2. First rocks on a dead Earth

Fluvial Erosion Characterisation in the Juqueriquerê River Channel, Caraguatatuba, Brazil

Earth Science Research ◽

10.5539/esr.v5n2p105 ◽

2016 ◽

Vol 5 (2) ◽

pp. 105 ◽

Cited By ~ 2

Author(s):

Victor F. Velázquez ◽

Viviane D. A. Portela ◽

José M. Azevedo Sobrinho ◽

Antonio C. M. Guedes ◽

Mikhaela A. J. S. P Letsch

Keyword(s):

River Channel ◽

Crystalline Basement ◽

Morphological Features ◽

Igneous Rocks ◽

Effective Capacity ◽

River Systems ◽

Field Surveys ◽

Fluvial Erosion ◽

Excellent Opportunity ◽

Different Types

The Juqueriquerê River channel was formed in a Precambrian crystalline basement. The lithological association is largely composed of ancient metamorphic and igneous rocks, with several overlapping tectonic episodes. Field surveys along the upper and middle course allowed for cataloguing a wide variety of fluvial erosion features. A sizable amount of morphological features have been sculpted on different types of rocks, including furrows, potholes, percussion marks, polishing and smoothing boulders as the most representative. The sizes and shapes of these scour marks are also diverse, and their study has provided important results for better understanding the erosive processes. Given their wide variety, the erosive morphological features offer an excellent opportunity to explore the mechanisms of fluvial erosion and evaluate their effective capacity to remove cobbles and boulders in bedrock river systems.

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On the fate of water in the formation of rocky planets

10.5194/egusphere-egu21-3315 ◽

2021 ◽

Author(s):

Lindy Elkins-Tanton ◽

Jenny Suckale ◽

Sonia Tikoo

Keyword(s):

Water Content ◽

Upper Mantle ◽

Lower Mantle ◽

Plate Tectonics ◽

Solidus Temperature ◽

Magma Ocean ◽

Excess Water ◽

Low Degree ◽

Giant Impacts ◽

Rocky Planets

Rocky planets go through at least one and likely multiple magma ocean stages, produced by the giant impacts of accretion. Planetary data and models show that giant impacts do not dehydrate either the mantle or the atmosphere of their target planets. The magma ocean liquid consists of melted target material and melted impactor, and so will be dominated by silicate melt, and also contain dissolved volatiles including water, carbon, and sulfur compounds.As the magma ocean cools and solidifies, water and other volatiles will be incorporated into the nominally anhydrous mantle phases up to their saturation limits, and will otherwise be enriched in the remaining, evolving magma ocean liquids. The water content of the resulting cumulate mantle is therefore the sum of the traces in the mineral grains, and any water in trapped interstitial liquids. That trapped liquid fraction may in fact be by far the largest contributor to the cumulate water budget.The water and other dissolved volatiles in the evolving liquids may quickly reach the saturation limit of magmas near the surface, where pressure is low, but degassing the magma ocean is likely more difficult than has been assumed in some of our models. To degas into the atmosphere, the gases must exsolve from the liquid and form bubbles, and those bubbles must be able to rise quickly enough to avoid being dragged down by convection and re-dissolved at higher pressures. If bubbles are buoyant enough (that is, large enough) to decouple from flow and rise, then they are also dynamically unstable and liable to be torn into smaller bubbles and re-entrained. This conundrum led to the hypothesis that volatiles do not significantly degas until a high level of supersaturation is reached, and the bubbles form a buoyant layer and rise in diapirs in a continuum dynamics sense. This late degassing would have the twin effects of increasing the water content of the cumulates, and of speeding up cooling and solidification of the planet.Once the mantle is solidified, the timeclock until the start of plate tectonics begins. Modern plate tectonics is thought to rely on water to lower the viscosity of the asthenosphere, but plate tectonics is also thought to be the process by which water is brought into the mantle. Magma ocean solidification, however, offers two relevant processes. First, following solidification the cumulate mantle is gravitationally unstable and overturns to stability, carrying water-bearing minerals from the upper mantle through the transition zone and into the lower mantle. Upon converting to lower-mantle phases, these minerals will release their excess water, since lower mantle phases have lower saturation limits, thus fluxing the upper mantle with water. Second, the mantle will be near its solidus temperature still, and thus its viscosity will be naturally low. When fluxed with excess water, the upper mantle would be expected to form a low degree melt, which if voluminous enough with rise to help form the earliest crust, and if of very low degree, will further reduce the viscosity of the asthenosphere.

