= Contraction : Fig. 3.16 Exposed granite in the Wicklow Mountains : Fig. 3.17 Contracting basalt
The Leinster Batholith is an Irish example of such a formation. It was formed over 400 million years when the North American Plate collided with the Eurasian Plate. When the plates collided, layers of sedimentary rock were pushed upwards, forming fold mountains. Magma rose upwards underneath the fold, before cooling slowly over millions years. A layer of sedimentary rock above the magma was subjected to heat and pressure, forming a layer of metamorphic rock above the batholith. Over time, the overlying mountains were weathered and eroded away to expose the underlying batholith. This metamorphic rock acts like a shield that protects the underlying granite from erosion. An example of this is the peak of the Sugarloaf in the Wicklow Mountains, which is protected by the metamorphic rock quartzite. The Leinster Batholith occupies over 1500 km² encompassing the Dublin, Wicklow and Blackstairs Mountains.
Sills
Sills are horizontal layers of granite rock that form when magma moves through the crust and forces its way between layers of sedimentary rock before cooling slowly. Slieve Gullion in Co. Armagh is an example.
Dykes
Dykes are formed when magma forces its way upwards through layers of overlying rock before cooling. Once cooled, a layer of granite is formed perpendicular to the layers of rock.
Laccoliths
Laccoliths are formed when magma forces its way through layers of rock. As more magma intrudes (forces its way) between the layers, the pressure of the magma forces the rock to push upwards. It cools slowly to form a dome- shaped layer of granite rock, e.g. Pine Valley Laccolith, Utah in the US.
Lopoliths
Lopoliths are formed in a similar way to laccoliths. As magma intrudes, its weight forces the rock downwards to form a bowl-shaped layer of granite, e.g. Great Dyke, Zimbabwe.
