Intrusive volcanic activity
Batholiths are formed deep below the surface when large masses of magma cool and solidify. As the magma cools slowly, large crystals are formed in the rock (e.g. granite). Batholiths are often dome-shaped and exposed by later erosion. This is the case on Dartmoor (left) and on the Isle of Arran. Batholiths can be several hundreds of kilometres in diameter. The area surrounding the batholith is altered by the heat and pressure of the intrusion to form a metamorphic aureole (limestone, for example can be transformed into marble). Batholiths are unaffected by the characteristics and structure of existing rock. Sometimes smaller injections of magma fom a lens shape that is intruded between layers of rock. This then forces the overlying strata (layers of rock) to arch upawards, forming a dome. This feature is knwon as a laccolith, and it may be exposed by later weathering and erosion to form a small range of hills, for example the Eildon Hills on the Scottish Borders.
Dykes - these are vertical intrusions with horizontal colloing crakcs. They cut across the bedding planes of the rocks into which they have been intruded. Dykes often occur in groups where they are knwon as dyke swarms. Many Scottish Islands, such as Mull and Skye have clusters of dykes all associated with one instrusive event.
Sills - these are horizontal intrusions along the lines of beeding planes. Sills have vertical cooling cracks. Examples include the Great Whin Sill (which carries part of Hadrian's Wakk) and Drumadoon on the Isle of Arran. Both sills and dykes are commonly made up of dolerite.