Igneous Petrology and Volcanology (L14-18) Flashcards
Rank these settings based on igneous rock production (extrusive and intrusive): Consuming plate boundaries Divergent plate boundaries Intra-oceanic plate Intra-continental plate
Divergent plate boundaries
Consuming plate boundaries
Intra-oceanic plate
Intra-continental plate
What is modal mineralogy?
Volumetric proportions of minerals forming igneous rocks are the basis for classification
What are the questions asked in classifying a coarse-grained rock by mode?
Proportions of alkali to plagioclase feldspars? Presence of quartz? Presence of feldspathoids? Which Fe,Mg mineral is present? Plagioclase composition?
What is normative mineralogy used for?
Why is it used?
Fine-grained or glassy rocks
To allow comparison with coarse-grained rocks
When does mode and norm classification agree?
Rocks cooled slowly, at low P and dry
What is the basalt tetrahedron used for?
Makes the distinction between the basic alkaline and tholeiitic magma series
What are the components of the basalt tetrahedron?
Diopside Olivine Hypersthene Nepheline Plagioclase Quartz
How is the basalt tetrahedron divided?
Into three volumes by two interior planes
‘Plane of silica saturation’ separates normative quartz compositions from normative olivine and hypersthene
‘Plane of silica undersaturation’ separates normative olivine and hypersthene from normative nepheline
What do the three volumes in the basalt tetrahedron correspond to?
Normative quartz = quartz tholeiites
Normative olivine and hypersthene = olivine tholeiites
Normative nepheline = alkali basalts
What is ‘silica saturation’ based on?
What is the principle control of it?
Nominal reactions in the Qz-Ol and Qz-Ne systems
Proportions of Si:Na:(Mg,Fe)
Define silica oversaturated
Rocks with enough SiO2 that Qz is in the norm
Define silica saturated
Rocks with insufficient SiO2 for normative Qz, but have normative Hy and Ol
Define silica undersaturated
Rocks with insufficent SiO2 for normative Qz or Hy, instead has Ne
What can the character of a magma series be defined by?
Abundances of different diagnostic elements
What is the TAS plot?
Total alkalis (Na2O + K2O) vs silica (SiO2) plot
What are the three typical differentiation trends on the TAS plot?
Which two are the most important?
Kenya Rift, Hawaii, Cascades
Hawaii and Cascades
How can basalts, andesites, dacites and rhyolites be further subdivided beyond the TAS plot?
Based on potassium content
Into low, medium and high K
Define peralkaline
Na2O + K2O > Al2O3
Alkali minerals in the norm
Define peraluminous
CaO + Na2O + K2O < Al2O2
Aluminous minerals in the norm
How important are peralkaline and peraluminous rocks?
Unimportant
Most rocks are neither
What is an AFM diagram?
Where is it used to make a distinction?
What are present on it?
Alkalis-iron-magnesium diagram
Within the silica-saturated rock series
Tholeiitic and calcalkaline trend
What distinguishes the tholeiitic and calcalkaline trends on an AFM diagram?
Where are calcalkaline trends common?
Tholeiitic: strong iron enrichment
Calcalkaline: Little or no iron enrichment
Subduction zones
What is the difference between batch and fractional processes (melting and crystallisation)?
Batch: closed system, bulk composition remains constant, equilibrium maintained between melt and xals
Fractional: open system, melt extracted as it forms/xals removed by settling as they form
Which factors control magma composition and evolution?
Source region composition
Depth, T and extent of melting
Melting process and melt extraction
Cooling and crystallisation history as magma ascends, ponds or erupts and freezes
What is the dihedral angle?
What is dependent on?
The angle at which two grains intersect with a pool of melt
Relative surface energies of the solids and the melt
What role does the dihedral angle in partially molten rocks?
Controlling melt distribution
What happens when dihedral angles are lesser than or greater than 60°?
Less than: melt forms an interconnected network
Greater than: isolated pools of melt form at grain-grain-grain junctions and boundaries
What happens during compaction to different parts of a partially melted column?
Base: melt expelled as matrix compacts and melt fraction decreases
Above compaction zone: region with constant melt fraction
Top: melt accumulates
What is Darcy’s law used for?
What is the equation?
To describe the flow of melt out of a porous and permeable medium Q = (κA/η)(dP/dz) Q = melt flux out κ = permeability A = x sectional area η = viscosity dP/dz = pressure gradient
In a system where the rock pile is compacted by gravity, which three characteristic scales can be calculated?
