Plate Tectonic Activity Research Articles

Palaeobiogeography of Famennian (Late Devonian) bryozoans

Bryozoans were widely distributed in marine habitats of the Famennian (Late Devonian) worldwide. This time interval was marked by gradual change in taxonomic composition of bryozoan faunas, which were insignificantly affected by global events (Upper Kellwasser, Handerberg). Distribution of bryozoan faunas was controlled by intrinsic factors (macroevolution) as well as extrinsic factors (eustasy, plate tectonic and volcanic activity). Palaeobiogeographic analysis of bryozoan associations reveals closest similarities between Tian Shan and Kazakhstan during the early to middle Famennian and between the Altay-Sayan Fold Belt, Transbaikalia and Mongolia during the late Famennian. Migration patterns of bryozoan faunas during the Famennian are reconstructed, showing main dispersions from the marine basins of Northern China to Kazakhstan and western offshoots of Tian Shan, and then to other areas. Biostratigraphic markers based on bryozoan taxa are established for each substage (lower, middle and upper Famennian) within relevant regions.

Indonesian Landforms and Plate Tectonics

DOI: 10.17014/ijog.v5i3.103The horizontal configuration and vertical dimension of the landforms occurring in the tectonically unstable parts of Indonesia were resulted in the first place from plate tectonics. Most of them date from the Quaternary and endogenous forces are ongoing. Three major plates – the northward moving Indo-Australian Plate, the south-eastward moving SE-Asian Plate and the westward moving Pacific Plate - meet at a plate triple-junction situated in the south of New Guinea’s Bird’s Head. The narrow North-Moluccan plate is interposed between the Asia and Pacific. It tapers out northward in the Philippine Mobile Belt and is gradually disappearing. The greatest relief amplitudes occur near the plate boundaries: deep ocean trenches are associated with subduction zones and mountain ranges with collision belts. The landforms of the more stable areas of the plates date back to a more remote past and, where emerged, have a more subdued relief that is in the first place related to the resistance of the rocks to humid tropical weathering Rising mountain ranges and emerging island arcs are subjected to rapid humid-tropical river erosions and mass movements. The erosion products accumulate in adjacent sedimentary basins where their increasing weight causes subsidence by gravity and isostatic compensations. Living and raised coral reefs, volcanoes, and fault scarps are important geomorphic indicators of active plate tectonics. Compartmental faults may strongly affect island arcs stretching perpendicular to the plate movement. This is the case on Java. Transcurrent faults and related pull-apart basins are a leading factor where plates meet at an angle, such as on Sumatra. The most complicated situation exists near the triple-junction and in the Moluccas. Modern research methods, such as GPS measurements of plate movements and absolute dating of volcanic outbursts and raised coral reefs are important tools. The mega-landforms resulting from the collision of India with the Asian continent, around 50.0 my. ago, and the final collision of Australia with the Pacific, about 5.0 my. ago, also had an important impact on geomorphologic processes and the natural environment of SE-Asia through changes of the monsoonal wind system in the region and of the oceanic thermo-haline circulation in eastern Indonesia between the Pacific and the Indian ocean. In addition the landforms of the region were, of course, affected by the Quaternary global climatic fluctuations and sea level changes.

Do Cenozoic analogues support a plate tectonic origin for Earth's earliest continental crust?

Archean continental crust largely comprises the trondhjemite, tonalite, and granodiorite/dacite (TTG/D) suite of igneous rocks. Formation of the earliest Archean (>3.5 Ga) TTG/Ds is controversial, being attributed to either subduction zone processes with active plate tectonics or thermochemical mantle convection with no plate tectonic processes. A suite of Cenozoic adakite-like lavas in Jamaica has geochemical compositions comparable to early Archean TTG/D. The data indicate that the adakites were generated by underthrusting (or subducting) and partial melting of oceanic plateau crust beneath Jamaica. This setting is analogous to proposed plate tectonic processes in the early Archean where hot, thick, and more buoyant Archean oceanic crust underthrusts adjacent plates. The new adakite data imply that earliest Archean TTG/D continental crust could have formed above primitive subduction zones.