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Spatial variations in cooling rate in the mantle section of the Samail ophiolite in Oman: Implications for formation of lithosphere at mid-ocean ridges

Earth and Planetary Science Letters ◽

10.1016/j.epsl.2017.02.038 ◽

2017 ◽

Vol 465 ◽

pp. 134-144 ◽

Cited By ~ 24

Author(s):

Nick Dygert ◽

Peter B. Kelemen ◽

Yan Liang

Keyword(s):

Cooling Rate ◽

Spatial Variations ◽

Ocean Ridges ◽

Mantle Section

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Evolution of the Martian Crust

10.1093/acrefore/9780190647926.013.18 ◽

2017 ◽

Author(s):

John C. Bridges

Keyword(s):

Plate Tectonics ◽

Chemical Weathering ◽

Compositional Data ◽

Crustal Contamination ◽

Planetary Science ◽

Mantle Metasomatism ◽

Igneous Rocks ◽

Forthcoming Article ◽

Physical Weathering ◽

Igneous Activity

This is an advance summary of a forthcoming article in the Oxford Encyclopedia of Planetary Science. Please check back later for the full article.Mars, which has a tenth of the mass of Earth, has cooled as a single lithospheric plate. Current topography gravity maps and magnetic maps do not show signs of the plate tectonics processes that have shaped the Earth’s surface. Instead, Mars has been shaped by the effects of meteorite bombardment, igneous activity, and sedimentary—including aqueous—processes. Mars also contains enormous igneous centers—Tharsis and Elysium, with other shield volcanoes in the ancient highlands. In fact, the planet has been volcanically active for nearly all of its 4.5 Gyr history, and crater counts in the Northern Lowlands suggest that may have extended to within the last tens of millions of years. Our knowledge of the composition of the igneous rocks on Mars is informed by over 100 Martian meteorites and the results from landers and orbiters. These show dominantly tholeiitic basaltic compositions derived by melting of a relatively K, Fe-rich mantle compared to that of the Earth. However, recent meteorite and lander results reveal considerable diversity, including more silica-rich and alkaline igneous activity. These show the importance of a range of processes including crystal fractionation, partial melting, and possibly mantle metasomatism and crustal contamination of magmas. The figures and plots of compositional data from meteorites and landers show the range of compositions with comparisons to other planetary basalts (Earth, Moon, Venus). A notable feature of Martian igneous rocks is the apparent absence of amphibole. This is one of the clues that the Martian mantle had a very low water content when compared to that of Earth.The Martian crust, however, has undergone hydrothermal alteration, with impact as an important heat source. This is shown by SNC analyses of secondary minerals and Near Infra-Red analyses from orbit. The associated water may be endogenous.Our view of the Martian crust has changed since Viking landers touched down on the planet in 1976: from one almost entirely dominated by basaltic flows to one where much of the ancient highlands, particularly in ancient craters, is covered by km deep sedimentary deposits that record changing environmental conditions from ancient to recent Mars. The composition of these sediments—including, notably, the MSL Curiosity Rover results—reveal an ancient Mars where physical weathering of basaltic and fractionated igneous source material has dominated over extensive chemical weathering.

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Juxtaposition of diverse, subduction-related tectonic blocks with contrasting metamorphic features and ages in the Paleoproterozoic Aketashitage orogen, NW China: Implications for Precambrian orogeny

Geological Society of America Bulletin ◽

10.1130/b35766.1 ◽

2020 ◽

Author(s):

Qian W.L. Zhang ◽

Jia-Hui Liu ◽

Zhen M.G. Li ◽

Meng-Yan Shi ◽

Yi-Chao Chen ◽

...

Keyword(s):

