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Forms of Igneous Rocks (Igneous Bodies)



Concordant and Discordant Forms of Igneous Rocks 



Igneous intrusions are classified based on their relationship to the pre-existing rock layers (country rocks). These intrusions can either conform to or disrupt the layering of the surrounding rocks, giving rise to concordant and discordant forms.


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Concordant Forms

Concordant intrusions occur when magma is injected along bedding planes or between layers of pre-existing rocks, aligning with the natural structure without significantly disturbing it.

1. Sills

Definition: Thin, horizontal sheets of magma that solidify parallel to the bedding planes.

Formation: Formed when magma intrudes between sedimentary or other layered rocks under low pressure.

Characteristics:

Typically uniform in thickness.

May vary in composition (basaltic sills are common).


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2. Laccoliths

Definition: Dome-shaped intrusions where magma pushes the overlying layers upward, forming a convex structure.

Formation: Result from magma of high viscosity (e.g., granitic magma) that pools and lifts the overlying layers.

Characteristics:

Flat base and domed top.

Often associated with volcanic activity.

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3. Lopoliths

Definition: Basin-shaped intrusions where the magma pools and causes the overlying layers to sag downward.

Formation: Typically formed by dense, mafic magma.

Characteristics:

Convex downward shape.

Large in size, often associated with layered mafic complexes.
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Discordant Forms

Discordant intrusions occur when magma cuts across the bedding planes or layers of the country rock, disrupting their natural arrangement.

1. Dikes

Definition: Vertical or steeply inclined sheets of magma that cut across pre-existing rock layers.

Formation: Formed when magma forces its way through fractures or faults in the rock.

Characteristics:

Typically narrow but can be very long.

Often found in swarms (multiple dikes in a region).

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2. Batholiths

Definition: Massive, irregularly shaped intrusions that form deep within the Earth's crust.

Formation: Result from the slow cooling and solidification of large volumes of magma.

Characteristics:

Extremely large (covering hundreds of square kilometers).

Composed mainly of granitic rock.

Often exposed at the surface due to erosion of overlying material.


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3. Volcanic Necks (Plugs)

Definition: Hardened magma within the vent of an extinct volcano, exposed after the surrounding volcanic material is eroded.

Formation: Forms when magma solidifies within the throat of a volcano.

Characteristics:

Resistant to erosion.

Often steep and prominent.



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