Annual Review of Earth and Planetary Sciences - Volume 38, 2010

During the past several decades, experimental studies on the genesis and properties of magma have been intensively carried out, and our understanding of the conditions for generation of major-type magmas and their behaviors at depths has been significantly advanced. The author was fortunate to participate in some of these studies. These and subsequent experimental studies should be amalgamated with the studies of magma in other disciplines to develop a field of multidisciplinary research on magma for a better understanding of the genesis and role of magmas in the evolution of Earth and other planets.

Aerosol particles in the atmosphere exert a strong influence on climate by interacting with sunlight and by initiating cloud formation. Because the tropospheric aerosol is a heterogeneous mixture of various particle types, its climate effects can only be fully understood through detailed knowledge of the physical and chemical properties of individual particles. Here we review the results of individual-particle studies that use microscopy-based techniques, emphasizing transmission electron microscopy and focusing on achievements of the past ten years. We discuss the techniques that are best suited for studying distinct particle properties and provide a brief overview of major particle types, their identification, and their sources. The majority of this review is concerned with the optical properties and hygroscopic behavior of aerosol particles; we discuss recent results and highlight the potential of emerging microscopy techniques for analyzing the particle properties that contribute most to climate effects.

The Cenozoic history of the retreating Hellenic subduction system in the eastern Mediterranean involves subduction, accretion, arc magmatism, exhumation, normal faulting, and large-scale continental extension from ∼60 Mya until the Recent. Ages for high-pressure metamorphism in the central Aegean Sea region range from ∼53 Ma in the north (the Cyclades islands) to ∼25−20 Ma in the south (Crete). Younging of high-pressure metamorphism reflects the southward retreat of the Hellenic subduction zone. The shape of pressure-temperature-time paths of high-pressure rocks is remarkably similar across all tectonic units, suggesting a steady-state thermal profile of the subduction system and persistence of deformation and exhumation styles. The high-pressure metamorphic events were caused by the underthrusting of fragments of continental crust that were superimposed on slab retreat. Most of the exhumation of high-pressure units occurred in extrusion wedges during ongoing lithospheric convergence. At 23–19 Mya large-scale lithospheric extension commenced, causing metamorphic core complexes and the opening of the Aegean Sea basin. This extensional stage caused limited exhumation at the margins of the Aegean Sea but accomplished the major part of the exhumation of high-grade rocks that formed between 21 and 16 Mya in the central Aegean. The age pattern of extensional faults and contoured maps of fission-track cooling ages do not show a simple southward progression. Our review of lithologic, structural, metamorphic, and geochronologic data is consistent with a temporal link between the draping of the subducted slab over the 660-km discontinuity and the large-scale extension causing the opening of the Aegean Sea basin.

Prevailing opinion assigns the Tibetan Plateau a crucial role in shaping Asian climate, primarily by heating of the atmosphere over Tibet during spring and summer. Accordingly, the growth of the plateau in geologic time should have written a signature on Asian paleoclimate. Recent work on Asian climate, however, challenges some of these views. The high Tibetan Plateau may affect the South Asian monsoon less by heating the overlying atmosphere than by simply acting as an obstacle to southward flow of cool, dry air. The East Asian “monsoon” seems to share little in common with most monsoons, and its dynamics may be affected most by Tibet's lying in the path of the subtropical jet stream. Although the growing plateau surely altered Asian climate during Cenozoic time, the emerging view of its role in present-day climate opens new challenges for interpreting observations of both paleoclimate and modern climate.

The 2004 Sumatra-Andaman earthquake has been extensively studied because of its great size and devastating consequences. Large amounts of high-quality seismic, geodetic, and geologic data have led to a number of proposed models for its length, duration, fault geometry, rupture velocity, and slip history. The latest of these models vary in their details but now largely agree in their large-scale features, which include significant coseismic slip along the entire 1300- to 1500-km rupture, the bulk of which occurred fast enough to radiate seismic waves. The earthquake's enormous size has challenged conventional processing approaches and stimulated the development of new analysis and inversion methods, including multiple-source inversions, high-frequency body-wave imaging, and satellite observations of tsunami heights and gravity changes. The Sumatra megathrust earthquake was the largest in 40 years and is by far the best documented, but it does not seem fundamentally different in its properties from other large subduction-zone earthquakes.

