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.