Lecture 2 - Rheology of magmas

Question 1

Controls of viscosity

Answer 1

Temperature, composition and gas content

Question 2

Network formers and breakers

Answer 2

Network formers increase viscosity (primarily SiO2)
Network breakers decrease the viscosity (Fe, Mg, Ca)

Question 3

What determines the viscosity of lava?

Answer 3

Linking of (SiO4)4- tetrahedral
Linkages through the O atoms
Increasing silica increases linkages
Additions of cations (Ca, Na) reduce linkages

Question 4

What does water do to lava?

Answer 4

Breaks up silica by adding an oxygen and a new terminus

Question 5

Rhylote lava compared to basalt

Answer 5

Rhyolite is much more viscous

Question 6

What happens as magma ascends a volcano?

Answer 6

• Decompression - the pressure exerted on it decreases
• Loss of H20 - the drop in pressure allows water that is dissolved to form steaming bubbles
• Viscosity increases - bubbles increase the viscosity

Question 7

How can bubbles can increase or decrease the viscosity of magma?

Answer 7

High viscosity - due to flow around spherical or slightly deformed bubbles
Low viscosity - due to the reduced volume fraction of viscous fluid due to presence of bubbles.

Question 8

How do crystals affect magma viscosity?

Answer 8

Adding solid particles of any shape always increases magma viscosity.
No slip at the crystal boundary.