Mass Spectrometry ◽

Inductively Coupled Plasma ◽

Subduction Zones ◽

Plate Tectonics ◽

Secondary Ion Mass Spectrometry ◽

Orogenic Belt ◽

Early Precambrian ◽

Inductively Coupled ◽

The North ◽

Depositional Age

The comprehensive investigation of orogenic-related litho-structural assemblages, metamorphism, and geochronology in early Precambrian orogens can help us better understand the features of plate tectonics in early Earth. The Paleoproterozoic Aketashitage orogenic belt is located at a key position in northwestern China and connects the North China craton, Tarim craton, Altaids orogen, and Tethys orogen. Garnet-bearing mafic and paragneissic granulite occur as interlayers or blocks preserved within paragneissic matrix, and two to three generations of metamorphic mineral assemblages were identified. Geothermobarometry and pseudosection modeling yielded clockwise metamorphic P-T paths passing from 7.5‒8.6 kbar/575‒715 °C (M1) through 7.4‒12.2 kbar/715‒895 °C (M2) and finally to 5.2‒7.3 kbar/710‒800 °C (M3) for the mafic and paragneissic granulite as well as amphibolite, which is indicative of metamorphic features of subduction/collision zones. Peak metamorphic P-T conditions of all the samples lie in the medium P/T facies series, suggesting that the thermal gradient (∼20‒31 °C/km) of this Paleoproterozoic orogenic belt was obviously higher than most of the Phanerozoic subduction zones. Secondary ion mass spectrometry (SIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of zircon and monazite yielded metamorphic ages of ca. 1.98−1.96 Ga in the eastern part of the orogen, ca. 1.86−1.85 Ga in the western part, and a maximum depositional age of ca. 2.06 Ga for paragneiss. Compared with previous studies, the Aketashitage orogen is composed of unordered juxtaposition of diverse, subduction-related tectono-metamorphic blocks with different protoliths, metamorphic grades, and ages preserved within the paragneissic matrix deposited in the Paleoproterozoic, which is highly similar to Phanerozoic mélange. A Paleoproterozoic subduction-metamorphic-exhumation-accretionary process was deciphered, similar to that found in accretionary/orogenic wedge in Phanerozoic orogens. The juxtaposition of diverse, subduction-related tectonic blocks with contrasting ages and metamorphic features can serve as a marker of early Precambrian orogens and plate tectonics.

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How do high school students teach geosciences to elementary students?

10.5194/egusphere-egu2020-2597 ◽

2020 ◽

Author(s):

Carole Larose

Keyword(s):

High School ◽

High School Students ◽

Young Children ◽

Plate Tectonics ◽

School Teacher ◽

Volcanic Rocks ◽

School Students ◽

Earthquake Resistant ◽

Hands On ◽

Different Types

I am Biology and Geology teacher in a high school and I teach for students between 15 and 18 years old. Geosciences are not very easy to understand because the concepts are complex. I try to interest my students by using different pedagogical materials including hands-on. At the end of the course, to make sure that they have a good understanding, I sometimes organize a meeting between my students and the children of a primary school. It is a way to assess them because if they are able to explain some geological issues to young children, they must before understand them.Before the meeting, the elementary school teacher and I did an educational notebook for young children. We have planned 5 activities on the topic "plate tectonics"<ul><li>Explosive and effusive volcanism : children identify different types of volcanism by watching two short videos</li> <li>Study the volcanic rocks : children observe the rocks and look under a polarizing microscope</li> <li>Earthquake-resistant buildings: children use a model to understand how a building can withstand an earthquake</li> <li>The different kind of faults: children use a model to create different types of faults.</li> <li>Identify the movement of Plate tectonics: children use software to do this exercise</li> </ul>The meeting lasted two hours. It was a great moment for all the students. My student's job was to help the youngest to answer the questions on their notebooks. They had to explain clearly and simply and it was a very interesting exercise for them because they needed knowledge to do it. Young students asked a lot of questions, they were very curious and interested in this topic.Here is an article in French. http://svt.spip.ac-rouen.fr/spip.php?article396&#160;

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Intraplate and petit-spot volcanism originating from hydrous mantle transition zone

10.5194/egusphere-egu2020-5804 ◽

2020 ◽

Author(s):

Jianfeng Yang ◽

Manuele Faccenda

Keyword(s):

Transition Zone ◽

Northeast China ◽

Subduction Zones ◽

Japan Trench ◽

Low Velocity ◽

Iranian Plateau ◽

Mantle Transition Zone ◽

Mantle Minerals ◽

Ocean Ridges ◽

Pacific Slab

Most magmatism occurring on Earth is conventionally attributed to passive mantle upwelling at mid-ocean ridges, slab devolatilization at subduction zones, and mantle plumes. However, the widespread Cenozoic intraplate volcanism in northeast China and the peculiar petit-spot volcanoes offshore the Japan trench cannot be readily associated with any of these mechanisms. Furthermore, the seismic tomography images show remarkable low velocity zones (LVZs) sit above and below the mantle transition zone which are coincidently corresponding to the volcanism. Here we show that most if not all the intraplate/petit-spot volcanism and LVZs present around the Japanese subduction zone can be explained by the Cenozoic interaction of the subducting Pacific slab with a hydrous transition zone. Numerical modelling results indicate that 0.2-0.3 wt.% H2O dissolved in mantle minerals which are driven out from the transition zone in response to subduction and retreat of a stagnant plate is sufficient to reproduce the observations. This suggests that critical amounts of volatiles accumulated in the mantle transition zone due to past subduction episodes and/or delamination of volatile-rich lithosphere could generate abundant dynamics triggered by recent subduction event. This model is probably also applicable to the circum-Mediterranean and Turkish-Iranian Plateau regions characterized by intraplate/petit-spot volcanism and LVZs in the underlying mantle.

