Effect of drainage density on the hydrograph

Crystallography Rotoinversion Projection

How Earth is retaining its magnetic field?

Image Source: Today I Found Out

First of all, the Earth shows it's magnetic field because of it's internal geodynamic nature. And it is believed that Earth's outer core is responsible for this kind of behavior. Unlike the mineral-rich crust and mantle, the core is made almost entirely of metals. Along with iron (Fe), silicon (Si) is also found at Earth’s core whose thermal conductivity has an impact on Earth’s thermal structure, evolution, and dynamics, as it controls the magnitude of thermal and compositional sources required to sustain the internal geodynamo of Earth. Scientists have directly measured the thermal conductivity of both solid Fe and Fe-Si alloys up to 144 GPa and 3300 K. 15 at% Si alloyed in Fe significantly reduces its conductivity by almost 2 folds at 132GPa and 3000 K. At similar pressure-temperature conditions, an outer core with 15 at% Si would have a conductivity of about 20 W m^-1 K^-1, which is lower than pure Fe. This suggests that a lower minimum heat flow, around 3TW, across the core-mantle boundary than previously expected, and thus less thermal energy needed to operate the geodynamo.

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Earth's Plate Tectonics Began Over 3.2 Billion Years Ago

Paleomagnetic evidence for modern-like plate motion velocities at 3.2 Ga

by  Alec R. Brenner et al.

An artistic cross-section through forming Earth's crust approximately 3-4 billion years ago. Image credit: Alec R. Brenner

Plate tectonics has been the dominant surface geodynamical regime throughout Earth’s recent geological history. One defining feature of modern plate tectonics is the differential horizontal motion of rigid lithospheric plates. The physiography and composition of Earth’s modern crust bear evidence for plate tectonic or “mobile-lid” processes including subduction, collisional orogeny, rifting, and ocean spreading. The case for the Archean Earth [4.0 to 2.5 billion years (Ga) ago] is not so clear. The surviving Archean crust consists of ~35 cratons, most with characteristic architecture of rounded granitoid intrusive domes rimmed by steeply dipping greenstone keels. The composition of extant Archean crust is substantially more mafic than modern oceanic crust, with a high fraction of ultramafic rocks such as komatiites. These structural and compositional differences have led to a number of proposals that the Archean crust was constructed by exotic processes, including plume tectonics, sagduction/drip tectonics, and a vertically overturning lithosphere. Since some of these processes are difficult to reconcile with plate mobility, alternative geodynamical regimes have been proposed for the Archean Earth, including stagnant-lid and sluggish-lid modes in which the lithosphere was rendered immobile, or at least slowed, due to decoupling from the asthenosphere under elevated geothermal gradients. Other studies argue for a uniformitarian model of the Archean Earth, in which some variant of modern plate tectonics was in operation at least locally throughout Earth history. Complete understanding of the Archean lithosphere, hydrosphere, atmosphere, and biosphere are predicated upon distinguishing between these proposed Archean geodynamic modes. Insights into these components of the early Earth are foundational to the inner workings of terrestrial planets generally and what surface conditions and environments hosted the development of the first life.

Arguments for alternative geodynamical regimes in the Archean are often based on inferences of a regime transition toward modern-style plate tectonics. Existing estimates for the age of such a transition range from the Neoproterozoic to the Hadean (see the Supplementary Materials) and invoke a range of observations including global and local geochemical records, field relations of possible syn-tectonic rocks, and paleomagnetic pole comparisons.  

A key discriminant between stagnant- and mobile-lid regimes is the rate of horizontal motion of plates over Earth’s surface. Absolute plate velocities have typically been ~2 to 10 cm/year (extremes from 0 to 25 cm/year) over the last 400 million years (Ma), while hypothesized velocities for stagnant- and sluggish-lid models are typically less than 2 cm/year. Paleomagnetic methods may constrain the velocity of crustal blocks in deep geological time by measuring their apparent polar wander histories. However, robust paleomagnetic evidence for latitudinal motion has been lacking thus far for times before 2.8 Ga. Here, we produce a new paleomagnetic pole from ~3180 Ma volcanics in the East Pilbara Craton of Western Australia and use this result to assess the presence of plate tectonic– like processes on Earth before that time.  

Original article: Paleomagnetic evidence for modern-like plate motion velocities at 3.2 Ga. DOI: 10.1126/sciadv.aaz8670. To read the article click here.