Intrusive features are formed underground as magma is injected into rocks and solidifies underground. These features tend to stay concealed unless exposed by erosion in the form of weathering.
The first main intrusive feature is called a Batholith, or Pluton. These are large dome shaped structures made of intrusive igneous rock. Formed when magma cools and crystallizes beneath Earth's surface. They are formed between convergent plate boundaries as one plate subducts under another; leading to partial melting of the denser plate at the Benioff zone. This creates magma less dense than that surrounding it. The magma then cools slowly as it reaches the crust forming large crystals of granite or granodiorite. Surrounding these features metamorphic rock is formed due to the intense heat and pressure. The subsequent weathering and erosion of the strata above the batholith can mean it is revelled at the surface. As they compromise resistant rocks such as granite they form prominent uplands such as the Batholith in Sierra Nevada which expands across California and is part of the Yosemite national park.
Another intrusive feature that forms as a result of a Batholith is a Dyke; this is a thin sheet of igneous rock, intruded at a high angle to the inclination of the older surrounding rocks. If the earth above a Batholith is faulted, magma may shoot up forming a horizontal intrusion. These Dykes usually occur in what is called Dyke swarms where many form in proximity to one another. Dyke swarms tend to occur around a single point of intrusion, volcano or fissure.. Large numbers of dykes occur in the Inner Hebrides on the islands of Mull and Skye. Some dykes extend over long distances. The Cleveland dyke runs from Mull in western Scotland to within a few kilometres of the North Sea near Scarborough.
Another intrusive landform radiating from a Batholith is a Sill. This occurs when there are horizontal faults shooting out from a Dyke and magma seeps in sideways and cools, and often form between sedimentary layers or country rock.
Sills are thin horizontal sheets of magma. Where erosion of valley sides has exposed sills they often form cliffs and escarpments. The Great Whin Sill in northern England has a major effect on relief. It forms the steep cliffs at High Cup Nick in Cumbria; and a prominent escarpment followed by sections of Hadrian’s Wall in Northumberland.
These formations can be observed at the North Arran Batholith in Scotland which forms most of the island giving it a rocky mountainous landscape. The Batholith also deformed much of the surrounding landscape, pushing rock away from the Batholith and exposing it and the metamorphic rock which surrounded it. Dykes and Sills have also been exposed on the island in swarms which are distinctive features protruding from the ground and from steep cliffs.