Batholiths

Recognise and describe.These are large igneous bodies that are >100 km2 in size.They are usually plutonic.Formation:Accumulation of large amounts of magma by:Permitted (cauldron subsidence/stoping) orForceful (doming).

Bosses and Stocks

These are smaller intrusions.

The dictionary definition of a boss is a knob like intrusion.

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A stock is like a batholith but < 100 km2 .

Dykes

You know these.

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.

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.

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.

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.

Transgressive 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.Transgressive sills are concordant for most of the time but occasionally cut up through the beds before becoming concordant again.

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.

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 it’s way along the beds like a sill but will also dome upwards forcing the overlying beds into a dome.