Plate Tectonics- Vulcanicity

Intrusive activity

With all volcanic regions, the majority of magma never reaches the surface but cools to form coarser grained igneous rocks beneath the ground. 

To form intrusive activity, magma cools slowly under the surface and changes the local rock forming metamorphic rocks, harder rocks and a metamorphic aureole surrounds this with precious metals such as gold zinc, tin and lead.

Features of Intrusive activity

Batholiths

• A Batholith is a large body of igneous rock formed beneath the Earth’s surface by the intrusion and solidification of magma.
• Deep seated
• Surrounded by hot rock the magma cools slowly
• Large crystals form 
• Large metamorphic contact zone. 
• E.g. Cornubian batholith; Dartmoor. Granite is only exposed due to erosion. 

Dyke

• A vertical intrusion with horizontal cooling cracks. 
• Cools rapidly on contact with surrounding colder rock. 
• Contracts and cracks, cuts cross bedding planes 
• Not usually visible as are small scale intrusive features. 
• Sometimes a swarm of dykes will form
• E.g. Isle of Arran- Kildonan shore a sandy beach with parallel walls of igneous rock acting like groynes

Sill

• A horizontal intrusion along bedding planes with vertical cooling cracks. 
• Cools rapidly on outside on contact with surrounding rocks. 
• Contracts and cracks. 
• These are not usually visible 
• E.g. Great Whin Sill, UK

Laccolith

• When magma cools and solidifies along the bedding plain 
• The volume of magma forces the overlying strata into a dome
• Visible at the surface as a small hill. 

Lopolith

• Magma cools and solidifies along the bedding plain between strata
• Underlying strata to warp downwards.