Compaction length d_c: distance over which compaction rate decreases by a factor of e
Compaction timescale t_0: melt fraction at the base of the compacting layer falls by a factor of e
Time, t_h, to reduce porosity in a layer of depth h by a factor of e (h»_space; d_c)
What are the rough compaction times for:
dry rhyolite, wet granite and basalt?
Dry rhyolite: 550 Ma
Wet granite: 180000 years
Basalt: 800 years
Define fractionation
Process of the formation of a variety of magma compositions from an initial, single parental composition
In liquids, what is the dominant fractionation process?
Define it
Diffusion
Elements might diffuse through a liquid at different rates in response to thermal, pressure or compositional gradients
When does fractional crystallisation occur?
What are the two stages of it?
As magma cools or degasses in the crust
The formation of crustals with a different composition to the bulk melt
Removal of the crystals
Define assimilation
Process by which magmas incorporate fragments of another rock
When are hybrid rocks produced?
What forms can they be in?
Two chemically distinct magmas mix
Completely homogenised or xenoliths of one type within another
What are the various major elements present in magma?
SiO2 Al2O3 FeO/Fe2O3 MgO CaO Na2O K2O H2O
What are major element variation diagrams or Harker plots useful for?
What is the convention?
When is one convention more useful?
Plotting major element data
Abundance of SiO2 or MgO plotted on the x axis
MgO useful on x axis for basaltic rocks
Which major elements usually correlate strongly?
Why?
MgO, CaO, FeO/Fe2O3
Low SiO2 rocks are typically rich in them, high SiO2 rocks are poor in them
How can rocks be classified by the colour index?
Leucocratic = pale-coloured Melanocratic = dark-coloured Mafic = rich in ferro-magnesian minerals
What is zoning in crystals in volcanic rocks a result of?
Fractional crystallisation of a solid solution, where diffusion is slower than crystallisation
Define phase
Chemically and physically homogeneous and distinct entity
Define component
Independent chemical variable
How can systems be distinguished by the number of components?
Minimum components to define the composition of all of the phases in the system Unary = 1 component Binary = 2 components Ternary = 3 components etc
What is a phase diagram a representation of?
What are they used to show?
The equilibrium phase assemblages of any system
How equilibria vary with P, T and composition
What is the Gibbs phase rule?
P + F = C + 2
P = phases
C = components
F = degrees of freedom
How can degrees of freedom be used to categorise parts of systems?
Divariant, univariant, invariant
When using an isobaric or isothermal phase diagram, what must be remembered w.r.t. Gibbs phase rule?
One degree of freedom has been implicitly used (P or T)
Most natural igneous rocks’ compositions can be expressed by which four endmember components?
CaO-MgO-Al2O3-SiO2:
Silica (Qz), olivine (Fo), feldspar (An), clinopyroxene (Di)
In binary phase diagrams of A and B, what are the four different idealities of mixing?
Give examples of each
A and B have v similar structures and mix nearly ideally: Fo-Fa, An-Ab
A and B have similar structures but mix non-ideally: anhydrous alkali feldspars
A and B are highly non-ideal when mixed as s.s.: pyroxenes
A and B have totally different structures, no s.s. tolerated: Di-An, Ab-Qz
Outline the path of equilibrium melting of plagioclase in a binary phase diagram
Rock has composition A
Heated until hits solidus and begins to melt
P1 rock in equilibrium with melt L1
T rises, more melt forms
At another T, xals P2 in equilibrium with L2
Rock T hits liquidus, final plag xal of composition P3 melts to form liquid L3
Outline the path of fractional melting of plagioclase in a binary phase diagram
Rock has composition X
Heated until hits solidus and begins to melt
Plag comp P1 melts to form melt comp L1
L1 is more Ab rich than solid
L1 removal moves bulk comp to more An rich
T rises for more melt to form
Melt and residue get more calcic
Continues until plag is 100% An and melts congruently
How can crystals produced by equilibrium and fractional crystallisation be told apart?