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An analytical model concerning the possible initiation of subduction between the India and Africa plates, caused by the Morondova plume

10.5194/egusphere-egu2020-6050 ◽

2020 ◽

Author(s):

Bernhard Steinberger ◽

Douwe van Hinsbergen

Keyword(s):

Subduction Zones ◽

Plate Tectonics ◽

Eastern Mediterranean ◽

Plate Boundary ◽

Plate Motion ◽

Mantle Flow ◽

Plate Motions ◽

Subduction Initiation ◽

Euler Pole ◽

Geodynamic Processes

Identifying the geodynamic processes that trigger the formation of new subduction zones is key to understand what keeps the plate tectonic cycle going, and how plate tectonics once started. Here we discuss the possibility of plume-induced subduction initiation. Previously, our numerical modeling revealed that mantle upwelling and radial push induced by plume rise may trigger plate motion change, and plate divergence as much as 15-20 My prior to LIP eruption. Here we show that, depending on the geometry of plates, the distribution of cratonic keels and where the plume rises, it may also cause a plate rotation around a pole that is located close to the same plate boundary where the plume head impinges: If that occurs near one end of the plate boundary, an Euler pole of the rotation may form along that plate boundary, with extension on one side, and convergence on the other.&#160; This concept is applied to the India-Africa plate boundary and the Morondova plume, which erupted around 90 Ma, but may have influenced plate motions as early as 105-110 Ma. If there is negligible friction, i.e. there is a pre-existing weak plate boundary, we estimate that the total amount of convergence generated in the northern part of the India-Africa plate boundary can exceed 100 km, which is widely thought to be sufficient to initiate forced, self-sustaining subduction. This may especially occur if the India continental craton acts like an &#8220;anchor&#8221; causing a comparatively southern location of the rotation pole of the India plate. Geology and paleomagnetism-based reconstructions of subduction initiation below ophiolites from Pakistan, through Oman, to the eastern Mediterranean reveal that E-W convergence around 105 Ma caused forced subduction initiation, and we tentatively postulate that this is triggered by Morondova plume head rise. Whether the timing of this convergence is appropriate to match observations on subduction initiation as early as 105 Ma depends on the timing of plume head arrival, which may predate eruption of the earliest volcanics. It also depends on whether a plume head already can exert substantial torque on the plate while it is still rising &#8211; for example, if the plate is coupled to the induced mantle flow by a thick craton.

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On the Margins of Science and Civilization: W. K. Gordon's 1895 Geological Reconnaissance in the Trans-Pecos Borderlands

Earth Sciences History ◽

10.17704/eshi.27.1.p456t258862533w9 ◽

2008 ◽

Vol 27 (1) ◽

pp. 113-130

Author(s):

Richard Francaviglia

Keyword(s):

Cross Section ◽

Plate Tectonics ◽

Igneous Rocks ◽

Mining Engineer ◽

Intrinsic Interest ◽

West Texas ◽

Coal Deposits ◽

Geological Cross Section ◽

Field Report ◽

Semi Arid

In 1895, self-trained mining engineer William K. Gordon, Sr (1862-1949) conducted a geological reconnaissance trip to far West Texas in search of coal deposits. A report from that trip reveals how Gordon's training in geology (acquired largely through reading) and his intrinsic interest in stratigraphy and geomorphology helped him effectively advise the Texas and Pacific Coal Company about the bleak prospects there. In 2005, using Gordon's never-before consulted field report, the author retraced, or rather re-hiked, Gordon's route. Gordon's report features hand-drawn maps and a geological cross-section that were field checked and compared to later data. The author concludes that Gordon enthusiastically, but often inaccurately, described the complex petrology in the rugged, semi-arid Eagle Mountains. Gordon was evidently vexed by how to identify some of the highly varied extrusive igneous rocks here. Nevertheless, Gordon's work should be recognized as the earliest serious geological reconnaissance in a remote area that would much later (1963) be studied in detail by geologists who had at their disposal considerably better tools to analyze the petrology, and possessed a growing awareness of plate tectonics that were unknown in Gordon's time.

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