Batholiths

1
Batholiths

• These are large igneous bodies that are gt100 km2
  in size.
• They are usually plutonic.
• Formation
• Accumulation of large amounts of magma by
• Permitted (cauldron subsidence/stoping) or
• Forceful (doming).

2
Bosses and Stocks

• These are smaller intrusions.
• The dictionary definition of a boss is a knob
  like intrusion.
• A stock is like a batholith but lt 100 km2 .

3
Dykes

• They are small scale intrusions (no more than a
  few 100 m wide).
• They form close to the surface Hypabyssal.
• Dykes are discordant cut across the beds.
• They often force their way along lines of
  weakness such as faults.
• In cross section or a map they will cross cut
  beds.

4
Ring Dykes

• These are associated with cauldron subsidence.
• This is a kind of permitted intrusion.
• The force of the intrusion causes the rocks above
  to form a cylindrical fracture.
• The centre of the cylinder sinks into the magma
  chamber and magma shoots up the cylindrical
  fracture to form a ring dyke.
• On a map they will look circular and cross
  cutting.

5
Cone Sheets

• These form inverted cones of dykes.
• They are focussed on the magma chamber and then
  widen upwards.
• Successive cone sheets form as the centre of the
  eruptions moves.

6
Sills

• You also know these.
• They are also small scale intrusions (no more
  than a few 100 m wide).
• They also form close to the surface Hypabyssal.
• Sills are concordant run parallel to the beds.
• They often force their way along lines of
  weakness such as bedding planes.
• In cross section and map views they run parallel
  to the beds.

7
Transgressive Sills

• They are also small scale intrusions (no more
  than a few 100 m wide).
• They also form close to the surface Hypabyssal.
• Transgressive sills are concordant for most of
  the time but may cut up through the beds before
  becoming concordant again.

8
Transgressive sills 2

• They often force their way along lines of
  weakness such as bedding planes, but will follow
  a weakness across beds if they find it (often a
  fault).
• In cross section and map views they run parallel
  to the beds for most of the time but will cut
  across beds briefly.

9
Laccoliths

• These occur where there is small scale intrusion
  that has a flat base and a domed top.
• These are often fed by Feeder Dykes.
• The magma forces its way along the beds like a
  sill but will also dome upwards forcing the
  overlying beds into a dome.