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Geophysical Flows

 Dynamics of ice sheets and ice shelves

Grounding line of golden syrup floating on less viscous solution

Credit: Rosie Robison

A viscous fluid (golden syrup) is introduced at the right-hand side and flows to the left, down the rigid slope and thence into a layer of denser, much less viscous fluid (a solution of potassium carbonate).  This laboratory-scale experiment mimics the flow of a glacial, marine ice sheet, which flows over continental bedrock towards the ocean, on which it floats to form an ice sheet.  We are studying the dynamical controls on the grounding line, where the viscous ice sheet first lifts away from the bedrock. See: Robison, Huppert & Worster (2010)Kowal, Pegler & Worster (2016).

 

Xanthan gum spreading on dense fluid

Credit: Roiy Sayag 

A shear-thinning fluid, Xanthan gum, is poured from a central, vertical pipe onto a shallow horizontal layer of denser fluid, seen here in plan view from below.  The Xanthan spreads axi-symmetrically while in contact with the rigid base of the fluid layer but breaks up once it begins to float on the fluid layer, where it experiences less stress and therefore becomes stiffer.  This fracturing is similar to that seen in ice shelves when they calve to form ice bergs. See: Sayag, Pegler & Worster (2012); Worster (2014).


 Coastal flow interaction with vegetation

Waves and marshes

Credit: James Tempest

Waves and vegetation

Credit: James Tempest

 

The action of sea water flows at salt marsh margins is crucial in determining the persistence and evolution of these important coastal landforms. The value of salt marshes has become more and more recognised, both in terms of their important role in providing diverse habitats for all kinds of species as well as providing important ecosystem services, including that of flood and erosion protection of their hinterland. The Cambridge Coastal Research Unit studies the hydrodynamic forces that such cliffs are exposed to and the sedimentary and biological responses to such forcing.

Top: Mid-tide with high wave energy at the marsh margin in Orplands, Essex, UK

Bottom: Rising tide with waves over the marsh margin at Tillingham, Dengie Peninsula, Essex, UK.

 Plant wave swirl

Credit: Iris Möller

The action of waves on submerged vegetation in intertidal zones can cause enhanced erosion where plants grow as individuals rather than in the form of a dense canopy. Understanding the interaction between fluid flows and flexible plant structures as well as the sediment surface on which they grow is critical in informing coastal management approaches that have to consider sediment budgets and longer term evolution of coastal wetlands for flood and erosion protection.

Above: Individual Salicornia seedling on the eroding marsh fringe at Tillingham, Essex, UK.

 


 Scree slope dynamics

Scree slope simulation mid-way through animation

Credit: Mike Bithell

Discrete element modelling of the collapse of a cliff to form a scree slope. Click on the image to see an animated gif of the simulation.

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Magma convection

Basalt with settled olivine grains - plane polarisation            Basalt with settled olivine grains - crossed polars

Basalt with convecting olivine grains - plane polarisation            Basalt with convecting olivine grains - crossed polars

Credit: Marian Holness

The detailed grain structure of basalt from an exposed horizontal tabular intrusion in the Shiant Isles (Outer Hebrides) has been used to infer the existence of convection in the magma. Images show microscopic view of basalt, using plane polarised light (left) and crossed polars (right). Field of view is 4.5mm tall.

Top row:  Olivine grains have been concentrated by gravitational settling. In the left-hand image, note the abundant rounded grains of olivine (colourless) set in a mixture of pyroxene (pale brown) and plagioclase (colourless). The olivine grains form loose clusters and poorly sintered chains with a wide range of grain sizes. This is essentially the crystal load that was brought into the intrusion and settled rapidly to the intrusion floor.

Bottom row: Slightly higher up in the same intrusion the olivine is now much coarser grained and forms extensive clusters of well-sintered grains. It takes time to accumulate grains into a cluster and to sinter the grain boundaries effectively. This can only be achieved if the olivine grains were suspended in a convecting magma, which slowed settling considerably.

 

Fieldwork on Mull - rock            Fieldwork on Mull - collecting samples

Credit: Marian Holness

Fieldwork on the Isle of Mull, collecting samples from vertical, cracks filled with now-solidified basalt. The work demonstrates that the microstructure of the basalt in these steep-sided bodies is very different to that of similar thickness horizontal tabular intrusions, consistent with crystallisation occurring predominantly in a strongly convecting magma.