Evidence For Plate Tectonics Research Articles

Tectono-stratigraphic terranes in Archaean gneiss complexes as evidence for plate tectonics: The Nuuk region, southern West Greenland

Prior to 1970 grey gneiss complexes were interpreted as partially-melted sedimentary sequences. Once it was recognised from the Nuuk region that they comprised calc-alkaline igneous complexes, it was understood that such complexes world-wide were dominated by TTG (trondhjemite-tonalite-granodiorite) initially found to have juvenile Sr, Nd and, subsequently, Hf isotopic signatures. Between 1970 and 1985 the Nuuk region gneiss complex was interpreted by the non-uniformitarian ‘super-event’ model of crust formation which proposed occasional but extensive crust formation, with craton-wide correlation of granulite facies metamorphism and deformational phases. The igneous rocks formed in a late- Meso- to early Neoarchaean super-event engulfed crust formed in an Eoarchaean super-event. Mapping and reinterpretation at Færingehavn showed there are three TTG gneiss domains, each with different early accretionary, metamorphic and tectonic histories, separated by folded meta-mylonites. This established the key feature of the tectono-stratigraphic terrane model; that each terrane has an early intra-terrane history of crust formation, deformation and metamorphism, upon which is superimposed a later deformation and metamorphic history common to several terranes after they were juxtaposed. Remapping and >250 U-Pb zircon age determinations have refined the geological evolution of the entire Nuuk region, and has confirmed at least four main crust formation events and two collisional orogenies with associated transient high pressure metamorphism within clockwise P-T-t loops. Via independent corroborative studies the tectono-stratigraphic terrane model has been accepted for the Nuuk region and, through the discovery of similar relations across other gneiss complexes, its mode of evolution is found to be applicable to Archaean high-grade gneiss complexes worldwide. The TTG and mafic components that dominate each terrane have geochemistry interpreted to indicate subduction-related magmatism at convergent plate boundaries. Each terrane is thus dominated by juvenile additions to the crust. Intra-terrane sedimentary rocks show near unimodal age distributions in contrast to those near the boundaries which are more diverse and complex. The combined geochronological, metamorphic and structural evidence of convergence of these terranes leading to collisional orogeny, this indicates that plate tectonic processes operated throughout the Archaean.

Combining meteorites and missions to explore Mars.

Laboratory studies of meteorites and robotic exploration of Mars reveal scant atmosphere, no evidence of plate tectonics, past evidence for abundant water, and a protracted igneous evolution. Despite indirect hints, direct evidence of a martian origin came with the discovery of trapped atmospheric gases in one meteorite. Since then, the study of martian meteorites and findings from missions have been linked. Although the meteorite source locations are unknown, impact ejection modeling and spectral mapping of Mars suggest derivation from small craters in terrains of Amazonian to Hesperian age. Whereas most martian meteorites are young (< 1.3 Ga), the spread of whole rock isotopic compositions results from crystallization of a magma ocean > 4.5 Ga and formation of enriched and depleted reservoirs. However, the history inferred from martian meteorites conflicts with results from recent Mars missions, calling into doubt whether the igneous histor y inferred from the meteorites is applicable to Mars as a whole. Allan Hills 84001 dates to 4.09 Ga and contains fluid-deposited carbonates. Accompanying debate about the mechanism and temperature of origin of the carbonates came several features suggestive of past microbial life in the carbonates. Although highly disputed, the suggestion spurred interest in habitable extreme environments on Earth and throughout the Solar System. A flotilla of subsequent spacecraft has redefined Mars from a volcanic planet to a hydrologically active planet that may have harbored life. Understanding the history and habitability of Mars depends on understanding the coupling of the atmosphere, surface, and subsurface. Sample return that brings back direct evidence from these diverse reservoirs is essential.

GEODYNAMICS AND RATE OF VOLCANISM ON MASSIVE EARTH-LIKE PLANETS

We provide estimates of volcanism versus time for planets with Earth-like composition and masses from 0.25 to 25 times Earth, as a step toward predicting atmospheric mass on extrasolar rocky planets. Volcanism requires melting of the silicate mantle. We use a thermal evolution model, calibrated against Earth, in combination with standard melting models, to explore the dependence of convection-driven decompression mantle melting on planet mass. Here we show that (1) volcanism is likely to proceed on massive planets with plate tectonics over the main-sequence lifetime of the parent star; (2) crustal thickness (and melting rate normalized to planet mass) is weakly dependent on planet mass; (3) stagnant lid planets live fast (they have higher rates of melting than their plate tectonic counterparts early in their thermal evolution) but die young (melting shuts down after a few Gyr); (4) plate tectonics may not operate on high mass planets because of the production of buoyant crust which is difficult to subduct; and (5) melting is necessary but insufficient for efficient volcanic degassing - volatiles partition into the earliest, deepest melts, which may be denser than the residue and sink to the base of the mantle on young, massive planets. Magma must also crystallize at or near the surface, and the pressure of overlying volatiles must be fairly low, if volatiles are to reach the surface. If volcanism is detected in the Tau Ceti system, and tidal forcing can be shown to be weak, this would be evidence for plate tectonics.

