Click here to return to the Earth Systems Revision Page

Impact of Intrusive Igneous Activity on the Landscape

Case Study: The Isle of Arran (Scotland)

Igneous intrusions often have no direct impact on the landscape (apart from some upward doming upon intrusion) until the overlying rocks have been weathered away. The Isle of Arran, has some excellent examples of the impact that intrusive igneous activity has had on the formation of landscape features.

The Isle of Arran has been an island longer than Britain has. The continental crust in the area was stretched and thinned by plate movements magam exploited points of weakness, forcing colums of magma towards the surface. As well as volcanic extrusions, some magma, whilst injected into the crust, did not reach the surface, forming igneous intrusions. Almost half of Arran is made up of igneous rocks (including granites and lavas) many of which date back 60 million years.

Over time, weathering and erosion in Arran has removed the characterisitcs volcanic shapes and worn the overlying rocks down, exposing the intruded granite below. It is this granit that now stands up as higher ground due to its greater resistance to erosion than the surrounding sedimentary rocks. The igneous intrusions on Arran, are made up of batholiths, dykes and sills - all of which have had their own impact on the landscape. The following table summarises some of the landforms that have been created since the removal of the overlying rock has exposed the intrusions.

THE IMPACT OF THE EXPOSED BATHOLITHS ON THE LANDSCAPE:

- The hills of Northern Arran – are made up of a granite batholith (see notes on batholith)

- The intruded granite is harder / more resistant to erosion than the surrounding sediment rocks.

- When the overlying rocks where eroded away, the granite was left upstanding and the erosion and chemical weathering of joints in the granite – have given rise to the rugged landscape of Arran.

THE IMPACT OF EXPOSED SILLS ON THE LANDSCAPE

- The Hills of Southern Arran are formed from the exposed sills. The granite sills have been tilted due to ground movements – as they are more resistant than the surrounding rocks they have remained and form the southern hills.

- Coastal Cliffs (formed due to presence of sills) - the exposure of sills has created cliffs (e.g. Drumadoon point) – here you can see the vertical joints formed during cooling.

Waterfalls (formed due to sills) - where rivers have crossed sills – the harder rock is not eroded very rapidly – and a step is formed in the landscape – over which the water flows as a waterfall – e.g. Glen Ashdale (staircase of waterfalls) - see diagram below.

THE IMPACT OF THE EXPOSED DYKES ON THE LANDSCAPE

Natural Breakwaters (formed due to presence of dykes) - Dykes formed of rocks that are harder than the surrounding sandstone – where they have been worn down – therefore stand above the sandstones – act as natural breakwaters – trap sand and form little beaches – e.g. Kildonan Shore (see diagram below).

Gorges - where dykes cut through granite – the dykes are less resistant and are worn aware to form lower ground – forms gorge and waterfalls – e.g. Witch’s Step. (see diagram below)

Download a more detailed account of how igneous intrusions have shaped the Isle of Arran's landscape (this is a pdf file and will require Adobe Acrobat Reader to view). This account was compiled by Mr C so is based around the case studies you have been taught.

Some good pictures of dykes, sills and other geological landforms on the Isle of Arran (Scotland)

Information on the Geology of Arran