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Characteristics and Formation of Igneous Intrusions

Only a small amount of magma actually reaches the surface, most is intruded into the crust where it solidifies and produces intrusive features. Intrusions (also known as plutons) tend to form where either the magma is rising slowly or if the crust is particularly thick with few weaknesses through which the magma is able to pass. Although intrusion may cause some initial doming of the surface due to the upthrust upon injection into the crust, intrusive activity has little or often no impact on the surface until overlying rocks are later worn aware, leaving landforms produced by the exposure of the ignous intrusions. As during intrusive activity, magma cools underneath the surface, the mineral crystals are coarse (large) due to the slow cooling (as can be seen in quartz crystals in granite).

Types of Intrusions:

1. BATHOLITHS

Type of Intrusion: Discordant (Vertical) Intrusion - cut across the sedimentary rock layers they intrude into.

Characteristics: large, igneous dome shaped features (deep-seated); coarse grained (due to slow cooling). These are of a significant size and usually cover at least 100km2.

Formation: created by the injection of magma into the crust - as the magma rises it fractures and melts the rock above. Surrounding the batholith is a "metamorphic contact zone" where the heat and pressure exerted by the intrusion on the surrounding sedimentary rock results in the metamorphosis of the surrounding rock - e.g. sandstone metamorphosed into schists.

2. DYKES:

Type of Intrusion: Discordant (Vertical) Intrusion - cut across the sedimentary rock layers they intrude into.

Characteristics: These are vertical intrusions which have horizontal cracks / joints which have formed parallel to the surface as the magma has cooled, contracted and solidified.

Formation: These usually form by forcing open existing fractures as well as opening up channels through cracks created by magmatic injection.

Where found in large numbers radiating from a batholith, they form a dyke swarm

3. SILLS

Type of Intrusion: Concordant (Horizontal) Intrusion - formed parallet to the bedding planes

Characteristics: boundaries of the intrusion are parallel to the exisiting layers and as the magma cools it forms vertical cooling cracks as it contracts

Formation: created by magma being intruded along bedding planes which provide a weakness for the flow of magma to exploit / flow along.

Not always formed as horizontal features and earth movements can also change the angle of sills

Igneous Intrusions - external weblink with summary of types of igneous intrusions

Explore the formation of Igneous rock formation (formed by extrusive and intrusive activity) (thank you to Dr S Hitch for the link!)

A detailed article - includes "What are Intrusions?" and "Types of Intrusions" - for the exam you are only expected to know batholiths, dykes and sills.