6 INTRUSIVE VULCANIC FEATURES




Vulcanism refers to movement of molten magma toward or onto the earth’s surface.

Igneous intrusive features are features formed when magma cool and solidify within the earth’s crust.

The following are feature resulting from intrusive igneous activities :

Dyke

this is a sheet of rock that is formed in the fracture of the pre-existing rock.

Dyke forms when a mass of magma cuts across the bedding planes and forms a wall-like structure.




An example of dyke is one that is found in Jos plateau in Nigeria this has been exposed to the surface through erosion.

Sill

this is formed when a sheet of magma lies along the bedding planes of the earth’s surface.

A sill is therefore a horizontal sheet of rock that solidifies from magma that has been injected concordantly between bedding planes; they may be of any thickness and extend for many squares of kilometers.

An example of a sill is a great Whill sill in great Britain.




Batholith

this is a large body of intrusive igneous rock believed to have been crystallized at the considerable depth below the earth’s surface.

They are the largest type of pluton.

An example of a batholith is Idaho batholith which covers a surface area of over 15500 kilometers square.

Lopoliths

these are saucer-shaped features formed beneath the earth’s surface through magma intrusion.

Lopolith forms a great shallow basin when magma solidifies within the crust a good example is the bushveld igneous complex of South Africa which is composed of both granite and basic rock.




Laccolith

this is a sheet intrusion that has been injected between two layers of sedimentary rock.

The pressure of magma is powerful enough that the overlying strata are forced upward giving laccolith a dome or mushroom-like form with a generally planar base.

Some laccolith can be found in Madagascar and Utah USA where they have been exposed by erosion.

Phacolith




this is a lens-shaped pluton that occupies either the crest of an anticline or trough of the syncline. A good example of phacolith can be seen in Corndon hills in the Shropshire United Kingdom.