Fall 2000 ASTER meeting

1
Physical Properties of Magmas
2
Physical Properties of Magmas

• Melt
• three phase system (liquid, solids (crystals,
  lithics), gas, ie volatiles)
• not one single melting temperature or
  crystallization temperature
• All components affects the viscosity
• All components affects the explosivity
• polymerization grouping/linking together of
  molecules to form chains in melt (SiO4 -- silica
  tetrahedra linked at the OO atoms)
• increase polymerization, increase viscosity
• reduce viscosity by depolymerizing the molecules
• addition of H20 etc reduces viscosity

3
Physical Properties of Magmas

• Melt
• melting a multi-component solid
• 3 factors influence the melting (including T of
  course)
• composition
• volatile content
• pressure
• increase P, increase Tmelt, at a given
  composition
• increase water , decrease Tmelt (for a given
  composition)
• melting initiates at crystal boundaries of the
  mineral with the lowest Tmelt

4
Physical Properties of Magmas

• Volatile effects on a melt
• most common volatiles H2O, CO2, SO2, CO, H2S ...
• only H2O effects on melting has been studied in
  detail by experimental petrologists
• in general, all volatile species are lumped into
  a percent value for the melt
• overall volatile in magmas (at surface) vary
  from lt 0.5 (basalts) to 6.0 (rhyolite) by
  weight
• at high P/T, volatile can be as high as 20
• at high pressure, volatiles are in solution (no
  bubbles), hence low viscosity
• however, volatiles have low molecular weights and
  therefore can be a substantial volume of the melt

5
Physical Properties of Magmas

• Volatile effects on a melt
• solubility decreases with decreasing pressure
• volatiles exsolve into gas (bubbles)
• Viscosity of remaining melt and potential
  explosivity increases
• bubbles can migrate and will therefore move to
  the top of conduit/magma chamber

6
Physical Properties of Magmas

• Phenocrysts effects on a melt
• for a melt that is cooling
• first minerals to crystallize are the ones with
  the highest Tmelt
• for basalts - olivine and pyroxene (both
  anhydrous)
• crystals increase BULK viscosity
• fractional crystallization
• removal of crystals from the melt through some
  process (ie, settling out)
• remaining fluid is enriched in volatiles and more
  silica-rich (have opposing effects on
  viscositycan be complex!)

7
Physical Properties of Magmas

• Phenocrysts effects on a melt
• study of phenocrysts
• insights into the pressure, temperature, time
  paths of the melt
• certain minerals can serve as geothermometers
  and/or geobarometers
• example Al in amphibole
• P (kbar) 4.76 AlT - 3.01
• where, AlT number of Al-cations in the
  amphibole structure

8
Physical Properties of Magmas

• Phenocrysts effects on a melt
• study of phenocrysts
• morphology of the mineral grain boundary
• embayments
• zonation in plagioclase (normal vs. reverse)
• used to track the evolution of the physical
  conditions in the magma chamber
• mixing of different lava compositions and
  contamination with surrounding rock
• xenocrysts

9
Physical Properties of Magmas

• How does one quantify a magma or lava?
• certain parameters derived from studies of fluids
  that are applicable
• stress s F/A (force/area) Pa kg/ms2
• strain e dL/L (change in length/length)
  unit-less
• strain rate ? e/time s-1
• Poissons ratio U dL/dw (change in
  length/change in width) unitless
• Modulus of Elasticity E s / e Pa
• viscosity ? s / ? Pas
• kinematic viscosity ?k ?/? (viscosity/density)
  m2/s

10
Physical Properties of Magmas

• Three types of stress
• compressive (eg convergent plate boundaries)
• tensional (e.g divergent plate boundaries)
• shear (e.g transform plate boundaries)

11
Physical Properties of Magmas

• Three types of strain
• elastic
• results in no permanent change in shape after
  force is removed
• example rubber band
• plastic
• results in a permanent change in shape after
  force is removed
• example upper mantle, silly putty
• brittle
• results in a failure/break during the application
  of force
• example lithosphere, glass

12
Physical Properties of Magmas

• Viscosity (?..greek letter eta)
• internal resistance to flow (strain) by a
  substance when subjected to shearing (stress)
• i.e., the sluggishness of a fluid
• example,
• basalt 102 103 Pas
• water
• olive oil
• dependence of stress on strain
• an applied stress produces a resulting strain
  through some proportionality (viscosity)
• what factors would affect the viscosity of a
  lava??

