Rock Topics

Extrusive and Intrusive (Part 3)

Intrusive Rocks

Intrusive igneous rocks form below the earth’s surface.  They are classified based on their shapes and relationships to the pre-existing or host rock.

• A dike is a tabular mass that cuts across surrounding rocks.

• A sill is a tabular mass that is parallel to planar structures (such as layers) in surrounding rocks.

• A batholith is a large body of irregular shape that cuts across surrounding rocks.  Batholiths are commonly composed of granite.

• A stock is like a batholith, but smaller. 

IntrusionsIntrusions (Diagram by Roy Sites)

This diagram shows the different kinds of igneous intrusions.  Note that the dike, batholith, and stock cut across the layers whereas the sill is parallel to the layers.