Subduction styles in the Precambrian: Insight from numerical experiments

Plate tectonics is a self-organizing global system driven by the negative buoyancy of the thermal boundary layer resulting in subduction. Although the signature of plate tectonics is recognized with some confidence in the Phanerozoic geological record of the continents, evidence for plate tectonics becomes less certain further back in time. To improve our understanding of plate tectonics on the Earth in the Precambrian we have to combine knowledge derived from the geological record with results from well-constrained numerical modeling. In a series of experiments using a 2D petrological–thermomechanical numerical model of oceanic subduction we have systematically investigated the dependence of tectono-metamorphic and magmatic regimes at an active plate margin on upper-mantle temperature, crustal radiogenic heat production, degree of lithospheric weakening and other parameters. We have identified a first-order transition from a “no-subduction” tectonic regime through a “pre-subduction” tectonic regime to the modern style of subduction. The first transition is gradual and occurs at upper-mantle temperatures between 250 and 200 K above the present-day values, whereas the second transition is more abrupt and occurs at 175–160 K. The link between geological observations and model results suggests that the transition to the modern plate tectonic regime might have occurred during the Mesoarchean–Neoarchean time (ca. 3.2–2.5 Ga). In the case of the “pre-subduction” tectonic regime (upper-mantle temperature 175–250 K above the present) the plates are weakened by intense percolation of melts derived from the underlying hot melt-bearing sub-lithospheric mantle. In such cases, convergence does not produce self-sustaining one-sided subduction, but rather results in shallow underthrusting of the oceanic plate under the continental plate. Further increase in the upper-mantle temperature (> 250 K above the present) causes a transition to a “no-subduction” regime where horizontal movements of small deformable plate fragments are accommodated by internal strain and even shallow underthrusts do not form under the imposed convergence. Thus, based on the results of the numerical modeling, we suggest that the crucial parameter controlling the tectonic regime is the degree of lithospheric weakening induced by emplacement of sub-lithospheric melts into the lithosphere. A lower melt flux at upper-mantle temperatures < 175–160 K results in a lesser degree of melt-related weakening leading to stronger plates, which stabilizes modern style subduction even at high mantle temperatures.

Magnetic World: The Historiography of an Inherently Complex Science, Geomagnetism, in The 20th Century

The 20th century witnessed a succession of remarkable developments in the history of geomagnetic research. After 1900, geomagnetic researchers increased their activity as they adopted theories by C. F. Gauss and J. C. Maxwell and developed means to bridge the gulf between these theories and masses of data of global phenomena. This paper outlines three main streams in 20th-century geomagnetic research: investigations of processes deep inside the Earth that produce the main geomagnetic field, examinations of crustal magnetism, and research into processes on the edge of space, where Earth's magnetic field interacts with the interplanetary environment. This discussion places these research streams in the historiographic context of disciplinary specialization and transformation. In the early 20th century, geomagnetic researchers thought of their domain as all of Earth's magnetic and electric phenomena. In mid-century, however, many researchers began to narrow their gaze to one problem area or another. This specialization contributed to a period of dramatic developments in the latter half of the century: geodynamo theory, paleomagnetic evidence of plate tectonics, computer modeling of magnetic reversals, and discovery of the solar wind, radiation belts, and magnetic substorms. But there was more to this period than simple specialization. As researchers gradually shifted their research programs, their methods, instruments, and theories moved from one program to another, researchers sometimes going with them. Chameleons and opportunists frequently left one research program for another, more promising one. This paper closes with a discussion of the possibility of "re-connection" among these specializations, as researchers have begun once again to communicate across inter-field lines.

