Annual Review of Earth and Planetary Sciences - Volume 26, 1998

We review recent developments in the study of volcanism and tectonics on Venus. Venus's crust is basaltic, dry, and probably about 30 km thick. The mantle convects, giving rise to plumes, and has a similar composition and mean temperature (∼1300°C), but a higher viscosity (∼10²⁰ Pa s), than that of the Earth. Inferred melt generation rates constrain the lithospheric thickness to between 80 and 200 km. The elastic thickness of the lithosphere is about 30 km on average. The present-day lack of plate tectonics may be due to strong faults and the high viscosity of the mantle. Most of the differences between Earth and Venus processes can be explained by the absence of water.

Venus underwent a global resurfacing event 300–600 Ma ago, the cause and nature of which remains uncertain. The present-day surface heat flux on Venus is about half the likely radiogenic heat generation rate, which suggests that Venus has been heating up since the resurfacing event.

Observations of suspected planet-forming disks provide estimates of protoplanetary disk masses, surface temperatures, and the rate at which mass is infalling onto the disks. Analyses of primitive meteorites and comets and their components constrain the solar nebula's temperature at the locations and times where those components were formed. Theoretical models of disks undergoing the accretion of mass from an infalling cloud envelope predict disk temperatures in good agreement with these constraints: a moderately warm (500–1500 K) inner disk, surrounded by a cool (50–150 K) outer disk. These models have important implications for the depletion of volatiles in the inner Solar System, for mechanisms of disk evolution, and for the orbital distances at which terrestrial and gas giant planets form.

Pahoehoe lava flows are common in every basaltic province, and their submarine variants, pillow lavas and sheet flows, cover the bulk of the Earth. Pahoehoe flows are emplaced by inflation—the injection of molten lava underneath a solidified crust. Only in the past few years has an understanding of the inflation process and the ability to recognize ancient inflated lava flows been achieved. All large terrestrial basaltic flow fields studied to date, including flood basalts, were emplaced as thermally efficient, inflated, compound pahoehoe sheet flows. This leads us to propose that this is the standard way of emplacing large lavas (the SWELL hypothesis). The atmospheric impact of such flood basalt eruptions could have been protracted and severe, providing a plausible link between flood basalt eruptions and mass extinctions.

This paper presents a general review of the recent research advances of the East Asian paleomonsoon, based mainly on studies of the loess-soil sequences in the Chinese Loess Plateau. In the last 2.6 million years, the paleomonsoonal history may be divided into about 166 events on a time scale of the Earth's orbital variations. During the last glacial period, millenial-scale oscillations of the monsoon system were prominent, which can be fairly well correlated with the Dansgaard-Oeschger cycles recognized in the Greenland ice cores. The monsoonal rainfall belt has experienced a wide, repeated advance-retreat change during the glacial-interglacial cycles of the Pleistocene. Both temporal and spatial changes of the monsoon system in the Quaternary could have been linked closely to global ice-volume variations.

Primitive meteorites contain grains of stardust that originated from stellar outflows and supernova ejecta prior to the formation of the Solar System. The study of these grains in the laboratory provide new information on stellar evolution, nucleosynthesis, mixing in supernovae, galactic evolution, and the age of the galaxy. Grains whose isotopically anomalous compositions indicate a stellar origin include diamond, silicon carbide, graphite, corundum, and silicon nitride. Most silicon carbide and corundum come from red giant and asymptotic giant branch stars (low-mass stars at the end of their evolution), and carry the isotopic signatures of H burning in the core and later of H and He burning in thin shells. Diamond carries a supernova isotopic signature in its Xe, and low-density graphite and silicon nitride, as well as a subgroup of silicon carbide, show evidence for a supernova origin in the form of extinct ⁴⁴Ti and large ²⁸Si excesses.

