What is a sill in geological terms?

In geological terms, a sill refers to a specific formation of igneous rock that occurs when magma intrudes between existing rock layers and solidifies. This process typically results in a horizontal or gently inclined layer of rock that can vary in thickness. By understanding this process, one can see that a sill represents a crystallized form of magma that was pushed into the surrounding strata, which influences geological structures and can be significant for studying the Earth’s crust.

The characterization of a sill as a slab of volcanic rock illustrates its specific formation process and distinguishes it from other geological features. This definition encompasses both the nature of the rock itself—being igneous—and its method of formation, which is integral to understanding how various rock layers interact within the Earth's geology.