Magma Rheology

Question 1

What is melt viscosity influenced by? (In terms of silicate networks)

Answer 1

Network formers - increase viscosity

Network breakers - decrease viscosity (e.g. water)

Question 2

What is the equation for a Newtonian Fluid?

Answer 2

Viscosity = stress / shear rate

For silicate melts

Question 3

How does shear rate impact bubbles?

Answer 3

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.

Question 4

If you have a bubbly melt, how do bubbles impact viscosity? (I.e. at walls)

Answer 4

At walls - get high shear, low viscosity.
In middle - get low shear, high viscosity.

This is known as shear thinning viscosity.

Question 5

What is a bubble free melt known as? How does this impact velocity?

Answer 5

Newtonian fluid. With no slip boundary conditions - means velocity is 0 at the walls.

Question 6

What a low concentration crystal contents known as?

Answer 6

Dilute crystal melt suspensions

Question 7

How do low crystal content melts impact viscosity?

Answer 7

Newtonian as long as crystals remain dilute.

Increases viscosity but crystals don’t interact.

Question 8

How do high crystal melt contents impact viscosity?

Answer 8

Non-Newtonian.
Increases viscosity and crystals interact.
Will have Bingham Rheology.

Question 9

What’s the equation for Bingham Rheology?

Answer 9

Stress = (plastic viscosity * shear)/ yield strength

Question 10

What causes a Newtonian fluid to become Non-Newtonian?

Answer 10

Increases crystal content. Greater than 40%.

Question 11

Basaltic lava flows cooling with time (flow diagram)

Answer 11

1. Magma cools with time (crosses liquids)
2. Crystallisation occurs
3. Melt viscosity increases
4. Magma viscosity increases

Question 12

Pahoehoe lava characteristics?

Answer 12

Thin deformable skin
Erratic advance as lava ruptures through crust
Fewer larger phenocrysts
Low viscosity, low shear

Question 13

A’a lava characteristics?

Answer 13

Thick crust 
Steady flow 
Rubble-like surface 
High viscosity, high shear 
Lots of microlites (not much time to grow)

Question 14

How to turn from pahoehoe to a’a?

Answer 14

Increased crystallisation (increases viscosity) 
Increase in slope (increased shear)

Question 15

What does decompression of water saturated magma cause?

Answer 15

Crystallisation

Question 16

Flow diagram for saturated water decompression

Answer 16

1. Pressure reduction as magma rises.
2. Water goes to bubbles - increases melt viscosity.
3. Increases liquidus temp.
4. Crystallisation.
5. Magma viscosity increases

Question 17

What is the main type of lava flow on steep slopes? And characteristics?

Answer 17

Open channel flow.
Flow velocity decreases with distance.
As you get cooling, crystallisation, viscosity increasing.

Question 18

What is the main type of lave flow on low slopes? Characteristics?

Answer 18

Tube fed flows.
Velocity constant with distance.
As crust insulates lava.
Cools slowly and travels further

Question 19

What’s the shear stress at the base of flow on inclined slope? For Bingham Rheology

Answer 19

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.

Question 20

Newtonian fluid on a slope characteristics?

Answer 20

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

Question 21

Bingham Rheology on a slope characteristics?

Answer 21

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