Deformed Messinian markers in the Cyprus Arc: tectonic and/or Messinian Salinity Crisis indicators?

This paper focuses on Messinian Salinity Crisis (MSC) evaporites in the Cyprus Arc (eastern Mediterranean) using high-resolution reflection seismic and multi-beam data. The results shed new light on the Miocene to Present tectonic evolution of this area and contribute to our general knowledge of the MSC in a deep basin setting. The evaporites and overlying formations show a complex deformation pattern due to a combination of thick- skinned plate-tectonic convergence and thin- skinned disharmonic deformation related to the mobile evaporite-bearing unit. Several MSC markers are identified and precisely mapped: the base of the MSC unit is a ‘decollement’ level, whereas the top is clearly identified as a toplap surface. Intra-MSC markers and two MSC subunits are identified and mapped over the entire study area. The geometry of MSC markers shows that the lower MSC subunit was deposited in a relatively quiet tectonic setting. The nature of the anisopachous upper unit indicates a syn-depositional phase of large- scale plate-tectonic activity. A thin- skinned phase of compressional deformation during the Late Miocene affected the entire MSC unit, overlain by undeformed Pliocene-Quaternary layers. A second thin- skinned phase, well expressed in the bathymetry, occurred from the Pliocene to Recent, resulting in extensional gravitygliding within the evaporites and the Pliocene-Quaternary sequence. We show that the MSC had a dramatic impact on the regional structure. For instance, the erosive nature of the top of the MSC unit is linked to the desiccation episode rather than to the cessation of tectonic activity. This particularly strong and short-lived erosion may have been enhanced by the regional effects of the MSC, owing to differential uplift/subsidence caused by the drawdown. The evaporites are essential markers for constraining the tectonic framework, provided that active deformation can be distinguished from passive gliding associated with extensional/contractional deformation.

Yin and yang of continental crust creation and destruction by plate tectonic processes

Earth's continental crust today is both created and destroyed by plate tectonic processes, a balance that is encapsulated by the traditional Chinese concept of yin–yang, whereby dualities act in concert as well as in opposition. Yin–yang conceptualizations of crustal growth and destruction are mostly related to plate tectonics; both occur mostly at subduction zones, by arc magmatic creation and by subduction removal. Crust is also created and destroyed by processes unrelated to plate tectonics, including losses by lower crust foundering and additions at hotspots. At present, creation and destruction of continental crust is either in balance (∼3.2 km3/year, or 3.2 AU) or more crust is being destroyed than created; the uncertainty comes from unknown deep losses of continental crust at collision zones and due to lower crustal foundering. The yin–yang creation–destruction balance changes over a supercontinent cycle, with crustal growth being greatest during supercontinent break-up due to high magmatic flux at new arcs and crustal destruction being greatest during supercontinent amalgamation due to subduction of continental material and increased sediment flux due to orogenic uplift. These conclusions challenge the widely held view that continental crust volume has increased over time due to plate tectonic activity; it is just as likely that this volume has decreased.

Hydrogen partitioning between iron and ringwoodite: Implications for water transport into the Martian core

We determined the exchange partition coefficients of hydrogen between solid iron and ringwoodite between 16.6 and 20.9 GPa at temperatures up to 1273 K using a Kawai-type multianvil high-pressure apparatus with synchrotron X-ray radiation at the BL04B1 beamline at SPring-8, Japan. The hydrogen concentration in iron hydride was estimated from the volume expansion of iron caused by hydrogenation determined by in situ X-ray diffractions at high pressure and high temperature, and the water content of ringwoodite in the recovered samples was estimated using the Fourier transform infrared spectroscopy (FTIR). According to our results, the exchange partition coefficients of hydrogen between the solid iron and ringwoodite were almost constant, 26, with pressure between 16.6 and 20.9 GPa and 1273 K. These results revealed that hydrogen was strongly partitioned to metallic iron and that iron hydride formed, coexisting with dry ringwoodite under the experimental pressures. Ringwoodite, found in the Martian core–mantle boundary region, is an important hydrogen reservoir. The pattern of quasi-parallel bands of uniformly magnetized crust with alternating positive and negative polarity measured by the Mars Global Surveyor spacecraft strongly shows that a magnetic field did exist in ancient Mars suggesting a possible plate tectonic activity on ancient Mars. Thus, water could have been transported to the deep Martian interior by hydrous minerals during the plate subduction process and stored in ringwoodite in the deep Martian slabs, as is suggested on the Earth today. Our experiments suggested that hydrogen stored in ringwoodite was absorbed by the Martian core at the Martian core–mantle boundary. Thus, water from the ancient Martian ocean may be stored now in the Martian core.

