Intrusive Structures

Batholith

• A Batholith is the largest intrusive volcanic landform, with a surface area of 100 square kilometres
• It has a domed top with gently sloping sides
• Formed when magma was forced into the crust, cooling and solidifying to form intrusive rock such as Granite
• It appears on the surface as Denudation (weathering and erosion) exposes the batholith and plates colliding (results in buckling and compression which creates fold mountains – example is Leinster Batholith from Kilkenny to Dublin)
• They are usually surrounded by metamorphic rocks
• Wicklow mountains were formed during the Caledonian folding – the core of the Leinster Batholith in the Wicklow mountains is Granite. The area surrounding the edge of the batholith is metamorphic rock such as quartzite and schist

Sills

• Formed when magma is injected horizontally between the layers of sedimentary rock that are found close to the surface
• The magma squeezes into the bedding planes, melting surrounding rock, then cools to form granite

Dykes

• Form when magma is injected vertically into fissures in the surrounding rock strata
• Overtime the magma widens this fissure and eventually cools, forming a vertical layer of granite

Laccoliths

• Form when magma pushes an overlying rock upwards
• They have a domed top with a flat base
• The magma then cools and solidifies to form granite

Case Study: The Leinster Batholith

This is a case study of an intrusive volcanic landform

Causes

• American and Eurasian plates collided, forming Leinster's fold mountains
• Part of these plates melted and subducted, allowing large volumes of magma to force its way into the crust
• Once this magma cooled and solidified, it formed a dome-shaped batholith underground
• This is made of granite - weathering and erosion removes outer layers of rock to expose the granite
• The hot magma underground deforms the other rocks, creating layers of metamorphic rock - Lugnaquilla in the Wicklow Mountains has a cap of resistant schist rock on top