Synopsis Lavas of the Lorne Plateau Volcanic Succession (LPVS) crop out on the island of Kerrera, NW Scotland, and were emplaced during the Devonian Period, synchronous with Old Red Sandstone (ORS) terrestrial sedimentation. These flows are discontinuously exposed for c . 3 km at Port a’ Chroinn, Kerrera, and typically comprise blocky, basal breccias, coherent, crystalline cores, and clinker flow tops, interpreted as basaltic andesite ’a’a lavas. Gradational zones are observed from coherent core, through highly vesicular core, to semi-coherent lava and normally graded clinker. Prismatically jointed, inflated pahoehoe lavas are also present, but have few characteristic morphological features. Localized domains of magma–sediment mingling occur at the bases of several lavas, with clasts of basaltic andesite within a sandstone matrix. These domains are interpreted as peperites, formed as lava flowed over and mingled with unconsolidated, wet sediment that had been deposited on the surfaces of underlying lavas during periods of volcanic quiescence.
This study is concerned with a fully nonlinear theoretical treatment of internal solitary waves in continuously stratified, incompressible, inviscid, shear-free Boussinesq fluids. Results are presented for wave propagation in both deep and shallow fluids with four different ambient stability profiles. Only the dominant mode with the greatest wave speed is considered. The morphology of finite-amplitude internal solitary waves in Boussinesq fluids is shown to be very sensitive to the precise form of the stability profile. The calculations indicate that a wave of maximum amplitude, which is less than the total fluid depth, exists for all internal solitary waves in continuously stratified Boussinesq fluids of finite depth. There is apparently no upper limit on the amplitude of internal solitary waves in many physically realistic unbounded fluids. Large amplitude waves of this type are mutually similar in form and the morphology of these waves appears to be independent of the ambient stability profile in the waveguide layer. It is shown that the properties of highly nonlinear waves with recirculating flow depend on the density distribution and vorticity of the trapped fluid inside the closed circulation cell. Fluid velocity components associated with the wave motion are evaluated and used to calculate the surface perturbation pressure. The surface perturbation pressure signature for internal solitary waves is found to change with the onset of recirculation from a single-crested profile at small wave amplitudes to a bimodal profile at large wave amplitudes. Results for solitary waves in finite-depth fluids differ from those found for deep fluids in that the surface perturbation pressure at the center of the wave eventually changes sign as wave amplitude increases.
Current good practice in geology is to identify volcaniclastic rocks using a descriptive classification scheme. The terms agglomerate, volcanic breccia and tuff are currently used in both a descriptive and genetic sense, in BS 5930 (1999) . However, the definitions used are now considered incorrect in both senses. This technical contribution redefines these terms and places them in a modern geological context. A new descriptive classification scheme for identifying and naming volcaniclastic rocks is proposed that will assist engineering geological descriptions of these rock types.
A variety of field studies, laboratory experiments and
modelling investigations have been undertaken to determine
the processes of ash aggregation during eruptions; however,
there is a paucity of data on the chemistry of the aggregates
and the role of eruption chemistry in their formation. Here
we present electron microprobe analyses of accretionary
lapilli (ash aggregates with a massive pellet at their core and
a cortex of multiple concentric fine ash laminae) collected
from a variety of pyroclastic density current deposits (Scafell
Caldera, English Lake District; Kilchrist Caldera, Isle of
Skye, NW Scotland; Poris Formation, Tenerife) These rocks
span the absolute time period ~460 Ma-273 ka and formed in
a variety of tectonic and environmental settings. In all cases
there are significant geochemical variations between cores
and laminae of individual accretionary lapilli and the matrix
of the tuffs/lapilli-tuffs in which they are found. We suggest
that ash pellets form in co-ignimbrite plumes and on
reaching a critical density fall under gravity into the
underlying turbulent pyroclastic density current. Here the
pellets accrete laminae that reflect the chemistry of the pulse
of the density current surrounding it at that time, and
therefore, the laminae of the growing accretionary lapilli
progressively record chemical flux during the eruption.
Complex chemical zoning of the laminae may record
repeated lofting and settling of the growing lapilli into
chemically diverse parts of the density current. The
accretionary lapilli therefore behave as “time capsules” that
provide a high-resolution record of the chemical evolution of
the eruption. These subtle changes may not be recorded by
the density current deposits themselves due to the effects of
current bypassing and/or erosion.
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