The Grenvillian and Pan-African orogens: World's largest orogenies through geologic time, and their implications on the origin of superplume

The Neoproterozoic Earth was shaped largely by the Grenvillian and Pan-African orogenies. Out of these, the Grenvillian orogeny has long been regarded to be of minor nature in terms of global-scale orogenic episodes, whereas the Pan-African orogeny has been widely recognized in many continental fragments, although not in major parts of Asia. Based on chronological information in zircons from major river mouths across several important terrains of the globe, we show here that the Grenvillian orogeny contributed significantly to the formation of the continental crust. The time period between 0.6 Ga and 0.8 Ga marked the climax at the dawn of the Pan-African orogeny. Continental crust formed in this period is concentrated in the Pan-African orogenic belts widely across the globe. These regions were widespread over the half hemisphere of the globe, and were subsequently reduced in size after they moved to form Laurasia. The normalized frequency distribution of zircon ages from river-mouth sand over the world clearly demonstrates that Neoproterozoic and (0.9–0.6 Ga) and Grenvillian (1.3–1.0 Ga) peaks define the largest population. This means that extensive subduction, and hence active plate tectonics, might have operated through these periods. The zircon study has also brought to light new regions of the Grenvillian orogenic belts, particularly in the continents which are now covered by thick Phanerozoic sedimentary basins. Based on the new locations of Grenvillian orogens identified in this study, and using the distribution patterns as a marker bed, we propose revised paleogeographic configurations of the Rodinia and Gondwana supercontinents. Our results demonstrate that the Neoproterozoic was the most active period of crust formation in the Earth. The cold basins, formed right after the assembly of Rodinia, exhibit a basin chain fringing the northern periphery of Rodinia, which turned into sites of mantle upwellings and led to the rifting and separation of the supercontinental assembly. The continents then moved northwards after the formation of Gondwana at ca. 540 Ma, and enlarged the northern half of the supercontinent Pangea since 250 Ma. Based on the results, we also evaluate the role of supercontinents in the mechanism of generation of superplumes addressing the enigma that the coldest mantle right above the Core–Mantle Boundary turns to the hottest one over a period of several hundreds of million years. Slab graveyard formed by the Pan-African subduction can be imaged through P-wave tomography. We postulate that the high-velocity anomaly in the D” layer underneath Gondwana has now transformed to the low-V regions to generate the African superplume. The tectonic history of solid Earth in the Phanerozoic seems to be controlled by the slab graveyards formed by the Grenvillian orogeny ca. 1.0 Ga.