Mantle plumes—which are usually, but not always, chemically distinct from the mid-ocean ridge basalt (MORB)—may be rooted in the core-mantle boundary and begin with large voluminous heads triggering massive eruptions or be headless and arise in the mid-mantle. Geochemistry provides convincing evidence that mantle plumes are 100–300°C hotter than normal upper mantle and that upwelling rates within the melting region are faster than beneath mid-ocean ridges. ¹⁸⁶Os/¹⁸⁸Os hints at the possibility of material from Earth's core in the Hawaiian plume, but this is not seen in other oceanic island basalt (OIB) and has not been confirmed by ¹⁸²W/¹⁸⁴W measurements. High ³He/⁴He in plumes does not require a primordial deep-mantle reservoir. The geochemical signature of mantle plumes originates primarily through melting in the upper mantle, probably through creation and subduction of oceanic lithosphere, but the details remain obscure. Plumes are lithologically heterogeneous, consisting of stringers of mafic material embedded in a more dominant peridotite.

Isotope ratios of actively cycled elements vary as a function of the biogeochemical processes in which they participate and the conditions under which those processes occur. The resultant spatiotemporal distribution of isotopes in environmental materials can be predicted using models of isotope-fractionating processes and data describing environmental conditions across space and time, and it has been termed an isoscape, or isotopic landscape. Analysis of isoscapes and comparison of isoscape predictions with observational data have been used to test biogeochemical models, calculate aerially integrated biogeochemical fluxes based on isotope mass balance, and determine spatial connectivity in biogeochemical, ecological, and anthropological systems. Isoscape models of varying quality are available for stable H, C, N, and O isotopes in a range of Earth surface systems, but significant opportunities exist to refine our understanding of biogeochemical cycles and our ability to predict isoscapes through the development of more mechanistic and more comprehensive isoscape models.

Whales are first found in the fossil record approximately 52.5 million years ago (Mya) during the early Eocene in Indo-Pakistan. Our knowledge of early and middle Eocene whales has increased dramatically during the past three decades to the point where hypotheses of whale origins can be supported with a great deal of evidence from paleontology, anatomy, stratigraphy, and molecular biology. Fossils also provide preserved evidence of behavior and habitats, allowing the reconstruction of the modes of life of these semiaquatic animals during their transition from land to sea. Modern whales originated from ancient whales at or near the Eocene/Oligocene boundary, approximately 33.7 Mya. During the Oligocene, ancient whales coexisted with early baleen whales and early toothed whales. By the end of the Miocene, most modern families had originated, and most archaic forms had gone extinct. Whale diversity peaked in the late middle Miocene and fell thereafter toward the Recent, yielding our depauperate modern whale fauna.

Frictional melting is the result of the conversion of mechanical deformation to heat under adiabatic conditions of slip. Within planetary materials, which are mainly natural ceramics, frictional melting occurs at high strain rates (typically >10⁻² s⁻¹) and at slip velocities greater than 0.1 m s⁻¹. The pathway to friction melting is controlled by the mechanical properties of a rock's constituent minerals, especially fracture toughness. Minerals with the lowest fracture toughnesses and breakdown temperatures are preferentially comminuted and fused to form the melt. The product is a polyphase suspension comprising mineral and rock fragments enclosed in a liquid matrix. This cools to form the rock type known as pseudotachylyte, and at even higher strain rates, it forms shock veins in meteorites and in impact craters, which may contain high-pressure mineral polymorphs. The generation of melt on sliding surfaces can lubricate earthquake faults, facilitate the post-shock modification of impact craters, and make landslides more hazardous.