Noble gas isotopic ratios in mantle-derived samples require variability in the time-integrated ratio of volatile to lithophile elements in the Earth. Documentation of mantle ³He/⁴He variability is becoming increasingly complete, but for the heavier noble gases, the picture is still partly clouded by the effects of atmospheric contamination of mantle samples. Nevertheless, clear variations in mantle Ne, Ar, and Xe isotopic ratios exist, are apparently correlated with ³He/⁴He, and may be the product of varying degrees of mantle degassing. However, uncertainties in noble gas geochemical behavior and several conflicting observations leave open other possibilities. Recent Ne isotopic data are particularly important because they require that the atmosphere has not been closed to exchange with space. Derivation of much of the atmosphere from a source other than degassing of the mantle is a strong possibility that complicates efforts to model the geochemical evolution of the Earth.

For technical reasons, the general circulation of the ocean has historically been treated as a steady, laminar flow field. The recent availability of extremely high-accuracy and high-precision satellite altimetry has provided a graphic demonstration that the ocean is actually a rapidly time-evolving turbulent flow field. To render the observations quantitatively useful for oceanographic purposes has required order of magnitude improvements in a number of fields, including orbit dynamics, gravity field estimation, and atmospheric variability. With five years of very high-quality data now available, the nature of oceanic variability on all space and time scales is emerging, including new findings about such diverse and important phenomena as mixing coefficients, the frequency/wavenumber spectrum, and turbulent cascades. Because the surface elevation is both a cause and consequence of motions deep within the water column, oceanographers soon will be able to provide general circulation numerical models tested against and then combined with the altimeter data. These will be complete three-dimensional time-evolving estimates of the ocean circulation, permitting greatly improved estimates of oceanic heat, carbon, and other property fluxes.

Stable isotopic, mineralogical, and chemical alteration in metamorphic terranes is evidence for reactive fluid flow during metamorphism. In many cases, the amount and spatial distribution of the alteration can be quantitatively interpreted using transport theory in terms of fundamental properties of metamorphic flow systems such as time-integrated flux, flow direction, and Peclet number. Many estimates of time-integrated flux in the upper and middle crust are surprisingly large, 10⁵–10⁶ cm³ fluid/cm² rock; estimates for the lower crust are much smaller. Rather than pervasive and uniform, reactive fluid flow in all metamorphic environments is channelized on scales of <1–10⁴ m. Channelization results from heterogeneous permeability structures controlled by features such as lithologic layering, contacts, folds, fractures, and faults. Consequently flow may be in the direction of either decreasing or increasing temperature or isothermal. Site-specific thermal-hydrologic models of metamorphic terranes that explicitly consider chemical reactions and dynamic permeability structures will help resolve outstanding questions with regard to the driving forces and duration of flow, metamorphic permeability distributions, and how deformation controls fluid flow.

This review proceeds from Luna Leopold's and Ronald Shreve's lasting accomplishments dealing with the study of random-walk and topologically random channel networks. According to the random perspective, which has had a profound influence on the interpretation of natural landforms, nature's resiliency in producing recurrent networks and landforms was interpreted to be the consequence of chance. In fact, central to models of topologically random networks is the assumption of equal likelihood of any tree-like configuration. However, a general framework of analysis exists that argues that all possible network configurations draining a fixed area are not necessarily equally likely. Rather, a probability P (s) is assigned to a particular spanning tree configuration, say s, which can be generally assumed to obey a Boltzmann distribution: P(s) ∝ e^−H(s)/τ, where τ is a parameter and H (s) is a global property of the network configuration s related to energetic characters, i.e. its Hamiltonian. One extreme case is the random topology model where all trees are equally likely, i.e. the limit case for τ → ∞. The other extreme case is τ → ∞, and this corresponds to network configurations that tend to minimize their total energy dissipation to improve their likelihood. Networks obtained in this manner are termed optimal channel networks (OCNs). Observational evidence suggests that the characters of real river networks are reproduced extremely well by OCNs. Scaling properties of energy and entropy of OCNs suggest that large network development is likely to effectively occur at zero temperature (i.e. minimizing its Hamiltonian). We suggest a corollary of dynamic accessibility of a network configuration and speculate towards a thermodynamics of critical self-organization. We thus conclude that both chance and necessity are equally important ingredients for the dynamic origin of channel networks—and perhaps of the geometry of nature.