Late Triassic and Early Jurassic palaeogeography of the world

A new version of global and regional palaeogeographic maps is presented for two time intervals. These maps depict the plate tectonic configuration, palaeoenvironment and lithofacies during the Late Triassic (Carnian–Norian) and Early Jurassic (Hettangian–Toarcian) time. The individual maps illustrate the general conditions present during the maximum marine transgressions of higher frequency cyclicity within the Absaroka sequence of Sloss. During Triassic time Pangaea began to stretch, initiating the rifting and future break-up of the supercontinent. The continued northward drift of the Cimmerian continent corresponded with the progressive closure and consumption of Palaeotethys oceanic crust, and the opening of the Neotethys Ocean. The most significant Late Triassic convergent event was the Indosinian orogeny, occurring as a result of the consolidation of South China and North China blocks. Also, Indochina and Indonesia were sutured to South China. At the same time the Qiangtang block approached the Eurasian margin. The consolidation of the North Chinese and Amurian blocks left open a large embayment of Panthalassa, between Amuria and Laurasia, the so-called Mongol-Okhotsk Ocean. Active subduction existed along the margin of this ocean, dipping cratonwards towards East Siberia. The last collisional events of the Uralian orogeny took place during the Triassic and Early Jurassic time. The conclusion of the Uralian orogeny was accompanied by uplift of the adjacent areas of Eastern Europe and Western Siberia. During the Early Jurassic the Palaeotethys Ocean was finally closed and the Cimmerian continent collided with Asia causing the Cimmerian orogeny. The time around the Triassic–Jurassic boundary marked an important biotic extinction event. Plate tectonic activity caused palaeogeographic and palaeoclimatic change, which may have contributed to the mass extinction. From the plate tectonic and palaeogeographic point of view the following events could have influenced the extinction: 1) the closure of Palaeotethys and assembly of the Asian part of Pangaea; 2) the break-up of Pangaea in the future Central Atlantic area and transition from rifting to drifting; and 3) the very extensive basaltic volcanism of the Central Atlantic Magmatic Province.

Mud volcanoes of Italy

AbstractThe locations and information about the sizes of 61 mud volcanoes on the Italian mainland and Sicily, plus an area of mud diapirism in the Italian Adriatic Sea, are presented. Data about the emission products are also provided. The majority of these mud volcanoes are found where thick sedimentary sequences occur within a zone of tectonic compression associated with local plate tectonic activity: the movement of the Adriatic microplate between the converging African and Eurasian plates. The principal gas emitted by these mud volcanoes is methane, which probably originates from deep within the sediments. Other mud volcanoes, associated with igneous volcanism, produce mainly carbon dioxide. The mud diapirs in the Adriatic Sea are thought to form as a result of the mobilization of shallow gassy sediments. It has been shown that radon emissions from mud volcanoes are indicators of forthcoming earthquake events. Copyright © 2004 John Wiley & Sons, Ltd.

Dual aspects of mantle convection through time: Changes in global upper mantle temperature, Earth’s rotation, and global plate motion acceleration induced by mantle avalanches

The mean depth of the mantle transition zone depends on the global mantle temperature through the Clapeyron slope. Numerical models of mantle convection with endothermic phase change at 670 km are characterized by quiet periods of partial layering alternating with catastrophic events. During an avalanche, global thermal anomalies, reaching several tens of degrees, and lasting several tens of million years occur that induce a global deepening of the mantle transition zone. The resulting inertia tensor perturbations lead to acceleration of the Earth’s rotation. The bottom heat fluxes remains strong as a consequence of the steep thermal gradient at the CMB level; whereas it decreases to a low level during quiet periods. The upper surface heat flux displays a more complex behavior due to the re-heating of upper mantle by plumes generated at the transition zone and at the CMB. Following these numerical results, two significantly different regimes of mantle convection may have alternated several times during the Earth’s history. The avalanche periods are also associated with a global increase of plate tectonic activity (ridge accretion and continental break-up) due to an increase of the surface velocity during the avalanche process. This work shows the richness of the behavior in surfaces and bottom heat fluxes that arises from the alternation of quiet periods of partial layering with catastrophic avalanche events. Complexities in the real Earth displays are surprisingly simular to those caused by the consequences of mantle avalanches.

Global thermal and dynamical perturbations due to Cretaceous mantle avalanche

The numerical models of mantle convection agree to depict avalanches behaviour according to the level of endothermicity of the spinel → perovskite phase change. Their potential effects on the global thermal and dynamical states of the mantle have been computed thanks to a numerical code, which takes into account both the 400-km exothermic and the 660-km endothermic phase changes. The cycle followed by the avalanches is: local layering, destabilization of the 660-km thermal layer, travelling and spreading on the core, and reappearing of the local layering. Therefore, mantle convection is characterized by quiet periods of partial layering embedded in catastrophic events. During the avalanche, the amplitude of the surface velocity is multiplied by two, which would imply an enhanced plate tectonic and ridge activities. The global thermal effects of the avalanche are compatible with a high mantle temperature and an acceleration of Earth's rotation during the Cretaceous. They also offer a coherent explanation to locate the origin of mantle plumes both within the CMB and just below the transition zone.