Equilibrium: homogeneous phenocrysts
Fractional: zoned phenocrysts
Outline the path of equilibrium crystallisation of Di+Fo in a binary phase diagram (no s.s. is possible)
L of composition X Cools to hit Fo+L liquidus Xals of Fo nucleate and grow L enriched in Di until eutectic point reached L -> Di + Fo No cooling until all L used up Aggregate then cools
Outline the path of equilibrium melting of Di+Fo in a binary phase diagram (no s.s. is possible)
Aggregate of composition Y
Aggregate melts at eutectic point, Di + Fo -> L until Di used up
L in equilibrium with Fo xals
T rises, melt moves up liquidus, more enriched in Fo as Fo continues to melt
Last Fo xal melts when L = Y
Outline the path of fractional melting of Di+Fo in a binary phase diagram (no s.s. is possible)
Aggregate of composition Y
Starts to melt at eutectic T
Eutectic liquid L1 formed and instantly removed
Bulk composition becomes more enriched in Fo
When Di used up, system is only Fo
No melting until T hits liquidus T of pure Fo
What is the difference between congruent and incongruent melting?
Congruent: 3 stable solid endmembers, 2 eutectic points
Incongruent: 1 eutectic point, 1 peritectic point
How can bulk composition be worked out in a ternary system?
Same principle as the lever rule
What is the conventional projection for a ternary system?
A triangle
T is shown through labelled contours (not always)
Boundaries between phase fields have arrows indicating down-T direction
What are the straight lines in isothermal sections of ternary systems?
Tie lines
Join compositions of coexisting solid and liquid
How do cotectic boundaries work in ternary systems?
When L = A + B
Unvariant reaction
Evolving liquid moves down T
Back projection of tangent to cotectic gives proportion of A:B
How do invariant points work in ternary systems?
Ternary eutectic point
Can be L = A + B + C
Invariant point at fixed P
System remains at constant T until L crystallised
What is Alkemade’s Theorem?
L composition always moves along a cotectic in a direction away from the line joining the two precipitating phases
What is a thermal divide?
Given that:
Ternary system of ABCD where D is between A + C
B to D = thermal divide if:
Compositions in ABD will never crystallise C and vice versa
What is a resorptional boundary?
Given that:
Ternary system of ABCD where D is between A + C
BD =/= thermal divide
BD = resorptional boundary if:
Boundary between B + L and D + L is not intersected by BD line
What happens at a resorptional boundary?
One of the xals is consumed along this boundary during equilibrium crystallisation
What is a simple way to tell if a boundary is resorptional or not?
If the back projection of the tangent to the curve joining the 2 components lies outside the subsolidus tie line joining them
What are subsolidus triangles used for?
What is required for every subsolidus triangle?
Determining solid mineral assemblages for a bulk composition of L
Corresponding ternary univariant point, eutectic or resorptional
Outline equilibrium crystallisation for a composition outside its subsolidus triangle
Use Fo-An-Qz-En where En-An is not a thermal divide
Composition A
Crystallises Fo
L composition evolves towards B, where extension of Fo-A hits resorptioal boundary
En crystallised, Fo resorbed until C, extension of En-A hits resorptional boundary
L moves across En + L field to D
At D, En and An crystallise
At eutectic L -> An + En + Qz
Outline fractional crystallisation for a composition outside its subsolidus triangle
Use Fo-An-Qz-En where En-An is not a thermal divide
Composition A
Fo crystallises as L moves along extension of Fo-A until resorptional boundary hit at B
No Fo to resorb at B so only En crystallises, moves across En+L field to C
C to E, En + Qz crystallise
At eutectic L -> An + En + Qz
What are the four components of the basalt tetrahedron?
An, Qz, Fo, Di
Which face of the basalt tetrahedron should be used to look at the evolution of low P basaltic melts?
Why?
Fo-An-Qz
Starts in Fo+L field and crystallise from silica saturated L compositions to silica-oversaturated
Which geological process is the basalt tetrahedron Fo-Di-Qz face used in relation to?
Melting of dry and wet peridotite
Subduction-related magmas
What are phase equilibria in the granite system strongly influenced by?
Why?
The extent of s.s. between Na-rich and K-rich feldspars
Which is sensitive to P and water-content:
Low P and dry = complete s.s. below the solidus
Wet = solidus falls, can intersect solvus
How is the Ne-Fo-Qz system related to the rock series evolution of basalt to rhyolite and basalt to phonolite?
Coincides closely with the thermal divide Fo-Ab (Ab is between Ne and Qz)
Basalt = Fo, Albite = trachyites, Qz = rhyolites, Ne = phonolites
What causes melt to rise?
How are melts kept less dense than their surroundings?
Buoyancy
Crystallisation and removal of dense minerals & growth of gas bubbles
What are the two ways magma can erupt?
Explosively
Effusively
What conditions favour effusive eruptions?
