Annual Review of Earth and Planetary Sciences - Volume 24, 1996

Although the temperature at the top of the lower mantle is well constrained by phase equilibrium data for the transformation of transition zone minerals to the denser perovskite polymorphs, the temperature distribution in the lower mantle is poorly known. Models depend strongly on the assumptions of the amount of internal heating and the viscosity profile. New melting data on iron to pressures of the outer core (2 Mbar) and the observed strong decrease of eutectic melting depression in the Fe-FeO-FeS system with increasing pressure, however, tightly constrain the temperature at the inner-core boundary to slightly less than 5000 K. This estimate can be reconciled with all recent static melting measurements on iron and lays within the uncertainty of shock temperature measurements. The resulting temperature at the core-mantle boundary of about 4000 K then requires a large temperature gradient at the bottom of the lower mantle of about 1500 K. Recent findings of the very high melting temperatures of the major lower-mantle materials Mg-Si-perovskite and magnesiowüstite indicate that this increase in temperature does not cause melting in the lower mantle.

Over the past decade, significant progress has been made in understanding the rheological properties of partially molten mantle rocks. Laboratory experiments demonstrate that a few percent of melt can have an unexpectedly large effect on viscosity both in the diffusional creep regime and in the dislocation creep regime. In both cases, the enhancement in creep rate is much larger than anticipated based on deformation models because melt wets at least a fraction of the grain boundaries. For diffusion creep, the wetted interfaces provide a rapid diffusion path that is not included in analyses based on melt distribution in isotropic melt-crystal systems. For dislocation creep, two points require consideration. First, even without a melt phase present, fine-grained samples deformed in the dislocation creep field flow a factor of ∼10 faster than coarse-grained rocks due to contributions from grain boundary mechanisms to the deformation process. Second, melt has only a small effect on creep rate for coarse-grained rocks but has a relatively large effect for fine-grained samples. Thus, because olivine has only a limited number of slip systems, grain boundaries contribute significantly to deformation of fine-grained rocks in the dislocation creep regime, provided that deformation occurs near the transition between diffusional creep and dislocation creep. Based on laboratory measurements, this transition is expected to occur at a grain size of about 6 mm for a differential stress of 0.1 MPa. Therefore, under mantle conditions, even a few percent melt should reduce the viscosity by as much as a factor of 10. A broad range of problems related to deformation beneath mid-ocean ridges and in the mantle wedge above subducting slabs can now be addressed using experimentally determined rheologies for partially molten mantle rocks.

Recent advances in computational physics allow numerical simulation of three-dimensional complex flows through arbitrarily complex geometries. Moreover, new technology for noninvasive imaging provides detailed three-dimensional tomographic reconstructions of porous rocks with a resolution approaching one micron. These two innovations are leading to new understanding of how the microscopic complexity of natural porous media influences fluid transport at a larger, macroscopic scale. This review describes new insights concerning single-phase and multiphase porous flow derived from numerical simulation. In particular, results concerning scaling relations between macroscopic parameters, the scale dependence of transport properties, and viscous coupling in multicomponent flow are emphasized.

Earth-based stellar occultations probe the temperature, pressure, and number-density profiles of planetary atmospheres in the microbar range with a vertical resolution of a few kilometers. Depending on the occultation data available for a given body and other information, the technique also allows determination of local density variations, extinction by aerosols and molecules, rotation period and zonal winds, atmospheric composition, and the temporal and spatial variability of an atmosphere. A brief quantitative description of the interaction of starlight with a planetary atmosphere is presented, and observational techniques are discussed. Observational results through 1995 are presented for Venus, Mars, Jupiter, Saturn, Titan, Neptune, Triton, Pluto, and Charon.

Io, innermost of Jupiter's large moons, is one of the most unusual objects in the Solar System. Tidal heating of the interior produces a global heat flux 40 times the terrestrial value, producing intense volcanic activity and a global resurfacing rate averaging perhaps 1 cm yr⁻¹. The volcanoes may erupt mostly silicate lavas, but the uppermost surface is dominated by sulfur compounds including SO2 frost. The volcanoes and frost support a thin, patchy SO2 atmosphere with peak pressure near 10⁻⁸ bars. Self-sustaining bombardment of the surface and atmosphere by Io-derived plasma trapped in Jupiter's magnetosphere causes escape of material from Io (predominantly sulfur, oxygen, and sodium atoms, ions, and molecules) at a rate of about 10³ kg s⁻¹. The resulting Jupiter-encircling torus of ionized sulfur and oxygen dominates the Jovian magnetosphere and, together with an extended cloud of neutral sodium, is readily observable from Earth.

