Intrusive Plutonic Features (Leaving Cert Geography): Revision Notes
Intrusive Plutonic Features
What are intrusive plutonic features?
Intrusive plutonic features form when molten magma cools and hardens beneath the Earth's surface within the crust. Unlike volcanic eruptions that occur above ground, these features develop deep underground over millions of years. The slow cooling process allows large crystals to form, typically creating granite rock formations.
During periods of mountain building, folding, and faulting, cracks develop in the Earth's crustal rocks. These fractures can extend deep into the mantle where magma exists under enormous pressure. When this molten material is released, it rises and intrudes into the crustal rocks above, where it gradually cools and solidifies to form various types of intrusive features known as plutons.
Over geological time, weathering and erosion remove the overlying rock layers, eventually exposing these intrusive features at the Earth's surface where we can observe them today.
Types of intrusive plutonic features
Batholiths
Batholith: A massive body of intrusive igneous rock that develops when magma cools slowly within the Earth's crust over millions of years, typically forming granite. Batholiths are usually larger than 100 km² and commonly develop beneath mountain ranges, often protected by a cap of metamorphic rock.
Batholiths represent the largest type of intrusive feature. As rising magma reaches the crust, it melts the surrounding rock layers and gradually cools to form enormous granite bodies. These features typically develop underneath mountain fold systems and become capped by metamorphic rock layers that act like a protective shield, preventing erosion of the underlying granite.
Case Study: The Leinster Batholith
- Location: Covers over 1,500 km² across Dublin, Wicklow, and Blackstairs Mountains in Ireland
- Age: Formed approximately 400 million years ago
- Formation process: Created when the North American Plate collided with the Eurasian Plate, causing sedimentary rocks to fold upward into mountains. Magma rose beneath these folds and cooled slowly over millions of years
- Protection: A metamorphic rock layer (including quartzite) protects the underlying granite, as seen at Sugarloaf Mountain in the Wicklow Mountains
- Exposure: Weathering and erosion have gradually removed overlying material, exposing granite formations like those visible in the Wicklow Mountains today
Sills
Sill: Horizontal sheets of granite rock that develop when magma intrudes between existing layers of sedimentary rock and cools slowly in place.
- Sills form when magma moves through crustal cracks and spreads horizontally between existing rock layers, following the natural bedding planes of sedimentary rocks. The magma cools gradually in these horizontal positions, creating sheet-like granite formations parallel to the surrounding rock layers.
- Example: Slieve Gullion in County Armagh demonstrates this type of horizontal intrusion
Dykes
Dyke: Vertical walls of granite rock formed when magma cuts upward through existing rock layers and cools, creating formations perpendicular to the surrounding rock structure.
Dykes develop when magma finds vertical pathways through overlying rock layers, cutting across existing geological structures. Once the magma cools and solidifies, it creates wall-like granite formations that stand perpendicular to the original rock layers.
Laccoliths
Laccolith: Dome-shaped granite formations created when intruding magma forces overlying rock layers to arch upward before cooling into a mushroom-like structure.
When magma intrudes between rock layers but encounters resistance, the pressure causes the overlying rocks to bulge upward, creating characteristic dome shapes. The magma then cools slowly in this position, forming granite bodies with distinctive mushroom-like profiles.
- Example: Pine Valley Laccolith in Utah, USA
Lopoliths
Lopolith: Bowl-shaped granite formations that develop when the weight of intruding magma causes overlying rock layers to sag downward before the magma cools.
Lopoliths form in a similar way to laccoliths, but instead of pushing rocks upward, the weight and volume of the magma causes the overlying rock layers to sink downward, creating bowl-shaped depressions that fill with cooling magma.
- Example: Great Dyke in Zimbabwe
Key Points to Remember:
- Intrusive plutonic features form when magma cools slowly beneath the Earth's surface, typically creating granite rock formations
- Batholiths are the largest intrusive features (>100 km²) and commonly develop beneath mountain ranges with protective metamorphic rock caps
- Sills intrude horizontally between rock layers, whilst dykes cut vertically through existing rock structures
- Laccoliths create dome shapes by pushing overlying rocks upward, while lopoliths form bowl shapes by causing rocks to sag downward
- The Leinster Batholith in Ireland demonstrates how these features can be exposed through millions of years of weathering and erosion