Although research on modern plant-arthropod associations is one of the cornerstones of biodiversity studies, very little of that interest has percolated down to the fossil record. Much of this neglect is attributable to dismissal of Paleozoic plant-arthropod interactions as being dominated by detritivory, with substantive herbivory not emerging until the Mesozoic. Recent examination of associations from some of the earliest terrestrial communities indicates that herbivory probably extends to the Early Devonian, in the form of spore feeding and piercing-and-sucking. External feeding on pinnule margins and the intimate and intricate association of galling are documented from the Middle and Late Pennsylvanian, respectively. During the Early Permian, the range of external foliage feeding extended to hole feeding and skeletonization and was characterized by the preferential targeting of certain seed plants. At the close of the Paleozoic, surface fluid feeding was established, but there is inconclusive evidence for mutualistic relationships between insect pollinivores and seed plants. These data are gleaned from the largely separate trace-fossil records of gut contents, coprolites, and plant damage and the body-fossil records of plant reproductive and vegetative structures, insect mouthparts, and ovipositors. While these discoveries accentuate the potential for identifying particular associations, the greatest theoretical demand is to establish the spectrum and level of intensity for the emergence of insect herbivory in a range of environments during the Pennsylvanian and Permian.

The first flowering plant fossils occur as rare, undiverse pollen grains in the Early Cretaceous (Valanginian-Hauterivian). Angiosperms diversified slowly during the Barremian-Aptian but rapidly during the Albian-Cenomanian. By the end of the Cretaceous, at least half of the living angiosperm orders were present, and angiosperms were greater than 70% of terrestrial plant species globally. The rapid diversification of the group, and its dominance in modern vegetation, has led to the idea that the Cretaceous radiation of angiosperms also represents their rise to vegetational dominance.

Paleoecological data cast a different light on the Cretaceous radiation of angiosperms. Analyses of sedimentary environments indicate that angiosperms not only originated in unstable habitats but remained centered there through most of the Cretaceous. Morphology of leaves, seeds, and wood is consistent with the status of most Cretaceous angiosperms as herbs to small trees with early successional strategy. The diversification of flowering plants in the Cretaceous represents the evolution of a highly speciose clade of weeds but not necessarily a major change in global vegetation.

The Re-Os isotope sytem, based on the long-lived β⁻ transition of ¹⁸⁷Re to ¹⁸⁷Os, has matured to wide use in cosmochemistry and high-temperature geochemistry. The siderophilic/chalcophilic behavior of Re and Os is different from that of the elements that comprise most other long-lived radiogenic isotope systems. Magmatic iron meteorites (IIIAB, IIAB, IVA, and IVB) have Re-Os isochrons that indicate asteroidal core crystallization within the first 10–40 million years of Solar System evolution. Rocks from Earth's convecting mantle show generally chondritic Re/Os evolution throughout Earth history that is explained by the addition of highly siderophile elements to the mantle after core formation via late accretion. Oceanic basalts have Os-isotope systematics that improve the detailed geological interpretation of extant mantle components. Some portions of ancient subcontinental lithospheric mantle are severely depleted in Re and have correspondingly subchondritic ¹⁸⁷Os/¹⁸⁸Os, indicating long-term isolation from the convecting mantle during the Archean-Proterozoic. Magmatic ore deposits have differences in initial Os isotopic composition traceable to the crustal vs mantle sources of the platinum-group elements and base metals.

We examine the dynamics of the angular momentum balance of the Earth's core. Not only is this balance of great importance to theories of the geodynamo process that is responsible for the generation of the Earth's magnetic field, but recent work has shown that angular momentum variations in the core have broad geophysical implications, ranging from studies of the travel times of seismic waves through the inner core to attempts to account for a possible phase discrepancy between atmospheric and oceanic angular momentum. In this review, we present a simple account of the underlying dynamics and review the relevant observations and their interpretation.

Fission track analysis as a geological dating tool was first proposed in the early 1960s. The past 10 years has seen a major expansion in application to more general geological problems. This reflects advances in understanding the temperature dependence of fission track annealing and of the information contained in fission track length distributions. Fission track analysis provides detailed information on the low-temperature thermal histories of rocks, below ∼120°C for tracks in apatite and below ∼350°C for zircon. Fission track analysis has been applied to a variety of geological problems, including sedimentary provenance, thermal history modeling of sedimentary basins, structural evolution of orogenic belts, and long-term continental denudation.

