Magma Rheology Flashcards
What is melt viscosity influenced by? (In terms of silicate networks)
Network formers - increase viscosity
Network breakers - decrease viscosity (e.g. water)
What is the equation for a Newtonian Fluid?
Viscosity = stress / shear rate
For silicate melts
How does shear rate impact bubbles?
Low shear rate - bubbles remain spherical. High viscosity, as there is a resistance to flow.
High shear rate - bubbles deformed. Low viscosity as reduced volume fraction of viscous fluid. No slip at bubble boundary.
If you have a bubbly melt, how do bubbles impact viscosity? (I.e. at walls)
At walls - get high shear, low viscosity.
In middle - get low shear, high viscosity.
This is known as shear thinning viscosity.
What is a bubble free melt known as? How does this impact velocity?
Newtonian fluid. With no slip boundary conditions - means velocity is 0 at the walls.
What a low concentration crystal contents known as?
Dilute crystal melt suspensions
How do low crystal content melts impact viscosity?
Newtonian as long as crystals remain dilute.
Increases viscosity but crystals don’t interact.
How do high crystal melt contents impact viscosity?
Non-Newtonian.
Increases viscosity and crystals interact.
Will have Bingham Rheology.
What’s the equation for Bingham Rheology?
Stress = (plastic viscosity * shear)/ yield strength
What causes a Newtonian fluid to become Non-Newtonian?
Increases crystal content. Greater than 40%.
Basaltic lava flows cooling with time (flow diagram)
- Magma cools with time (crosses liquids)
- Crystallisation occurs
- Melt viscosity increases
- Magma viscosity increases
Pahoehoe lava characteristics?
Thin deformable skin
Erratic advance as lava ruptures through crust
Fewer larger phenocrysts
Low viscosity, low shear
A’a lava characteristics?
Thick crust Steady flow Rubble-like surface High viscosity, high shear Lots of microlites (not much time to grow)
How to turn from pahoehoe to a’a?
Increased crystallisation (increases viscosity) Increase in slope (increased shear)
What does decompression of water saturated magma cause?
Crystallisation
Flow diagram for saturated water decompression
- Pressure reduction as magma rises.
- Water goes to bubbles - increases melt viscosity.
- Increases liquidus temp.
- Crystallisation.
- Magma viscosity increases
What is the main type of lava flow on steep slopes? And characteristics?
Open channel flow.
Flow velocity decreases with distance.
As you get cooling, crystallisation, viscosity increasing.
What is the main type of lave flow on low slopes? Characteristics?
Tube fed flows.
Velocity constant with distance.
As crust insulates lava.
Cools slowly and travels further
What’s the shear stress at the base of flow on inclined slope? For Bingham Rheology
Yield Stress = Density * gravity * thickness (d) * sin(alpha)
Is yield stress as there will only be flow at the base if shear stress is greater than the yield stress.
Newtonian fluid on a slope characteristics?
At base - y = 0, u = 0, shear rate is highest. Have no slip conditions hence why velocity is 0.
Shear rate decreases with thickness upwards. In middle - ŷ > 0, stress > 0
At top - shear rate ŷ = 0, y = d, u > 0, stress = 0
Bingham Rheology on a slope characteristics?
At base - y = 0, u = 0. Shear rate is highest.
Middle 1 - ŷ > 0, stress > yield stress
At Ycrit - stress is equal to yield stress.
Middle 2 - above Ycrit, stress is less than yield stress so shear rate is 0. Means this section is moving at a constant velocity. No shearing. Ŷ =0, stress < yield stress.
Top - u > 0, y = d
