Types of Fluids

How a fluid or a sediment-water mixture behaves when a force is applied to it depends on its rheology - specifically, on the relationship between the shear stress applied to the material and the rate at which it deforms in response. ^[1] This relationship is not the same for all materials, and the differences are geologically consequential: whether a sediment-water mixture flows like water, flows sluggishly like wet concrete, or refuses to move at all until a critical force threshold is crossed determines how far it will travel, what kind of deposit it will leave, and whether it will persist on a gentle slope or arrest immediately.

Newtonian Fluids

The simplest fluid type is a Newtonian fluid - one that has no strength and whose viscosity remains constant regardless of how fast it is being sheared or stirred. ^[1] Ordinary water is the canonical geological example: whether water is barely trickling or rushing through a turbulent channel, its viscosity stays the same. ^[1] On a shear-stress vs. deformation-rate graph, a Newtonian fluid plots as a straight line passing through the origin - flow begins the instant any force is applied, and doubling the force doubles the rate of deformation proportionally.

Non-Newtonian Fluids

Non-Newtonian fluids also lack yield strength - they will begin to flow under any applied stress - but their viscosity is not constant: it changes as the shear rate changes. ^[1] Geologically important examples include water carrying dispersions of sand at concentrations greater than roughly 30% by volume, or even lower concentrations of cohesive clay. ^[1] This means that highly water-saturated, non-compacted muds - freshly deposited seafloor muds, for example - can display non-Newtonian behaviour, moving sluggishly at low flow velocities but becoming noticeably less viscous at higher velocities. ^[1] The variable viscosity reflects the way high sediment concentrations or clay particles interact with and resist shear in a way that changes as the intensity of shearing changes.

Bingham Plastics

Some extremely concentrated sediment dispersions do not behave like fluids at all in the ordinary sense - they behave as plastic solids until a critical stress is applied. ^[1] A Bingham plastic is the specific case where, once that yield strength is exceeded, the material flows with a constant viscosity - it deforms at a rate proportional to the excess stress above the yield point. ^[1] Debris flows are the prime geological example: large cobbles and boulders can be suspended in a matrix of interstitial fluid and fine sediment, and the whole mass will not move until the downslope gravitational stress exceeds the yield strength of that matrix. ^[1] This yield-strength behaviour is why a debris flow can halt abruptly mid-slope the moment gravity can no longer overcome the matrix strength, leaving a lobate deposit with steep, unsupported margins.

Pseudoplastics

Pseudoplastics differ from Bingham plastics in that their viscosity continues to change after flow has begun, rather than becoming constant. ^[1] Water with dispersed sediment and ice are both cited as geological pseudoplastics. Like Bingham plastics, they require a yield stress to be exceeded before flow begins, but unlike Bingham plastics their resistance to flow continues to evolve with the rate of shearing once movement is underway.

Thixotropic Substances

Thixotropic substances are a special category of pseudoplastic with a particularly important geological consequence: they possess strength at rest, but shearing destroys that strength, after which the material flows as a non-Newtonian fluid until it is allowed to rest and gradually rebuilds its strength. ^[1] Freshly deposited muds commonly display thixotropic behaviour, which has a directly important geological outcome: an earthquake that sends shear waves through a deposit of recently laid-down, water-saturated mud can momentarily destroy the mud’s internal strength, causing it to liquefy and flow downslope even though, without that disturbance, the deposit would have remained stable indefinitely. ^[1] Thixotropy thus links seismic activity directly to sediment remobilisation on continental slopes - a mechanism that may have been responsible for many submarine landslide events throughout geological time.

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