The role of Antarctica in the development of plate tectonic theories: from Scott to the present

One hundred years of geological research in and around Antarctica since Scott's Discovery expedition of 1901–1904 have seen the continent move from a great unknown at the margins of human knowledge to centre stage in the development of plate tectonics, continental break-up and global climate evolution. Research in Antarctica has helped make the Gondwana supercontinent a scientific fact. Discoveries offshore have provided some of the key evidence for plate tectonics and extended the evidence of global glaciation back over 30 million years. Studies of Antarctica's tectonic evolution have helped elucidate the details of continental break-up, and the continent continues to provide the best testing ground for competing scientific models. Antarctica's deep past has provided support for the “Snowball Earth” hypothesis, and for the pre-Gondwana, Rodinia supercontinent. Current research is focusing on Antarctica's subglacial lakes and basins, the possible causes of Antarctic glaciation, the evolution of its surrounding oceanic and mantle gateways, and its sub-ice geological composition and structure. None of this would have been possible without maps, and these have provided the foundation stone for Antarctic research. New mapping and scientific techniques, and new research platforms hold great promise for further major contributions from Antarctica to Earth system science in the twenty-first century.

Magnetism, the Earth as a Magnet, and Seafloor Banding—How Much Magnetism is Enough?

Although seafloor banding patterns are widely taught as a key piece of evidence for plate tectonics, little research has been conducted on student understanding of magnetism in a geological context. This study was conducted in two introductory geology courses to identify the effect of students' understanding of basic magnetism on their ability to build a simple conceptual model of seafloor banding consistent with the scientifically accepted model. Students were asked open-ended questions to elicit their reasoning about magnetism, Earth magnetism, and seafloor banding. Results suggest that many students in the study had weak or alternate models of basic magnetism. These models led students to reason incorrectly about magnetic interactions of the Earth's magnetic field and Earth materials during rock formation. Even students with robust magnetic models had difficulty applying them correctly to account for the origin of seafloor banding.

The STRATegy COLUMN for Precollege Teachers – Looking for Evidence of Plate Tectonics

The World Wide Web is an excellent source of data and images for teaching about plate tectonics. Students can use this data to develop their own explanations of plate-tectonic processes, then test their models against geologists' understanding of plate tectonics.

Earthquakes and Earth structure: a perspective since Hutton and Lyell

Abstract Lyell’s interest in earthquakes as part of the Principles of Geology continues to be justified many fold. A quarter century after Lyell’s death, seismology began to open the window on the contemporary structure and tectonic deformation of the Earth. Detailed non-biased observations of the global distribution of earthquakes played a crucial role in the attack on preplate theories of Earth dynamics. There were three critical seismological assault tools: reliable hypocentre catalogues, uniform magnitude estimates, and fault source mechanisms. Previously used as evidence for plate tectonics, seismicity is now often taken as predicted by it. Nevertheless, earthquake occurrence remains unforecastable in definite temporal terms. Interplate and intraplate spatial patterns show complexity in macro-crustal and micro-crustal structures. In particular, the mechanism and dynamic implications of deep-focus earthquakes and subduction remain a challenge. Local and global seismographic networks are increasingly enhanced by broadband digital seismometry. This modern instrumentation provides high resolution of strong ground shaking and crustal and deeper interior structure. Second-order structural variations are now being mapped in the upper mantle and more detailed boundary conditions for convection models are being resolved in the lithosphere and in the D″ mantle-core layer. Recently, seismological evidence for scattering anomalies throughout the mantle has become persuasive.

Report from IUGG Assembly: IAGA session on rock magnetism of fine particles

As a frontier science, rock magnetism provided crucial evidence for plate tectonics, but that was two decades ago and skeptics might ask whether the subject can still generate excitement, innovation, and controversy. They would have been reassured at the recent IAGA session on fine particles, the only meeting at Canberra that was devoted to rock magnetism per se, where 14 papers were presented.One of the few topics on which rock magnetists seem generally agreed is that the stable remanence, important in paleomagnetism, resides in effectively singledomained (SD) grains. However, their mode of occurrence and relationship to multidomained (MD) grains in rocks is not understood. For example, is SD caused by submicroscopic particles or by effective subdivision of much larger grains, or both? Other contentious questions raised at Canberra concern the role of grains that carry viscous remanence (VRM) and of superparamagnetism (SP) as a cause of magnetic instability.

New Evidence on the Palaeobiology of the Eureka Sound Formation, Arctic Canada

The Eureka Sound Formation, a thick sedimentary unit in the Canadian Arctic having a late Cretaceous and/or early Tertiary age, is known to contain plant fossils indicative of a continental origin of deposition and a relatively temperate climate. The Formation was selected for a palaeontological survey in order to determine whether it could, as suggested by distribution of fossil vertebrates in other areas and from evidence of plate tectonics, provide evidence on terrestrial migrations between North America and Europe in the Palaeogene. Fossils of plants, invertebrates and fish were found. They indicated that large parts of the Formation are marine in origin, although other parts are continental and thus could still be interpreted as representing part of a land connection between the northern landmasses.