Vertebrate fossils and continental sediments provide a rich record of variations in the isotopic composition of surface environments. To interpret these records, a greater understanding of isotopic sources, as well as fractionations associated with animal physiology, soil geochemistry, and diagenesis, has been essential. Tooth enamel and fish otoliths yield subannual records of surface environments, whereas soil minerals may integrate signals over many thousands of years. Carbon isotope variations in fossil vertebrates and soils record changes in the structure of vegetation and the isotope composition and concentration of atmospheric CO2. Oxygen isotope variations may be indirectly related to climate, through reconstruction of the oxygen isotope composition of meteoric water, or directly related to temperature, through application of oxygen isotope paleothermometry to soil minerals or otoliths. In Africa, nitrogen isotope variations show promise as a proxy for rainfall abundance, though the generality of this association elsewhere has not been demonstrated.

The central assumption of plate tectonics, that plate interiors are rigid, remains a useful but uncertain approximation. Strain rates of stable plate interiors are bounded between 10⁻¹²–10⁻¹¹ year⁻¹ and ∼4 × 10⁻¹⁰ year⁻¹. The narrowness of all plate boundaries, the other main assumption of plate tectonics as originally conceived, is contradicted by many observations, both in the continents and in the oceans. Some diffuse plate boundaries in both continents and oceans exceed dimensions of 1000 km on a side. Diffuse plate boundaries cover ∼15% of Earth's surface. The maximum speed of relative plate motion across any one diffuse plate boundary ranges from ∼2 to ∼15 mm/year, which is faster than some upper bounds on intraplate motion across stable plate interiors (≤2 mm year⁻¹). Strain rates in diffuse plate boundaries can be as high as ∼10⁻⁸ year⁻¹, ∼25 times higher than the upper bound on strain rates of stable plate interiors, but ∼600 times lower than the lowest strain rates across typical narrow plate boundaries. The poles of rotation of the plates flanking a diffuse oceanic plate boundary tend to be located in the diffuse boundary, which is a consequence of the strong coupling across the boundary.

This paper reviews rock friction and the frictional properties of earthquake faults. The basis for rate- and state-dependent friction laws is reviewed. The friction state variable is discussed, including its interpretation as a measure of average asperity contact time and porosity within granular fault gouge. Data are summarized showing that friction evolves even during truly stationary contact, and the connection between modern friction laws and the concept of “static” friction is discussed. Measurements of frictional healing, as evidenced by increasing static friction during quasistationary contact, are reviewed, as are their implications for fault healing. Shear localization in fault gouge is discussed, and the relationship between microstructures and friction is reviewed. These data indicate differences in the behavior of bare rock surfaces as compared to shear within granular fault gouge that can be attributed to dilation within fault gouge. Physical models for the characteristic friction distance are discussed and related to the problem of scaling this parameter to seismic faults. Earthquake afterslip, its relation to laboratory friction data, and the inverse correlation between afterslip and shallow coseismic slip are discussed in the context of a model for afterslip. Recent observations of the absence of afterslip are predicted by the model.

Ocean floor structures with horizontal scales of 10 to a few hundred kilometers and vertical scales of 100 m or more generate sea surface gravity anomalies observable with satellite altimetry. Prior to 1990, altimeter data resolved only tectonic lineaments, some seamounts, and some aspects of mid-ocean ridge structure. New altimeter data available since mid-1995 resolve 10-km–scale structures over nearly all the world's oceans. These data are the basis of new global bathymetric maps and have been interpreted as exhibiting complexities in the sea floor spreading process including ridge jumps, propagating rifts, and variations in magma supply. This chapter reviews the satellite altimetry technique and its resolution of tectonic structures, gives examples of intriguing tectonic phenomena, and shows that structures as small as abyssal hills are partially resolved. A new result obtained here is that the amplitude of the fine-scale (10–80 km) roughness of old ocean floor is spreading-rate dependent in the same way that it is at mid-ocean ridges, suggesting that fine-scale tectonic fabric is generated nearly exclusively by ridge-axis processes.