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Dickman argues that our model of the Mf ocean tide is unrealistic and that this invalidates conclusions regarding mantle anelasticity. His evidence is based on comparison with his own ocean modelling experiments and on his physical intuition regarding near-equilibrium tides. That evidence is unconvincing. Simple physical arguments alone are enough to rebut his main points. Furthermore, we test here his suggestion of increasing bottom-friction dissipation by a factor of 100 over our preferred solution, and we show that this results in unacceptable polar motion and poor agreement with independent tide-gauge data. In contrast, tests against independent data lend support to the realism of our Mf tide model and give confidence that our estimates of mantle anelasticity are reliable within stated error bounds. [ABSTRACT FROM AUTHOR]

The permeability of the upper mantle controls melt segregation beneath spreading centers. Reconciling contradictory geochemical and geophysical observations at ocean ridges requires a better understanding of transport properties in partially molten rocks. Using x-ray synchrotron microtomography, we obtained three-dimensional data on melt distribution for mantle peridotite with various melt fractions. At melt fractions as low as 0.02, triple junctions along grain edges dominated the melt network; there was no evidence of an abrupt change in the fundamental character of melt extraction as melt fraction increased to 0.2. The porosity of the partially molten region beneath ocean ridges is therefore controlled by a balance between viscous compaction and melting rate, not by a change in melt topology. [ABSTRACT FROM AUTHOR]

Cyclic sedimentary deposits link stratigraphic sequences that are now geographically distant but were once part of the same depositional environment. Some of these sequences occur at periods of 2 to 20 million years, and eustatic sea-level variations or regional tectonic events are likely causes of their formation. Using numerical modeling, we demonstrate that small-scale mantle convection can also cause the development of stratigraphic sequences through recurrent local and regional vertical surface movements. Small-scale convection-driven stratigraphic sequences occur at periods of 2 to 20 million years and correlate only at distances up to a few hundred kilometers. These results suggest that previous sequence stratigraphic analyses may contain erroneous conclusions regarding eustatic sea-level variations. [ABSTRACT FROM AUTHOR]

Bends in volcanic hotspot lineaments, best represented by the large elbow in the Hawaiian- Emperor chain, were thought to directly record changes in plate motion. Several lines of geophysical inquiry now suggest that a change in the locus of upwelling in the mantle induced by mantle dynamics causes bends in hotspot tracks. Inverse modeling suggests that although deep flow near the core-mantle boundary may have played a role in the Hawaiian-Emperor bend, capture of a plume by a ridge, followed by changes in sub-Pacific mantle flow, can better explain the observations. Thus, hotspot tracks can reveal patterns of past mantle circulation. [ABSTRACT FROM AUTHOR]

The article discusses the use of precision isotope analysis to reveal details about the dynamic processes involved in the formation of planet Earth's mantle. Topics include a belief among geologists that evidence of the first few hundred million years of Earth's history had been erased by mantle convection and plate tectonics, a study within the issue regarding geophysics research by M. Touboul and colleagues reporting measurements of the isotopic composition of tungsten (W) in 2.8-billion-year-old basaltic rock, called komatiite, which is known to be derived from source regions deep within Earth's mantle, and the abundance of an tungsten isotope 182W, which forms from the decay of an isotope of hafnium (182Hf).

The article discusses a report within the issue regarding geophysics research investigating global plate tectonics. A brief overview of how convention within planet Earth's mantle affects geological activity, such as tectonic motions on the surface of Earth, is presented. Topics include an overview of plate tectonics and the team's creation of a numerical model, which uses adaptive mesh refinement algorithms on parallel computers to observe global mantle flows through simulation methods, and the behavioral influence of the oceanic lithosphere on plate driving forces.