Lecture 2 Rheology

Question 1

What is viscosity of magma?

Answer 1

• viscosity is a fluid’s internal resistance to flow
• A measure of the consistency of a substance
  logarithmic scale - much more aggressive towards the RHS

Question 2

Classification by silica content (trend for named silica melts)

Answer 2

increased silica = mire viscous
less iron and magnesium and decrease temperature
least Si02
Basalt 42-52 (also hottest)
Andesite 52-63
Dacite 63-68
Rhyolite more than 68 = silica rich (coldest)

Question 3

why is there a viscosity trend for silica and iron and magnesium?

Answer 3

silica = (SiO4)4-, si is linked to all 0 atoms making it most viscous, the more silica the more linkages.
- positive cations interact with negative OH- anions to break apart the bonds.
adding cations like iron, magnesium, calcium reduces these linkages.

Question 4

what does temperature do to viscosity?

Answer 4

lower temperatures = more linkages (bonds) = more viscous.

Question 5

chemical composition of the magma os a fundamental control on…

Answer 5

its viscosity and its explosivity

Question 6

What happens as magma physically ascends?

Answer 6

there is decompression and loss of water (H20) into the bubbles and viscosity increases.

Question 7

bubble shape related to viscosity

Answer 7

Relatively high viscosity - due to flow around spherical or slightly deformed bubbles.
(round shape)

Relatively low viscosities from reduced volume fraction of viscous fluid due to presence of bubbles
free slip surfaces
(thin bubble)

Question 8

gas content and explosivity

Answer 8

high viscosity inhibits escape of expanding bubbles leading to pressure build up while magma is ascending.

Question 9

crystals and viscosity

Answer 9

whenever solid is added it increases magma viscosity.
different from bubbles as there is no slip at the crystal boundary.
more crystals always = higher viscosity.

Question 10

what’s the link between basalt surface morphology and crystal content?

Answer 10

pahoehoe = less crystals and a a = more crystal content

Question 11

pressure and viscosity

Answer 11

viscosity decreases when pressure increases

Question 12

Temperature and viscosity

Answer 12

• viscosity increases significantly on cooling
  (partly due to crystallization)

Question 13

Volatile Content and viscosity

Answer 13

• viscosity lower with increasing volatile content
• water depolymerizes melt by breaking Si–O–Si bond

Question 14

Crystal Content and viscosity

Answer 14

• suspended crystals increase the bulk viscosity

Question 15

Bubble Content and viscosity

Answer 15

• basalts - little effect
• rhyolite - big effect