Geophysical estimates of mid-ocean ridge axial heat fluxes (2–4 × 10¹² W) and of the total hydrothermal flux (9 ± 2 × 10¹² W) are well established. Problems arise in calculation of water fluxes because of uncertainties in (a) values of off-axis fluxes and (b) the partition of axial heat flow between high-temperature black smoker and lower-temperature diffuse flow. Of the various geochemical methods of estimating fluxes, ³ He/heat data are extremely variable, the Mg method is sensitive to flank fluxes, Sr isotopes agree with geophysical estimates only if flank fluxes are important, Li isotopes data are consistent with geophysical values, and Ge/Si ratios give low fluxes, which may reflect low-temperature processes not yet fully quantified. Estimates of hydrothermal heat and water fluxes derived from these approaches are presented as are hydrothermal chemical fluxes at the ridge axis, off axis, and as affected by hydrothermal plumes.

Changes of the isotopic composition of water within the water cycle provide a recognizable signature, relating such water to the different phases of the cycle. The isotope fractionations that accompany the evaporation from the ocean and other surface waters and the reverse process of rain formation account for the most notable changes. As a result, meteoric waters are depleted in the heavy isotopic species of H and O relative to ocean waters, whereas waters in evaporative systems such as lakes, plants, and soilwaters are relatively enriched. During the passage through the aquifers, the isotope composition of water is essentially a conservative property at ambient temperatures, but at elevated temperatures, interaction with the rock matrix may perturb the isotope composition. These changes of the isotope composition in atmospheric waters, surface water, soil, and groundwaters, as well as in the biosphere, are applied in the characterization of hydrological system as well as indicators of paleo-climatological conditions in proxy materials in climatic archives, such as ice, lake sediments, or organic materials.

Turkic-type orogeny is a class of collisional mountain building, in which the precollision history of one, or both, of the colliding continents involves the growth of very large, subcontinent-size subduction-accretion complexes, into which magmatic arc axes commonly migrate and thus enlarge the continent to which they are attached. A review of the evolution of two Phanerozoic (Altaids, Nipponides), one Neoproterozoic (East African), and one Archean (Yilgarn) Turkic-type orogens shows that this type of orogeny may have been the principal builder of the continental crust through recorded Earth history. The total juvenile material added to Turkic-type orogens at any one time in the Phanerozoic seems close to 1 km³/year, which about equals the amount of material annually fed into the mantle at subduction zones. As some 0.02 to 0.03% of that material is generally agreed to return to the crust by arc magmatism, these figures provide a minimum net growth rate for the continental crust during the Phanerozoic.

Continental North America's greatest earthquake sequence struck on the western frontier of the United States. The frontier was not then California but the valley of the continent's greatest river, the Mississippi, and the sequence was the New Madrid earthquakes of the winter of 1811–1812. Their described impacts on the land and the river were so dramatic as to produce widespread modern disbelief. However, geological, geophysical, and historical research, carried out mostly in the past two decades, has verified much in the historical accounts. The sequence included at least six (possibly nine) events of estimated moment magnitude M ≥ 7 and two of M ≃ 8. The faulting was in the intruded crust of a failed intracontinental rift, beneath the saturated alluvium of the river valley, and its violent shaking resulted in massive and extensive liquefaction. The largest earthquakes ruptured at least six (and possibly more than seven) intersecting fault segments, one of which broke the surface as a thrust fault that disrupted the bed of the Mississippi River in at least 2 (and possibly four) places.

…it is a riddle wrpped in a mystery inside an enigma.

     Winston Churchill

Seismic anisotropy beneath continents is analyzed from shear-wave splitting recorded at more than 300 continental seismic stations. Anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity. The observed coherence with crustal deformation implies that the mantle plays a major, if not dominant, role in orogenies. No evidence is found for a continental asthenospheric decoupling zone, suggesting that continents are coupled to general mantle circulation.

Paleontologists have always been concerned about the documentary quality of the fossil record, and this has also become an important issue for biologists, who increasingly look to accumulations of bones, shells, and plant material as possible ways to extend the time-frame of observation on species and community behaviors. Quantitative data on the postmortem behavior of organic remains in modern environments are providing new insights into death and fossil assemblages as sources of biological information. Important findings include: 1. With the exception of a few circumstances, usually recognizable by independent criteria, transport out of the original life habitat affects few individuals. 2. Most species with preservable hardparts are in fact represented in the local death assemblage, commonly in correct rank importance. Molluscs are the most durable of modern aquatic groups studied so far, and they show highest fidelity to the original community. 3. Time-averaging of remains from successive generations and communities often prevents the detection of short-term (seasons, years) variability but provides an excellent record of the natural range of community composition and structure over longer periods. Thus, although a complex array of processes and circumstances influences preservation, death assemblages of resistant skeletal elements are for many major groups good to excellent records of community composition, morphological variation, and environmental and geographic distribution of species, and such assemblages can record temporal dynamics at ecologically and evolutionarily meaningful scales.