Plutons (plutonic rock)

Definition• A pluton in geology is a body of intrusive igneous rock (called a plutonic rock) that

crystallized from magma slowly cooling below the surface of the Earth.• Plutons include batholiths, dikes, sills, laccoliths, lopoliths, and other igneous

bodies. • In practice, "pluton" usually refers to a distinctive mass of igneous rock, typically

several kilometres in dimension, without a tabular shape like those of dikes and sills.

• Batholiths commonly are aggregations of plutons. Examples of plutons include Cardinal Peak and Mount Kinabalu.

Batholiths• A batholith (from Greek bathos, depth + lithos, rock) is a large emplacement of

igneous intrusive (also called plutonic) rock that forms from cooled magma deep in the Earth's crust.

• It is formed as a result of magma forcing it’s way into the rock above. When it cools it forms granite and this can be exposed by weathering and erosion.

laccoliths• A laccolith is a sheet intrusion (or concordant pluton) that has been injected between two layers of

sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.

• Simply: mushroom-shaped body of igneous rocks with a flat bottom and domed top. It is parallel to the layers

lopoliths• A lopolith is a large igneous intrusion which is lenticular in shape with a depressed

central region. Lopoliths are generally concordant with the intruded strata with dike or funnel-shaped feeder bodies below the body.

• Essentially the opposite to laccoliths, with a flat top and curved bottom.

Sills• A sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of

volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet. This means that the sill does not cut across pre-existing rocks, in contrast to dikes which do cut across older rocks.

Dike• An intrusive dike is an igneous body with a very high aspect ratio, which means that its thickness is usually

much smaller than the other two dimensions. Thickness can vary from sub-centimetre scale to many metres, and the lateral dimensions can extend over many kilometres. A dike is an intrusion into an opening cross-cutting fissure, shouldering aside other pre-existing layers or bodies of rock; this implies that a dike is always younger than the rocks that contain it. Dikes are usually high angle to near vertical in orientation, but subsequent tectonic deformation may rotate the sequence of strata through which the dike propagates so that the latter becomes horizontal. Near horizontal, or conformable intrusions, along bedding planes between strata are called intrusive sills.