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Dike IntrusionDike Intrusion
Christiansen and Best,Chapter 9
How Can Dense Magma Rise?How Can Dense Magma Rise?
• Volumetric expansion on melting?
• Exsolution of bubbles?
• There must be another cause.
Magma OverpressureMagma Overpressure
• For a magma lens, pressure is equal to the lithostatic load
Pm = ρρρρr g z• The pressure can be greater in a conduit
connecting a deeper pocket to the surface• This overpressure can be great enough to
bring denser magma to the surface
Magma AscentMagma Ascent
• Dikes
– Sub-vertical cracks in brittle rock
• Diapirs
– Bodies of buoyant magma
– They squeeze through ductile material
DikesDikes
• Intrusions with very small aspect ratio
• Aspect: width/length = 10-2 to 10-4
• Near vertical orientation
• Generally 1 - 2 meters thick
Dike Swarms Dike Swarms
• Hundreds of
contemporaneous dikes
• May be radial
• Large radial swarms
associated with mantle
plumes
Sheeted Dike SwarmsSheeted Dike Swarms
• Form at ocean ridges
• Sub-parallel dikes intruded into other dikes
• Caused by long-term extension and intrusion
Sill SwarmsSill Swarms
• Underlie flood-basalt fields
• Can have huge volumes
• Diabase sills associated with breakup of
Gondwanaland
Intrusion into DikesIntrusion into Dikes
• Stress perpendicular to the fracture is less than magma pressure
• Pressure must overcome resistance to viscous flow
• Magma can hydrofracture to rock and propagate itself
Magma Velocity in a DikeMagma Velocity in a Dike
v = (wv = (w22 g g ∆ρ∆ρ∆ρ∆ρ∆ρ∆ρ∆ρ∆ρ)/3 )/3 ηηηηηηηη• Variables: width (w),
density differential (∆ρ),
and viscosity (η)
• Doubling the width
increases the velocity
four times
Magma Transport DistanceMagma Transport Distance
• Rate of magma cooling
– Heat transfer into wall rock
– Viscosity increase
• Rate of magma flow
Calculated Intrusion DistanceCalculated Intrusion Distance
d = (wd = (w44 g g ∆ρ∆ρ∆ρ∆ρ∆ρ∆ρ∆ρ∆ρ)/ (3 )/ (3 κκκκκκκκ ηηηηηηηη))• Variables include: the dike width (w),
density differential (∆ρ), thermal
conductivity (κ), and viscosity (η)
Dike LengthDike Length
• The distance depends on dike width to the
fourth power!
• Viscosity a strong factor too, because it
covers several orders of magnitude
Aplite DikesAplite Dikes
• Associated with granodiorites
• Minimum residual melt composition
• Intrude self-generated extensional fractures
• Formed during cooling of crystal mush
Stress for DikesStress for Dikes
• Dikes are hydraulic tensile fractures
• They lie in the plane of σ1 and σ2
• They open in the direction of σ3
• They are good paleostress indicators
σσσσσσσσ11111111 verticalvertical σσσσσσσσ33333333 verticalverticalOrientationOrientation
• Near-vertical dikes imply
horizontal σ3
• Typical in areas of tectonic
extension
• Can be used to interpret past
stress fields
En Echelon DikesEn Echelon Dikes
• Dikes commonly form
fingers upwards
• Sub-parallel overlapping
alignments
• Suggest a rotation of σ3 in
the horizontal
Radial DikesRadial Dikes
• Stress orientation around a central intrusion
• σ1 is perpendicular to the contact (radial)
• σ3 is horizontal and tangential to contact
• Radial dikes are radial from intrusion
• Far dikes assume the regional trend
Spanish Peaks, ColoradoSpanish Peaks, ColoradoSpanish Peaks, Colorado
Cone SheetsCone Sheets
• Stress orientation above an intrusion
• Planes containing σ1 and σ2 are cones
• Magma intruded along these form cone sheets
Ring DikesRing Dikes
• If magma pressure diminished• The roof of the chamber may subside• This forms a caldera• The bounding fault is a ring fault• If magma intrudes, this is a ring dike
Tectonic RegimeTectonic Regime
• Extensional regime
– Basalts common
• Compressional regime
– Andesites common
Extensional RegimeExtensional Regime
• σ1 is vertical• σ2 and σ3 are are
horizontal• Pm > σ3
• Vertical basaltic dikes rise to surface
Compressional RegimeCompressional Regime
• σ3 is vertical • σ1 and σ2 are are
horizontal• Pm < σ2
• Basalt rise limited by neutral buoyancy
DiapirsDiapirs InstabilitiesInstabilities
• A layer of less dense material overlain by a denser material is unstable
• The upper layer develops undulations and bulges (Rayleigh-Taylor instabilities)
• The spacing of the bulges depends on the thickness of the light layer and its density contrast with the heavy layer
Diapir AscentDiapir Ascent
• Velocity of ascent depends on diapir size and shape
• A sphere is the most efficient shape• Surface area ~ frictional resistance• Volume ~ buoyant driving force• Rise velocity proposrtional to area squared
Thermal effectsThermal effects
• The wake of a diapir is thermally softened• Subsequent diapirs may follow the same path• Diapirs stall because ductile strength of the rock
increases exponentially upward.