Minor Intrusions

• Dykes


  Is Discordant

  Results from crustal Tension

  Chilled margin is fine grained

  Middle will be medium grained

  Found as feeder vents under or as part of a volcano

  Create ridges as they are more resistant to erosion than the rocks they intrude
• Sills


  Concordant

  Intrude between and follow bedding planes
• Transgressive Sill


  Jump up to the next bedding plane where weakness is found
• Laccolith


  Forced between two layers of sedimentory Rocks

  Pressure of injecting the magma was high enough to push the rock layers above
• Volcanic Plug


  The feeder pipe of an ancient volcano - much more resistant to erosion than the surrounding country rock
• Ring Complex


  Roof above magma chamber collaplses

  Allows magma to intrude upwards into the fractured in sheets called ‘Ring Dykes’