10-3 Pas
10-1 Pas

• temperature, phenocryst and bubble content,
  volatile content, SiO2, pressure

13
Physical Properties of Magmas

• Viscosity (?) as a ..
• temperature
• increase in T gt decrease atomic bonds and
  viscosity

rhyolite
1010
105
andesite
Viscosity (h)
basalt
100
komatiite
500
1000
1500
Temp (C)
14
Physical Properties of Magmas

• Viscosity (h) as a
• crystal content
• increase in crystal content gt increase in
  viscosity
• general numeric relationship
• ?s ?o (1 - f)-2.5
• where,
• f 1.67 (scaling factor dependant on the
  packing factor of the phenocrysts)
• ?s viscosity with phenocrysts
• ?o viscosity without phenocrysts
• More complex than this as shape, interaction and
  distribution of crystals also important

15
Physical Properties of Magmas

• Viscosity (h) as a
• silica content (SiO2)
• increase in SiO2 gt increase in viscosity

16
Physical Properties of Magmas

• Viscosity (h) as a ..
• percent volatiles
• increase in volatile wt. gt decrease in
  viscosity
• breaks SiO2 chains
• as T decreases, volatile increases
• as P decreases, volatile decreases

rhyolitic melt composition
1010
105
Viscosity (h)
101
2
6
12
wt. H2O
17
Physical Properties of Magmas

• Viscosity (h) as a
• percent volatiles
• increase in volatile wt. gt decrease in
  viscosity
• breaks SiO2 chains
• as T decreases, volatile increases
• as P decreases, volatile decreases

basaltic melt composition
103
Viscosity (h)
101
2
6
12
wt. H2O
18
Physical Properties of Magmas

• Viscosity (?) as a ..
• pressure
• increase in P gt decrease in viscosity
• not a steady rate (ie more complex)
• associate with mineral phase changes and/or
• changes in the melt structure

Viscosity (?)
Pressure
19
Physical Properties of Magmas

• Density (?) as a .
• temperature
• increase in T gt decrease in density
• varies with composition
• more a function of the constituent minerals the
  partial molar volumes

3000
komatiite
basalt
2700
andesite
Density (kg/m3)
rhyolite
2400
600
1200
1800
Temp (C)
20
Physical Properties of Magmas

• Density (?) as a
• pressure
• increase in P gt increase in density

basaltic composition
1800 C
6000
2800 C
4200
Density (kg/m3)
2400
0
25
50
Pressure (GPa)
21
Physical Properties of Magmas

• Modes of Material Behavior
• already examined the factors that affect changes
  in viscosity
• recall, viscosity is defined as a resistance to
  flow (strain) by a substance when subjected to
  shear (stress)
• therefore, it is related to both stress and
  strain
• this relation means that an applied stress
  produces a resulting strain through some
  proportionality (viscosity)
• the relationship of this proportionality depends
  on the material
• could be linear, non-linear, exponential,
  discontinuous,

22
Physical Properties of Magmas

• Viscosity relationships
• linear elastic behavior (Newtonian behavior)
• stress is linearly proportional to strain rate
• most lavas only behave as a Newtonian fluid if
  they are very hot (very low viscosity)

rhyolite
basalt
h
stress (s)
23
Physical Properties of Magmas

• Viscosity relationships
• Bingham Plastic behavior
• stress is linearly proportional to strain rate
  (e) after an initial amount is applied
• s t h e
• this offset is known as the yield stress (t) or
  yield strength
• caused by

.
.
Internal resistance, soldifying crust, bubbles,
phenocrysts etc
stress (s)
h
t
24
Physical Properties of Magmas

• Viscosity relationships
• Power Fluid behavior
• stress is exponentially proportional to strain
  rate
• if exponential is less than 1.0 (termed
  pseudo-plastic or thixotropic)
• if exponential is greater than 1.0 (termed
  rheopectic)
• Most lavas tend toward pseudo-plastic