The cause of loss of lithospheric rigidity in areas far from plate tectonic activity

Significant losses of lithospheric strength are generally considered to be almost entirely associated with abnormal heating or steep lithospheric bending and/or stretching near to active plate boundaries. Several areas—the western Greater Caucasus, the North Crimean basin, the Carpathian foredeep, the Peri‐Caspian basin and the Trans‐Caspian areas—are shown to have steep basement slopes, usually comprising a difference in height of several kilometres over lateral distances of only 20–30 km, corresponding to very low, ∼3–5 km, effective elastic thicknesses of the lithosphere. Each of these areas is shown to have undergone rapid steepening of the basement slope, usually within 1–2 Myr but in up to 10 Myr in some areas. At such times, these localities were far from active plate boundaries and in positions where bending forces could not have been transmitted to them from far‐distant plate activities. Surface and/or subsurface loading can similarly be excluded as mechanisms for such steepening, and there is no apparent outflow of crustal materials into adjacent regions. It is suggested that such rapid subsidence far from plate tectonic activity is caused by rapid increases in the local density of the lithosphere. This could occur as a result of, for example, a gabbro‐eclogite transformation in the lower crust, catalysed by the infiltration of volatiles from the asthenosphere. The resultant contraction of the mafic rocks would be non‐uniform in space and produce high deviatory stresses, reducing the viscosity in the lower crust to ∼ 1023 Pa s. This would result in the rapid subsidence of the top of this layer, accompanied by steep ductile bending of the overlying upper crust. Such steep downwarping of the basement would be accompanied by a similar steepening of the underlying weakened mantle. The formation of such steep slopes thus indicates a weakening of the entire lithospheric layer, most probably due to the infiltration of volatiles from the asthenosphere, and unrelated to coeval plate tectonic activity.

Spatial Distribution and Ages of Alkaline Intrusive Rocks in China and Related Tectonics

Abstract: On the basis of the geological data from 220 intrusions and isotope ages of 115 rock bodies, the paper discusses the distribution of 18 alkaline intrusive rock belts along deep faults and their relations to the plate tectonic activity. In view of the presence of a large amount of Variscan alkaline rocks of deep‐source character, it is inferred that mantle diapirism was very strong in China during the Variscan. Besides anorogenic and postorogenic alkaline granites, there also occur alkaline granites formed in the terminal orogenic stage when the regional tectonic activity was of compressional nature. Finally, the paper discusses the tectonic environment of the Maoniuping nordmarkite in Mianning county, Sichuan province, and the authors consider it to be a compressional environment originating after rift closing.

Study provides data on active plate tectonics in southeast Asia region

A major geodynamic study has provided significant new information about the location of active plate boundaries in and around Southeast Asia, as well as deformation processes in the Sulawesi region of Indonesia and tectonic activity in the Philippine archipelago. Results also have confirmed the existence of the so‐called Sunda Block, which appears to be rotating with respect to adjacent plates.The study, known as the Geodynamics of South and South‐East Asia (GEODYSSEA) project, has been a joint venture of the European Commission and the Association of South‐ East Asian Nations. It began in 1991 and involved a large team of European and Asian scientists and technicians studying the complex geodynamic processes and natural hazards of the region from the Southeast Asia mainland to the Philippines to northern Australia. Earthquakes, volcanic eruptions, tsunamis, and tectonically induced landslides endanger the lives of millions of people in the region, and the tectonic activity behind these natural hazards results from the convergence and collision of the Eurasian, Philippine, and Indo‐Australian Plates at relative velocities of up to 10 cm per year.

