the broadband radiance channel extends from 5.5 to 100 μm. There are six 8.3-mrad fields of view for each sensor arranged in a 3 × 2 array, each with 3-km resolution at the nadir. Uncooled deuterated triglycine sulphate (DTGS) pyroelectic detectors provide a signal-to-noise ratio (SNR) of over 500 at 10 μm for daytime spectral observations at a surface temperature of 270 K. The SNR of the albedo and thermal bolometers will be approximately 2000 at the peak signal levels expected. The instrument is 23.6 × 35.5 × 40.0 cm, with a mass of 14.4 kg and an average power consumption of 14.5 W. The approach will be to measure the spectral properties of thermal energy emitted from the surface and atmosphere. Emission phase angle studies and day-night observations will be used to separate the spectral character of the surface and atmosphere. The distinctive thermal infrared spectral features present in minerals, rocks, and condensates will be used to determine the mineralogic and petrologic character of the surface and to identify and study aerosols and volatiles in the atmosphere.

Earthquake Prediction: Variation of Seismic Velocities before the San Francisco Earthquake

A large precursory change in seismic body-wave velocities occurred before the earthquake in San Fernando, California. The discovery that this change is mainly in the P-wave velocity clearly relates the effect to the phenomenon of dilatancy in fluid-filled rocks. This interpretation is supported by the time-volume relation obtained by combining the present data with the data from previous studies. The duration of the precursor period is proportional to the square of an effective fault dimension, which indicates that a diffusive or fluid-flow phenomenon controls the time interval between the initiation of dilatancy and the return to a fully saturated condition which is required for rupture.

An alternative mechanism of flood basalt formation

All large continental igneous provinces and most high-temperature magmas (picrites, komatiites) are found on the margins of cratonic lithosphere. The standard plume model of flood basalt formation offers no explanation for this observation. We propose that thick lithosphere (usually Archean) adjacent to thinner lithosphere may control the locations of flood basalt provinces. The boundary between thick and thin lithosphere focuses both the strain in the lithosphere and the upwelling convection. In addition, the non-uniform boundary condition actually induces a small-scale form of convection that is not present in simple convection and plume models. Whereas plumes are a form of convective instability rising from the base of a convecting system heated from below, the form of convection we are discussing is triggered from above. Unlike other lithospheric mechanisms, the asymmetric lithosphere does not require convective thinning or heating of the plate in order to produce melting. This eliminates time delay between the arrival of the plume head and the onset of volcanism in the stretching model. We consider a series of calculations with a step-function change in thickness of the boundary layer and an externally imposed pull-apart. The flow in our models is shallow and sub-horizontal, and brings hot material from under the thicker (cratonic) boundary layer towards the pull-apart. A simple estimate of the amount of melt generated by this mechanism suggests that it is capable of producing a large igneous province, even for a dry mantle.

The thermal state of the upper mantle; No role for mantle plumes

A variety of geophysical data indicates that long wavelength temperature variations of the asthenosphere depart from the mean by ±200°C, not the ±20°C adopted by plume theoreticians. The ‘normal’ variation, caused by plate tectonic processes (subduction cooling, continental insulation, small‐scale convection) encompasses the temperature excesses that have been attributed to hot jets and thermal plumes. Geophysical estimates of the average potential temperature of the upper mantle are about 1400°C. Asthenospheric convection at ridges, rifts and fracture zones and at the onset of continental breakup is intrinsically 3D, giving rise to shallow pseudoplume‐like structures without deep thermal instabilities. Deep narrow thermal plumes are unnecessary and are precluded by uplift and subsidence data. The locations and volumes of ‘midplate’ volcanism appear to be controlled by lithospheric architecture, stress and cracks.