.
n gt 1.0
n lt 1.0
stress (s)
25
Physical Properties of Magmas

• Viscosity relationships
• Hybrid behavior
• best estimate for erupting/flowing magmas
• stress is exponentially proportional to strain
  rate
• addition of a yield stress (combination of a
  Power Fluid and pseudo-plastic)
• if n1.0
• if n1.0 and t 0
• if t gets very large, plug flow initiates
• behavior of most silicic domes
• large yield stress caused by very thick, cool
  carapace

Bingham Plastic
Newtonian Fluid
26
Physical Properties of Magmas

• Viscosity relationships
• Hybrid behavior
• best estimate for erupting/flowing magmas
• stress is exponentially proportional to strain
  rate

n lt 1.0
stress (s)
t
27
Physical Properties of Magmas

• Rheology
• study of flowing material (fluid dynamics)
• different flow regimes depending on velocity,
  channel depth and viscosity
• laminar particles in the fluid all move with a
  constant velocity and direction
• turbulent particles become highly disorganized
  with variable speeds and directions
• relationship of velocity, channel depth and
  viscosity is by way of the unitless Reynolds
  Number (Re)
• Re U h / hk
• where, U velocity and h channel depth

28
Physical Properties of Magmas

• Rheology
• at Re gt 1000-2000, laminar flow transitions to
  turbulent
• silicic lavas have low velocities and high
  viscosities
• therefore low Reynolds Numbers (flow laminarly)
• more mafic lavas have higher velocities and lower
  viscosities
• therefore higher Reynolds Numbers (but still
  laminar flows)
• evidence that some komatiite flows were turbulent

29
Lava Flows

• Examine Lava Flows
• What are flows?
• range considerably
• depending on the composition/rheology/size
• rheology is a function of T, flow rate, viscosity
  velocity
• size can range from several m2 to 100s km2
• begin with examining basalt flows
• READ Chapter 6 in Francis and p291-306 in
  Encyclopedia. BOTH ARE EXAMINABLE!!

30
Lava Flows

• Basalt Flows

• provide excellent tests of numerical models
• more easily accessible and measured
• conform to most models adapted from other fluid
  flows
• time scales are predictable
• on average
• Terup 1200C
• verup 60 km/hr

31
Lava Flows

• Basalt Flows
• cooling quickly produces a darker surface crust
• increases viscosity and decreases velocity
• produces distinct flow fronts, channels and
  eventually tubes
• if source dies or is diverted
• flow advance stops after some lag period
• cooling of the interior continues
• can be months to years depending on thickness of
  the flow

32
Lava Flows

• Basalt Flows
• pahoehoe versus aa

33
Lava Flows

• Basalt Flows
• pahoehoe versus aa
• different surface textures on basalt flows
• must be a function of some other properties
  within the flow
• observations
• all basalts erupt as pahoehoe
• can quickly transition into aa
• never see the opposite case
• how does this occur?
• stiff clots form at areas of high shear stress
  (edges of channels, etc)
• pahoehoe breaking by solid fragmentation to form
  aa (flow over very steep topography)
• ponded/stored pahoehoe becomes remobilized
  (highly degassed/high viscosity)

34
Lava Flows

• Basalt Flows
• What is the cause?
• examine the parameters that directly affect flow
  morphology
• change in velocity (slope) produce changes in
  viscosity and strain rate
• how?
• h changes cooling, crystallization, degassing
• e changes topography changes, rough pre-flow
  surface
• therefore, if h is low and e is very high, could
  get aa
• if h is very high and e is very low, could get
  pahoehoe

35
Lava Flows

• Basalt Flows
• pahoehoe versus aa

aa
pahoehoe
36
Lava Flows

• Rheology
• flow thickness
• controlled by viscosity and velocity
• how does one measure these properties?
• difficult
• must derive models which fit the properties of
  existing flows
• flow initiation
• shear stress must exceed the yield stress (sshear
  gt t)
• this can be related to thickness of the flow by
• t r g t (tan a)

37
Lava Flows

• Rheology
• flow thickness
• flow initiation
• shear stress must exceed the yield stress (sshear
  gt t)
• this can be related to thickness of the flow by
• t r g t (tan a)
• looked at another way, a flow will attain a
  thickness which results in a stress balance

rg sin a
rg cos a
a