Granitic magma dynamics

Of what importance is granite? If we take an imaginary journey through our solar system we can visit terrestrial planets like Venus and Mars, our Moon and various rocky satellites orbiting the outer gas giants. In all of these places there are marvels and ‘geological’ wonders, but none of these worlds has any granite. Earth is unique in our solar system in having these rocks—the fundamental stuff of continents. Another unique aspect of Earth is its combination of active plate tectonics with a 70% covering of liquid H2O. It is also unique (as far as we know) in supporting DNA-based life, notwithstanding recent descriptions of what may be Martian nanobacterial fossils. Is it fantastic to suggest that all of these unique features might be linked in some way? If Earth’s mantle did not convect and there were no spreading ridges, we would not have plate tectonics. If the ridges were not covered in water, there would be no hydrothermal ridge processes; sea-floor basalts would not undergo alteration to hydrous minerals like clays and chlorites. If altered basalts had not formed in the Archaean then the first voluminous granitic magmas could not have formed by partial melting of these materials. If the early granitic rocks had not formed then ‘indestructible’ continental masses would not have grown. Although the distribution and elevation of continents plays a role in climate control, their role in the evolution of life is unclear. However, if granitic continents had not existed, we would not be able

Three generations of anorthosite-mangerite-charnockite-granite (AMCG) magmatism, contact metamorphism and tectonism in the Saguenay-Lac-Saint-Jean region of the Grenville Province, Canada

Four A-type granitoid plutons from the Saguenay-Lac-Saint-Jean region of the Grenville Province, lacking regionally imposed solid-state deformation, give UPb zircon igneous crystallisation ages of 1146 ± 3 Ma, 1082 ± 3 Ma, 1067 ± 3 Ma, and 1020 +4 −3 Ma. These ages indicate that the region has not been affected by the Ottawan period of the Grenville orogeny. A compilation of these and existing UPb age data reveals three periods of Mesoproterozoic magmatism in this region: 1160-1140 Ma, 1082-1050 Ma and 1020-1010 Ma. Each of these periods appears to have had some or all of the components of the well-known anorthosite-mangerite-charnockite-granite suite, and the first two periods can be correlated with magmatism elsewhere in the Grenville Province. Sparse UPb metamorphic ages also fall within these periods and are interpreted to reflect regional-scale contact metamorphism produced by the large plutons. In addition, the few ages of strike-slip movements on major shear zones also fall within these time periods. It is difficult to interpret these data as reflecting separate orogenic events, as there is little definitive evidence of calc-alkaline magmatism, thrusting or regional metamorphism in this region. Perhaps during the interval 1160 to 1010 Ma magmatism was driven by repeated upwelling of hot mantle, either in a plume or above a sinking lithospheric slab, but access to the mid-levels of the crust currently exposed was controlled by strike-slip movements on major vertical shear zones that resulted from plate-tectonic activity.

Magma generation on Mars constrained from an 40Ar degassing model

The degassing history of 40Ar on Mars is reconstructed from a model based on the Martian volcanic record. Accumulation history of 40Ar is influenced by the volcanic eruption rate at each Martian epoch in addition to the production rate of 40Ar due to potassium decay in the mantle. In order to degas the present amount of 40Ar to the Martian atmosphere, the volume fraction of erupted magma to the original mantle materials averaged over history should be about 0.006–0.018. The integrated volume of mantle materials that produced the observed volcanic materials and atmospheric 40Ar is about (4–11) × 1018 m3, hence the extent of mantle differentiation after the formation of Mars is only 0.03–0.08. A significant amount of 40Ar (about 73% of the present atmospheric content) has apparently been degassed during the formation of the major volcanic regions on Mars, such as Tharsis and Elysium. This means that a significant portion of 40Ar in the Martian atmosphere would have been degassed by relatively recent volcanic activity, whereas large fractions of other volatile components, such as H2O and CO2, might have been degassed by an early catastrophic degassing. The possibility of ancient plate tectonic activity on Mars, which was proposed recently to explain the origin of the Martian crustal dichotomy, is also examined by using the 40Ar degassing model. Although the possibility of Martian plate tectonics may not be ruled out from the model, we can constrain an upper limit for the duration of plate tectonics. The duration should be much shorter than 6–350 Ma (probably ≪100 Ma), which is rather short compared with the ranges of the northern lowlands ages (600–750 Ma).