Low-viscosity/volatile-poor melts
If a melt contains dissolved gases, what happens as it rises?
Magma decompresses
Gases become saturated
Bubbles nucleate
What is the density of magma?
The ratio of the sum of the contributions to mass and volume from each oxide component
Why does the density of igneous rocks only slightly decrease with increased T?
Thermal expansion coefficient is small
Partial molar volume of SiO2 is ~ independent of T at magmatic T’s
How does SiO2 content relate to magma structure?
Higher SiO2 content = higher level of polymerisation of SiO4 tetrahedra
How does water influence silicate melt structure?
H2O + SiOSi = 2SiOH
Decreases polymerisation
How do Newtonian and Non-Newtonian fluids differ?
Newtonian: τ = η(du/dy), shear stress proportional to shear rate by viscosity
Non-Newtonian: τ = τ0 + η(du/dy) , must overcome yield strength before flow
Define these terms w.r.t. shear & viscosity: Newtonian Shear thickening Shear thinning Bingham plastic
Newtonian: constant viscosity
Shear thickening: viscosity increases with rate of shear
Shear thinning: viscosity decreases with rate of shear
Bingham plastic: solid at low stress, viscous fluid at high stress
What is the viscosity of magma dependent on?
T: exponential decrease as T rises
SiO2 content: rhyolite several orders more viscous than basalt at same T
Water: decreases polymerisation = easier flow
Xals: non-linear increase with xal content, big increase at 45-50 vol%
What do non-Newtonian magmas form?
Lava channels bounded by levees
How does yield strength of a non-Newtonian magma affect the lava channels formed?
Height and width of levee proportional to yield strength
What are the ways in which lava can erupt effusively?
What controls it?
Pahoehoe & a’a
A’a = higher yield strength and viscosity
Under what condition does pahoehoe lava dominate?
When is the transition from pahoehoe to a’a?
Low rates of strain
At a constant strain rate due to cooling and crystallisation = viscosity increase OR strain increase (steeper terrain)
How does lava behave in a pahoehoe eruption?
Flows through earlier-formed toes
Toes coalesce to form a lava tube
Only 1°C/km lost
Like a river delta at the flow front
Effusive eruptions:
Eruption rate?
Flow length?
Time scale?
10-100 m^3 s^-1
5-10km long
Intermittently for 100s of years
What are explosive eruptions of low viscosity magma dependent on?
How melt and gas are coupled in the conduit
Melt/gas volume ratio
What distinguishes Hawaiian and strombolian explosive eruptions?
Hawaiian: gas-rich ascending rapidly up a conduit yielding explosive lava fountaining
Strombolian: Low gas flux and slow melt ascent rates
What is fragmented magma?
What are they indicative of?
Ash/tephra
More silicic magmas
What is believed to be the driving force for explosive eruptions?
How?
Expansion of a volatile phase, usually water
Melts get supersaturated in volatiles as they rise, bubbles form, density falls, greater buoyancy, melt accelerates -> eruption
What are the various regions of an explosive eruption?
Magma reservoir with dissolved volatiles
Magma with exsolved volatiles
Fragmentation region
Source mixture of hot pyroclasts and gas
Jet phase: jet density > atm
Convective phase: rises by buoyant convection
Umbrella region: level of natural buoyancy
What happens to an eruption plume once it reaches a level of natural buoyancy?
Spreads out as a gravity current
Pumice and ash fallout
What does eruption plume height depend on?
What is required to double column height?
Mass eruption rate
2x column height = 16x eruption rate
How can a convective column collapse?
Increase vent radius
Decrease velocity
What are the two ways explosive eruptions disperse tephra?
Buoyant plumes leading to tephra fall deposits
Pyroclastic flows or surges
What hazards are produced by volcanoes?
Pyroclastic flows Lava flows Ash fall Lahars Earthquakes Landslides Gases (CO2 + H2S)
Define pyroclastic flow
Velocity?
How can a pyroclastic surge be related?
Fast-moving current of hot gas and volcanic ash
Typically over 50 mph, up to 100s mph
May detach from the main body of the flow
What is a lahar?
Effect on surroundings?
A type of mudflow or debris flow composed of pyroclastic material, rocky debris and water
Significant yield strength and erosive power
What is the anatomy of a lahar?
Front to back: Streamflow Lahar pushes water at the front Peak flow Water mixing with sediment Sediment-rich debris flow More dilute, coarse sediment Muddy streamflow