The Gogo Formation of Western Australia preserves a unique Late Devonian (Frasnian) reef fauna. The exceptional three-dimensional preservation of macrofossils combined with unprecedented soft-tissue preservation (including muscle bundles, nerve cells, and umbilical structures) has yielded a particularly rich assemblage with almost 50 species of fishes described. The most significant discoveries have contributed to resolving placoderm phylogeny and elucidating their reproductive physiology. Specifically, these discoveries have produced data on the oldest known vertebrate embryos; the anatomy of the primitive actinopterygian neurocranium and phylogeny of the earliest actinopterygians; the histology, radiation, and plasticity of dipnoan (lungfish) dental and cranial structures; the anatomy and functional morphology of the extinct onychodonts; and the anatomy of the primitive tetrapodomorph head and pectoral fin.

“Booming” sand dunes have a remarkable capacity to produce sounds that are comparable with those from a stringed instrument. This phenomenon, in which sound is generated after an avalanching of sand along the slip face of a dune, has been known for centuries and occurs in at least 40 sites around the world. A spectral analysis of the sound shows a dominant frequency between 70 and 110 Hz, as well as higher harmonics. Depending on the location and time of year, the sound may continue for several minutes, even after the avalanching of sand has ceased. This review presents historical observations and explanations of the sound, many of which contain accurate and insightful descriptions of the phenomenon. In addition, the review describes recent work that provides a scientific explanation for this natural mystery, which is caused by sound resonating in a surface layer of the dune.

We explore the role of large impacts in creating the Martian valley networks. Recent dating shows that some large impact basins are contemporaneous with the valley networks. The mass deposited (and volatiles released) by impacts is large, and comparable with the mass from the Tharsis volcanic construct. Steam atmospheres formed after large impacts can produce more than 600 m of rainfall, followed by rainfall from water-vapor greenhouse atmospheres, and snowmelt. The erosion rates from impacts that created the currently visible craters are somewhat less than the erosion rates suggested for the Noachian (4.2 to 3.82 Gya). There are several possible explanations for this difference, and it is possible that erosion rates are overestimated because the burial of small craters by global debris layers from impacts has not been considered. Rainfall after the Noachian was low because the impact rate and CO2 pressure declined. We suggest tests of the hypothesis that impacts caused the river valleys.

The present kinematics in the eastern Mediterranean and Middle East is now well constrained by GPS measurements and dominated by a circular counterclockwise motion. While Arabia rotates rigidly, the rotational velocities increase from the Levant to Aegea, causing extension in western Anatolia. We place the present pattern in light of the post–Middle Miocene geological history and propose that in addition to the effects of the Hellenic subduction and Zagros collision-subduction zones on driving the surface motion, an underlying asthenospheric flow is also required. This counterclockwise toroidal flow is expected to exist around the eastern edge of the African slab owing to subduction rollback, and it helps explain the initiation of the North Anatolia Fault, the extrusion and uplift of Anatolia, and the Mid-Miocene to Recent volcanism in eastern Anatolia. The Afar plume had a similar effect on the acceleration of Arabia at 40 Mya.

The 2008 Wenchuan earthquake occurred on imbricate, oblique, steeply dipping, slowly slipping, listric-reverse faults. Measurements of coseismic slip, the distribution of aftershocks, and fault-plane solution of the mainshock all confirm this style of deformation and indicate cascading earthquake rupture of multiple segments, each with coseismic slip occurring in the shallow crust above a depth range of 10 to 12 km. Interactions among three geological units—eastern Tibet, the Longmen Shan, and the Sichuan basin—caused slow strain accumulation in the Longmen Shan so that measurable preearthquake slip was minor. Coseismic deformation, however, took place mostly within the interseismically locked Longmen Shan fault zone. The earthquake may have initiated from slip on a fault plane dipping 30–40° northwest in a depth range from 15 to 20 km and triggered oblique slip on the high-angle faults at depths shallower than 15 km to form the great Wenchuan earthquake.

