Igneous intrusions

Igneous intrusions

1. Dyke

   • A Dyke is a sheet of igneous rock that is formed shown magma is injected into a fracture or fault. Dykes cut across or are discordant, the structure (e.g bedding planes) of the country rock. The width of the dyke rages from a few cm to several km's. Heat from the intrusion creates a thin baked margin composed of metamorphic rock.
   • The rocks in dykes are general medium grained and include micro granite, and dolerite such as in the Cleveland dyke. All dykes display chilled margins composed of fine grained crystals, use to he more rapid cooling of magma in contact with the cold country rock

2. Sill

   • A sill is a sheet of igneous rock that is formed when magma is injected along the bedding plane of sediment. Hence sills are concordant with the structure of the country rock. The width of Sills range from a few cm to 100m. Heat from the intrusion creates a think baked margin composed of metamorphic rock, such as the sugar limestone adjacent to the whin sill.
   • The rocks in Dykes are generally medium grained and include micro granite, and dolerite such as in the whin sill. All sills display chilled margins composed of fine grained crystals, due to the more rapid cooling of magma in contact with the cool country rock.

3. Batholith/Pluton

   • A batholith or pluton is very large intrusion caused by the slow rise of a huge mass of magma. They vary widely in dimensions and can be 100s of km in width. The considerable amount of heat from the intrusion creates a wide zone of metamorphic rocks around the intrusion called a aureole.
   • The rocks in batholiths are coarse grained due to prolonged mineral crystallization during magma cooling and include granite and gabbro. Classic examples of plutons in Britain are the granite batholiths of south-west England and the Skiddaw and shap granite batholiths in the lake district.

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