University of Liverpool

About: University of Liverpool is a education organization based out in Liverpool, United Kingdom. It is known for research contribution in the topics: Population & Context (language use). The organization has 40406 authors who have published 94388 publications receiving 3188970 citations. The organization is also known as: Liverpool University & The University of Liverpool.

A reappraisal of ignimbrite emplacement: progressive aggradation and changes from particulate to non-particulate flow during emplacement of high-grade ignimbrite

Abstract: We propose a mechanism by which massive ignimbrite and layered ignimbrite sequences — the latter liable to have been previously interpreted as multiple flow units-form by progressive aggradation during sustained passage of a single particulate flow. In the case of high-temperature eruptive products the mechanism simplifies interpretation of problematic deposits that exhibit pronounced vertical and lateral variations in texture, including between non-welded, eutaxitic, rheomorphic (lineated) and lava-like. Agglutination can occur within the basal part of a hot density-stratified flow. During initial incursion of the flow, agglutinate chills and freezes against the ground. During sustained passage of the flow, agglutination continues so that the non-particulate (agglutinate) layer thickens (aggrades) and becomes mobile, susceptible to both gravity-induced motion and traction-shear imparted by the overriding particulate part of the flow. The particulate to non-particulate (P-NP) transition occurs in and just beneath a depositional boundary layer, where disruptive collisions of hot viscous droplets give way, via sticky grain interactions, to fluidal behavior following adhesion. Because they have different rheologies, the particulate and non-particulate flow components travel at different velocities and respond to topography in different ways. This may cause detachment and formation of two independent flows. The P-NP transition is controlled by factors that influence the rheological properties of individual erupted particles (strain rate, temperature, and composition including volatiles), by cooling and volatile exsolution during transport, and by the particle-size population and concentration characteristics of the depositional boundary layer. At any one location along the flow path one or more of these can change through time (unsteady flow). Thus the P-NP transition can develop momentarily or repeatedly during the passage of an unsteady flow, or it can occur continuously during the passage of a quasi-steady flow supplied by a sustained explosive eruption. Vertical facies successions developed in the deposit (high-grade ignimbrite) reflect temporal changes in flow steadiness and in material supplied at source. The P-NP transition is also influenced by factors that affect flow behaviour, such as topography. It may occur at any location laterally between a proximal site of deflation (e.g. a fountain-fed lava) and a flow's distal limit, but it most commonly occurs throughout a considerable length of the flow path. Up-sequence variations in welding-deformation fabric (between oblate uniaxial to triaxial and prolate) reflect evolving characteristics of the depositional boundary layer (e.g. fluctuations from direct suspension-sedimentation to deposition via traction carpets or traction plugs), as well as possible modifications resulting from subsequent, post-depositional hot loading and slumping. Similar processes can also account for lateral lithofacies gradations in conduits and vents filled with welded tuff. Our consideration of high-grade ignimbrites has implications for ignimbrite emplacement in general, and draws attention to the limitations of the widely accepted models of emplacement involving mainly high-concentration non-turbulent transport and en masse ‘freezing’ of high-yield-strength plug flows.

The numerous modified nucleotides in eukaryotic ribosomal RNA.

Abstract: Publisher Summary This chapter discusses the currently available information on the modified nucleotides, especially in man, the vertebrates and in yeast. Information on the modified nucleotides cannot be obtained simply by sequencing rDNA. Instead, a number of structural and kinetic techniques have been applied to the direct analysis of the modified nucleotides in rRNAs and ribosomal precursor RNAs. The most detailed knowledge has come from the studies on vertebrates including man, and from the yeast, Saccharomyces. Almost all of the modified nucleotides that have been located are in the conserved structural core of rRNA. This brings the modified nucleotides into the central arena of rRNA structure and function. There is no simple theme of primary or secondary structure among the modification sites. The diversity of modification sites is discussed in the chapter, and this diversity raises challenging questions concerning the molecular recognition processes that bring about the modifications. Further insight into these processes may come from the long established finding that most modifications occur very rapidly upon ribosomal precursor RNA. A related finding, also long established, is that inhibition of methylation of precursors of rRNA is associated with failure of ribosome maturation. However, the precise roles of the modified nucleotides in the maturation and working life of the ribosome remain unknown.