Intrusive Activity

Question 1

What is intrusive activity?

Answer 1

Intrusive activity takes place place beneath the Earth’s surface. It includes the formation of large magma chambers and magma being forced into the crust through cracks in the rock.
Magma in intrusive volcanic features cools, crystallizes and solidifies into igneous rocks below the surface. Slow cooling results in large crystals forming­ typical of rocks such as granite and dolerite.
The resulting features may then only become part of the landscape once later erosion removes the overlying rocks. This activity is likely to be the case if magma is rising slowly, if there is a great thickness of crust to pass through, and if there are few weaknesses in the crust through which it can flow easily.

Question 2

Give an example of a landscape with several intrusive landforms.

Answer 2

Intrusive volcanic activity (batholiths vs dykes and sills) - here it is the scale of the process which results in distinctive landscapes - batholiths create landscapes, whereas dykes and sills are seen more as landforms, but both are created by igneous intrusion.

The Isle of Arran off the west coast of Scotland was created by the intrusion of large granite batholiths that domed the sandstone surface about 60 million years ago as Greenland separated from Scotland during the creation of the Atlantic Ocean.
The fractured overlying sandstone and metamorphic rock have subsequently been weathered and eroded to expose the resistant granite, peaking at a height of 874 m on Goatfell.

Question 3

Batholiths

Answer 3

Batholiths are large masses of intrusive rock that may cause a general doming­ up of the surface as they are forming. The heat transferred from the magma to the country rock causes metamorphic rock to be produced around the intruding magma.

Question 4

Dykes

Answer 4

A series of dykes, 2­3 m wide, are exposed across the beach at Kildonan. Dykes are discordant because they cut across the bedding planes of the country rock, often vertically. Magma flows through cracks and weaknesses but again cools and solidifies before reaching the surface. Contraction joints develop parallel to the surface as the magma solidifies. Once exposed, the dykes can appear as linear outcrops of resistant rock.

Question 5

Sills

Answer 5

At Drumadoon a sill has been exposed on the coast, forming a cliff 50 m high. Sills are intrusions that are formed parallel to bedding planes in the country rock (ie concordant), often, but not always, lying horizontally. The bedding planes provide a line of weakness along which the magma flows before cooling and solidifying. As it cools, the magma contracts, producing cracks in the resultant rock.