Which term refers to a slab of volcanic rock formed when magma forces itself across rock layers?

The term that specifically refers to a slab of volcanic rock formed when magma forces itself across existing rock layers is a "sill." A sill is formed when magma intrudes into the layers of rock and solidifies, lying parallel to the existing strata. This process often creates horizontal layers of igneous rock between sedimentary layers above and below.

In contrast, a dike is similar in that it is also an intrusion of magma, but it cuts across pre-existing rock layers, rather than lying parallel to them. A batholith is a much larger mass of igneous rock that forms deep underground and can encompass several sills and dikes. A butte refers to an isolated hill with steep, often vertical sides, which is not related to volcanic processes in the same way.

Understanding these definitions clarifies why "sill" is the correct term for this specific geological formation.