SUMMARY / RELATED TOPICS

Pressure ridge (ice)

A pressure ridge develops in an ice cover as a result of a stress regime established within the plane of the ice. Within sea ice expanses, pressure ridges originate from the interaction between floes, as they collide with each other. Currents and winds are the main driving forces, but the latter are effective when they have a predominant direction. Pressure ridges are made up of angular ice blocks of various sizes; the part of the ridge, above the water surface is known as the sail. Pressure ridges are the thickest sea ice features and account for about one-half of the total sea ice volume. Stamukhi are pressure ridges that are grounded and that result from the interaction between fast ice and the drifting pack ice; the blocks making up pressure ridges are from the thinner ice floe involved in the interaction, but it can include pieces from the other floe if it is not too thick. In the summer, the ridge can undergo a significant amount of weathering, which turns it into a smooth hill. During this process, the ice loses its salinity.

This is known as an aged ridge. A consolidated ridge is; the term consolidated layer is used to designate freezing up of the rubble just below the water line. The existence of a consolidated layer depends on air temperature — in this layer, the water between individual blocks is frozen, with a resulting reduction in porosity and an increase in mechanical strength. A keel's depth of an ice ridge is much higher than its sail's height - about four times; the keel is 2-3 times wider than the sail. One of the largest pressure ridges on record had a sail extending 12 metres above the water surface, a keel depth of 45 metres; the total thickness for a multiyear ridge was reported to be 40 metres. On average, total thickness ranges between 5 metres and 30 metres, with a mean sail height that remains below 2 metres; the physical characterization of pressure ridges can be done using the following methods: Mechanical drilling, whereby augers designed for ice are driven through the ridge, the core retrieved for analysis.

Surveying, whereby a level, theodolite or a differential GPS system is used to determine sail geometry. Thermal drilling – drilling involving melting of the ice. Observation of the ice canopy by scuba divers. Upward looking sonars. A series of thermistors. Electromagnetic induction, from the ice surface or from an aircraft. From an offshore engineering and naval perspective, there are three reasons why pressure ridges are a subject of investigation. Firstly, because the highest loads applied on offshore structures operating in cold oceans by drift ice are associated with these features. Secondly, when pressure ridges drift into shallower areas, their keel may come into contact with the seabed, thereby representing a risk for subsea pipelines and other seabed installations. Thirdly, they have a significant impact on navigation. In the Arctic, ridged ice makes up about 40% of the overall mass of sea ice. Drift ice Finger rafting Iceberg Ice volcano Offshore geotechnical engineering Sea ice Seabed gouging by ice Stamukha Submarine pipeline

Andrew D. Huxley

Andrew D. Huxley is a chair of the physics department of the University of Edinburgh. Of no relation to Sir Andrew F. Huxley, Huxley is known in the field of condensed matter physics. While at the CEA laboratory in Grenoble, Huxley was involved in the revolutionary discovery of superconductivity in the ferromagnet UGe2 under applied pressure, in collaboration with a team at the University of Cambridge; this was followed up by a series of breakthroughs in another ferromagnetic material, URhGe, found to turn superconducting under the application of an external magnetic field. This emergence of an unconventional superconducting state by the application of an external tuning parameter such as magnetic field or pressure is hypothesised to be related to a'Quantum critical point' - a special phase transition that occurs at temperatures approaching zero kelvins. Quantum fluctuations are enhanced at the QCP, destabilising the conventional phase that dominates under ambient conditions, making conditions propitious for the emergence of a novel unconventional phase such as superconductivity, or even more exotic states.

Huxley graduated with a BA from Churchill College, Cambridge, an MS from the University of Pennsylvania, a PhD from the University of Cambridge. He was subsequently a postdoctoral fellow and a scientist at CEA, Grenoble before joining the University of Edinburgh as a Professor of Physics in 2006. Huxley is an alumnae of the Quantum Matter Group of the Cavendish Laboratory, University of Cambridge that have gone on to become leading physicists. 1. Realignment of the flux-line lattice by a change in the symmetry of superconductivity in UPt3. Andrew Huxley, Pierre Rodière, Donald McK. Paul, Niels van Dijk, Robert Cubitt and Jacques Flouquet. Nature 406, pp. 160 – 164 2. Superconductivity on the border of itinerant-electron ferromagnetism in UGe2. S. S. Saxena, P. Agarwal, K. Ahilan, F. M. Grosche, R. K. W. Haselwimmer, M. J. Steiner, E. Pugh, I. R. Walker, S. R. Julian, P. Monthoux, G. G. Lonzarich, A. Huxley, I. Sheikin, D. Braithwaite and J. Flouquet. Nature 406, pp. 587 – 592 3. Coexistence of superconductivity and ferromagnetism in URhGe.

Dai Aoki, Andrew Huxley, Eric Ressouche, Daniel Braithwaite, Jacques Flouquet, Jean-Pascal Brison, Elsa Lhotel and Carley Paulsen. Nature 413, pp. 613 – 616 4. Magnetic Field-Induced Superconductivity in the Ferromagnet URhGe. F. Lévy, I. Sheikin, B. Grenier and A. D. Huxley. Science 309, pp. 1343–1346 5. Acute enhancement of the upper critical field for superconductivity approaching a quantum critical point in URhGe. F. Lévy, I. Sheikin and A. Huxley. Nature Physics 3, pp. 460 – 463 University of Edinburgh School of Physics and Astronomy

Ventarura

Ventarura is a genus of extinct vascular plants of the Early Devonian. Fossils were found in the Windyfield chert, Scotland; some features, such as bivalved sporangia borne laterally and the anatomy of the xylem, relate this genus to the zosterophylls. Other features are unclear due to poor preservation. Fossils of Ventarura were found in the Windyfield chert separate from the better-known Rhynie chert, both located near the village of Rhynie, Scotland. Only fragmentary fossils were found, the longest being around 12 cm long. Stems of two kinds were found, although without clear connections between them. Aerial stems were leafless and dichotomously branched, they contained. Alone among the Rhynie chert plants, there was evidence of sclerenchyma – supporting tissue made up of dead cells with thick cell walls. Spore-forming organs or sporangia were borne on the sides of the stems, attached without clear stalks, they consisted of two circular to pear-shaped'valves', one narrower facing the stem, one away from it.

Sporangia may have formed strobilus. Spores were shed via a slit at the top of the sporangium between thickened valve borders. Spores were about 67 µm in diameter, without trilete marks. What are thought to be the underground stems of Ventarura had single-celled hairs rhizoids, on all sides, they branched irregularly and more compared to aerial stems. Ventarura was described too late to be included in the cladistic studies published by Kenrick and Crane in 1997 which are the source of much of the information on the phylogeny of early land plants; the shape of the sporangia and their lateral position as well as the anatomy of the xylem show a clear relationship to the zosterophylls. However, the arrangement of the sporangia is difficult to determine from the fossil specimens. Zosterophylls are thought to be the earliest diverging group of lycophytes, a group which includes modern clubmosses and relatives