Dike Intrusion Dike 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 Overpressure Magma Overpressure • For a magma lens, pressure is equal to the lithostatic load P m = ρ r 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 Ascent Magma Ascent • Dikes – Sub-vertical cracks in brittle rock • Diapirs – Bodies of buoyant magma – They squeeze through ductile material Dikes Dikes • 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
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Magma Overpressure Magma Ascent P r g zMagma Velocity in a Dike v = (w2 g g ∆ρ∆ρ)/3 η • Variables: width (w), density differential (∆ρ), and viscosity (η) • Doubling
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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),
• 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