Cassini observations confirm that Saturn's rings are predominantly water ice. The particles in Saturn's rings cover a range of sizes, from dust to small moons. Occultation results show the particles form temporary elongated aggregates tens of meters across. Some of the ring structure is created by moons, others by various instabilities. Data from future Cassini measurements can help investigators decide if the rings are remnants of the Saturn nebula or fragments of a destroyed moon or comet.

As with many mountain belts globally, the Alps have seen a large increase in sediment yield in the late Neogene. The hypothesis that this increase results from climate-change impacts on erosion rate over the past ∼5 Ma is testable, given the extensive work completed in the Alps. Sediment budgets, thermochronology-based cooling rates, and estimates of modern rock uplift and erosion in the Western Alps are reviewed to search for correlation in space and time that might fingerprint climatic events. Major increases in sediment yield are apparent starting in the late Messinian (5.5 Mya) and accelerating toward the present day; this suggests the occurrence of a series of climatic changes that increased erosion rates, even as tectonic processes appear to have slowed or stopped. At present, tight correlations in time are difficult to establish and there remain open questions about the role of tectonics in the coupled system.

Although fault and magmatic processes have achieved plate spreading at mid-ocean ridges throughout Earth's history, discrete rifting episodes have rarely been observed. This paper synthesizes ongoing seismic, structural, space-based geodetic, and petrologic studies from the subaerial Red Sea rift in Ethiopia where a major rifting episode commenced in September 2005. Our aims are to determine the length and timescales of magmatism and faulting, the partitioning of strain between faulting and magmatism, and their implications for the maintenance of along-axis segmentation. Most of the magma for the initial and subsequent 12 intrusions was sourced from the center of the Dabbahu-Manda Hararo rift segment. Strain is accommodated primarily by axial dike intrusions fed from mid-segment magma chamber(s). These findings show that episodic (approximate century interval), rapid opening of discrete rift segments is the primary mechanism of plate boundary deformation. The scale (∼65 km × 8 km) and intensity of crustal deformation (∼6 m), as well as the volume of intrusive and extrusive magmatism (>3 km³), provokes a re-evaluation of seismic and volcanic hazards in subaerial rift zones.

Glacial earthquakes are a new class of seismic events, first discovered as signals in long-period seismograms recorded on the Global Seismographic Network. Most of these events occur along the coasts of Greenland, where they are spatially related to large outlet glaciers. Glacial earthquakes show a strong seasonality, with most events occurring during the late summer. The rate of glacial-earthquake occurrence increased between 2000 and 2005, with a stabilization of earthquake frequency at 2003–2004 levels in 2006–2008. Recent observations establish a strong temporal correlation between the distinct seismic signals of glacial earthquakes and large ice-loss events in which icebergs of cubic-kilometer scale collapse against the calving face, linking the seismogenic process to the force exerted by these icebergs on the glacier and the underlying solid earth. A sudden change in glacier speed results from these glacial-earthquake calving events. Seasonal and interannual variations in glacier-terminus position account for general characteristics of the temporal variation in earthquake occurrence. Glacial earthquakes in Antarctica are less well studied, but they exhibit several characteristics similar to glacial earthquakes in Greenland.

Planets are built from planetesimals: solids larger than a kilometer that grow by colliding in pairs. Planetesimals themselves are unlikely to form by two-body collisions alone; subkilometer objects have gravitational fields individually too weak, and electrostatic attraction is too feeble for growth beyond a few centimeters. We review the possibility that planetesimals form when self-gravity brings together vast ensembles of small particles. Even when self-gravity is weak, aerodynamic processes can accumulate solids relative to gas, paving the way for gravitational collapse. Particles pile up as they drift radially inward. Gas turbulence stirs particles but can also seed collapse by clumping them. Whereas the feedback of solids on gas triggers vertical-shear instabilities that obstruct self-gravity, this same feedback triggers streaming instabilities that strongly concentrate particles. Numerical simulations find that solids ∼10–100 cm in size gravitationally collapse in turbulent disks. We outline areas for progress, including the possibility that still smaller objects self-gravitate.