Magellan and comparative planetology

Magellan data have been widely used to provide explanations for the tectonics and volcanism on Venus. However, it is also important to compare our understanding of these processes on Venus with the Earth, Moon, and Mars. One essential question concerning the behavior of these bodies is, How does the heat get out? Internal heat generation and planetary cooling are the energy sources responsible for planetary tectonics and volcanism. On the Earth the plates of plate tectonics are the thermal boundary layers of mantle convection cells. The subduction of these cold plates at ocean trenches contributes about 75% of the mantle heat flux. It is now quite clear that active plate tectonics is not occurring on Venus. The surface of Venus is a single rigid plate, as is the case for the Moon, Mars, and Mercury. Prior to Magellan, the general consensus was that Venus was in a near‐uniformatarian state, with conductive heat losses through the lithosphere balancing internal heat generation and secular cooling of the planet. However, crater studies from Magellan images provide direct evidence for a catastrophic or episodic evolution of Venus. One explanation is that Venus died tectonically 500 m.y. ago; plate tectonics occurred during the early evolution of the planet but ceased as the radioactive heat production decreased. An alternative explanation is that Venus experiences episodic global subduction events. Two major questions concerning the Earth remain largely unanswered, particularly when compared with Venus. (1) Why does the Earth have continents? (2) Why does the Earth have plate tectonics? A major difference between the Earth and Venus is the presence of liquid water (oceans) during much of the evolution of the Earth. It has been hypothesized that water is an essential ingredient for the silicic volcanism that produces continental rocks on the Earth, and the absence of well‐defined continental masses on Venus supports this hypothesis. It has long been recognized that plate tectonics cannot evolve in time without internal deformation of plates. Plate evolution in time results in geometrical incompatibilities that require overlaps or gaps. On the Earth these incompatibilities lead to broad zones of continental deformation, i.e., the western United States and China. The absence of plate tectonics on Venus suggests that rheologically soft continents are necessary for plate tectonics. A reasonable conclusion is that the Earth is the remarkable planet in terms of tectonics and volcanism, not Venus.

Taphonomy of spores and pollen in Gondwana Sequence of India

Theoretically only few sedimentary rocks are devoid of spores and pollen; however, it is the taphonomic factors viz., necrolysis, stratinomy and diagenesis, that make them barren, impoverished or polleniferous. Therefore, high fossilization potential of spores-pollen from predominantly terrestrial and complex package of sediments spaning from Early Permian through Early Cretaceous has been utilized to build a comprehensive data-base on the Gondwana palynostratigraphy and also to assess their loss by taphonomic factors. Advent of Gondwana sedimentation witnesses glacio-fluvial-lacustrine environments and rare marine transgressions during Asselian-Sakmarian Talchir Formation. Adverse effect of moraine transport on preservation of spores-pollen from boulder bed matrix of Talchir Formation is evident. In the subsequent Permian Sub-period, depositional conditions favour the formation of enormous coal deposits. Low grade coals, shales and fine-grained sandstones normally contain less degraded spores-pollen from the Upper Gondwana red beds of Lower Triassic Panchet Formation and Intertrappean beds of Lower Cretaceous Rajmahal Formation are interpreted to have undergone chemical and thermal diagenesis, respectively. Spores-pollen from sediments deposited under anoxic and alkaline conditions (with high contents of pyrite, siderite and phosphorites) show that their preservation is related to syn- and post-depositional conditions. Whereas, sediments influenced by plate movement and tectonic activity in Peri-Gondwana Tethyan Himalaya have spores-pollen showing the effect of post-depositional diagenetic factors.

Boundary layer theory and subduction

Numerical models of thermally activated convective flow in Earth's mantle do not resemble active plate tectonics because of their inability to model successfully the process of subduction, other than by the inclusion of artificial weak zones. Here we show, using a boundary layer argument, how the “ rigid lid ” style of convection favored by thermoviscous fluids leads to lithospheric stresses which may realistically exceed the yield stress and thus cause subduction to occur through the visoc‐plastic failure of lithospheric rock. An explicit criterion for the failure of the lid is given, which is sensitive to the internal viscosity ηa below the lid. For numbers appropriate to Earth's mantle, this criterion is approximately ηa > 1021 Pa s.